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Using old laptop batteries to build homemade powerwalls (futurism.com)
313 points by jonbaer on Mar 21, 2018 | hide | past | web | favorite | 197 comments

The comments here are more evidence the "hacker" part of hacker news really no longer applies :)

In seriousness - of course putting this in your house is a a pretty risky and likely dumb idea. I absolutely have plans to do this though, just with a fire resistant (concrete block) "power shed" 50 yards away from my house.

Someone that can build a working DIY powerwall can cheaply build a cinder-block shack and run conduit back to the living quarters. If you can't afford to do that I'd say you can't afford to do the project at all.

"Hackers" historically developed such a depth of understanding of some system that it allowed them to modify and subvert the system to suit their needs. The people here who have built power systems for Battlebots and are familiar with the variability in specifications for 18650 cells and are saying that this isn't a good idea for reason X, Y, and Z, those people are the hackers.

The people that are saying, "a battery's a battery, I'll just put it in a shed" are dabblers. They don't understand the technology well enough to do it safely, so they just build it with the assumption that it's going to go FOOF someday.

> They don't understand the technology well enough to do it safely, so they just build it with the assumption that it's going to go FOOF someday.

Accommodating for failure modes is not lack of understanding... most chemical batteries have some inherent combustive or explosive danger, lead acid can produce hydrogen explosions, LiPo can slow burn when the layers are punctured through expansion etc.

Failure in a car is one thing (make sure it doesn't happen violently so that it can be controlled and escape is possible), but in a house you just can't afford for it to happen at all, sticking it in a separate building is a very reasonable and simple precaution for such a large quantity of chemical batteries on one place.

It's bad enough when small laptop batteries burn up, and those have to pass safety test... imagine a few crates... in your house... under high load from having to power all of your appliances.

You've created quite the false choice there. You've completely left out the people who know that it is in idea for reasons, X, Y, and Z (and maybe some others too) but are doing in anyway with a full understanding of how to modify and subvert those risks.

You know, hacking with a depth of understanding. I think you are responding to a person like this.

This definition of a hacker doesn't sit well with me. When I was a kid I hacked on things I didn't understand for a long time in order to get a better understanding. Was I not hacking? What I not a hacker then? I've never really put too much thought into it but always assumed I was.

Who cares? It's all just gatekeeping. Keep doing what you enjoy doing and don't worry about what categories other people put you in.

There's a difference between "not fully understanding the functioning principles" of something and "not understanding what is potentially lethal about the functioning principles" of something. I messed around with plenty of things I barely understood as a kid but I knew where to draw the line when it came to messing around with things I didn't fully understand. I knew to hold off and do my research (basically, ask a competent adult) before messing around with anything to do with flammable and/or "powerful" chemicals (think heavy duty, old school brake dust cleaner from my grandfathers garage), electricity (this saved me from electrocuting myself with the capacitors in a CRT TV), and high temperatures.

There are certain things that will make sense to someone with a pretty extensive knowledge of the subject at hand but are still in the category where the factors at play should make one pause and consult a professional. One example would be people who hook up generators to their 220V dryer connection in order to power their house in an emergency (it's called backfeeding). Technically, that should work but if you don't think to throw the main you'll wind up injuring or killing the lineman expecting a deenergized line when they come to restore power to your neighborhood.

Trouble is that DC power at the levels required to be used as a replacement for mains is quite different to low voltage dc 5 /3.3v as say used by hackers who use ardunios etc.

Talk to any telecoms vet and they will have stories about accidents with DC power hopefully funny ones not tragic ones

Can you reference any stories? I can't find much.

But the implication that you need to use high voltages isn't true. You can easily feed 24/36/48 volts into an inverter. The high current potential only acts weirdly when you short it out, which is where a couple fuses can prevent issues.

Doesn't normaly make the news

One of they ones I heard was about a painter who when painting the walls of a small exchange put the paint can across the bus bars which shorted and exploded the can of paint.

"FOOF" is an interesting word choice. Have you heard of dioxygen difluoride, aka FOOF? If not, you may find this article interesting:


I did in fact borrow it from that blog. That's probably my favorite chemistry-related "dessert reading". :-)

Since you mentioned Battlebots, I wanted to mention that when I was on Battlebots in 2002, the options for batteries were pretty damn slim.

The was one company making a good product called Battlepacks (for lightweight - super heavyweight classes). If you were doing anything in the smaller weight classes (antweights through the 30lbers), you were typically hijacking a lot of remote aviation resources.

The options available now are much better than they used to be.

Well, the Powerwall, along with batteries, also has a ton of safety features that make it safe to install inside a home without worry.

One would argue even if you know exactly what you're doing, unless you're replicating those same safety features as well, you should put it in a shed anyway.

That said, totally agree with what you're saying.

> Well, the Powerwall, along with batteries, also has a ton of safety features that make it safe to install inside a home without worry.

The logical conclusion of this is someone mentioning ICE vehicles and gas tanks, so allow me to be that person. If you park your gar in the garage and it's an ICE vehicle, you're parking quite a lot of latent energy in a fairly volatile form in your house.

If you wouldn't feel comfortable rigging a bunch of old gas cans in tandem to a little engine your scrounged as a DIY generator and putting it in your garage, maybe you shouldn't rig a bunch of batteries up in a similar arrangement.

For that matter, with that perspective, how far away for your cinderblock outhouse is far enough?

The amount of energy isn't the most interesting thing to consider: a stack of logs contains way more energy than such a powerwall. What matters is “how stable” is that energy storage, and batteries are way less stable than wood ;)

Well, yes, and that's what I said "in a fairly volatile form" with regard to gasoline. :)

Any energy storage medium that holds a good amount of energy and has a methods to release it in a fairly short time frame (either through a failure mode or through intended use) deserves some real thought. Which is basically what you were saying...

And I think that's why you don't see that kind of energy storage happen in a home very often.

- Natural Gas is brought in through VERY small pipes, and is never kept anywhere, it's just used immediately.

- Jerry cans are found regularly but are also sealed up as much as possible while still remaining usable.

- Vehicles have sealed fuel systems, and are designed to not allow the fuel to combust all at once if at all possible.

In fact, now that I'm thinking about it, pretty much the only time you hear about homes exploding/bursting into flame/etc. is when one or more of these systems is compromised.

> In fact, now that I'm thinking about it, pretty much the only time you hear about homes exploding/bursting into flame/etc. is when one or more of these systems is compromised.

Medical oxygen plus ignition sources (especially cigarettes) seems to be another (for “bursting into flames” more than “exploding”.)

Where I live, Norway, if your garage is part of your house then it has to a have a firewall between it and the rest of the house.

The same is often required in America. Garage doors usually need to be solid as well.

That said, a firewall is not a fire proof wall, it's a wall designed to burn slower to prevent the fire spreading between those areas too quickly. Its purpose is to retard the fire u til it can be put out or so you have enough time to escape with your life. I suspect with failed batteries the latter is more likely your best chance.

Personally I have no expertise on the matter, but there are also other hackers who have been doing this for ages. Go to EVTV.me, a community about home built EVs, for example. You'll find plenty of documented projects of recycling laptop cells for both EVs and home energy storage.

From these comments, it seems like a "hacker" is either someone who understands the technology extremely well or not well at all... either way it's going to be fun ;) (and hopefully not too dangerous).

A battery is a battery, any lithium battery from a drill to a tesla is simply a 1.5 volt cylindrical cell stacked in series to produce the desired Voltage, then these stacks are connected in parallel or series again as needed (depends on application) to increase the load it can handle (Amperage).

The safety features you're so worried about are just fuses. And maybe some diodes if you're that worried. I can say that installing a bank on a piece of plywood on your garage wall is obviously stupid, but a simple opening of the NEC book will show the code for a battery bank, shall have clearance from a wall and rest on non-flammable/non-conductive material, usually in a nuclear power plant or a substation we would build a rack out of metal then install ABS plastic shields.

Source: I'm an industrial electrician and went through an IBEW apprenticeship,

Sorry, but no.

Most lithium chemistries have a nominal voltage near 3.3 - 3.7V per cell.

When you stack lithium cells in series for any sort of high-performance application, you need additional balancing circuitry to bleed off energy from the cells individually (tiny variations between cells results in them having different capacities, which means some will charge up at different rates, resulting in them DISCHARGING at higher rates, which results in pretty serious negative effects for the life of the battery pack). The only exception here is extremely well matched cells, which is how Tesla is able to get away with not using much balancing circuitry (they are very picky about the cells they use, and have enough cells in stock to pick ones with very similar capacities).

And the safety features include a lot more than fuses - there are over-voltage protection circuits (extremely important!), over-discharge protection circuits, temperature protection circuits (you can overheat a cell even without drawing too much power out of it).

In short, large stacks of lithium cells are a difficult and dangerous beast to handle, and a lot of work goes into keeping them safe. These cells tend to have very low internal impedance, and as a result a chain of them can produce staggering amounts of power.

Source: I work on the embedded protection systems for a company which makes specialty lithium-ion battery systems for industrial, aerospace, and defense customers.

Not to mention" that LiIon cells were known by Ford and Edison. Their first examples are in the museum in Florida.

Why didn't they use them in the first cars? When they either over or undervolted, they tended to explode. We didn't have the electrical circuitry to handle lithiums.

Now we do, mostly. Bad "spray fire and death" style accidents still happen.

> any lithium battery from a drill to a tesla is simply a 1.5 volt cylindrical cell

Incorrect. Lithium Ion batteries are 3.7V (nominal) with different chemistries varying from 3.2V (LiFePO4) to 3.8V (LiHV).

>The safety features you're so worried about are just fuses. And maybe some diodes if you're that worried.

Also incorrect. It won't just have fuses, but also low voltage (over-discharge) protection, high voltage (over-charge) protection, and transient current limiting with auto reset, typically done with an IC + FETs per small pack of cells or even per cell.

So before you can build your pack you need to make sure that the cells all have the same chemistry and charge level, and that the ICs are programmed to cut out at the same voltage and current, otherwise you get nasty surprises when connecting in parallel (the voltage mismatch) or in series (current mismatch) or any hybrid of the two.

Source: specced and sourced cells, safety ICs and chargers for portable payment terminals. And 15 years experience in electric RC aircraft.

nice, I cede all your points. Glad to learn from everyone and help my ignorance in the matter. Thanks all.

Nope, different lithium batteries have different chemistry compositions.

A LiFePO4 will happily catch on fire if you charge it with the same voltage/current curve as a LiPo(3.6v vs 4.2v).


To expand a bit, this[1] is a fantastic reference on exactly how complicated and involved the different chemistries can be. This is talking about Lead -> LiFePo4 but a lot of stuff applies to LiPo and other variants as well.

[1] https://marinehowto.com/lifepo4-batteries-on-boats/

From your linked article:

> Please note that in all 68 of these RUINED LFP banks, that I have to date, there was not one incident of fire, explosion or dangerous off-gassing.

So you can destroy them easily by overcharging or other mistake, but LiFePO4 is probably the safest of the lithium chemistries as far as fire goes.

Fair enough, I'm used to pushing the packs a bit harder on discharge so I swapped that around(charge/discharge).

You have to try really hard to make LiFePO4 catch on fire. LiIon just requires 0.5v of overcharge, but LiFePO4 usually also needs a case puncture, followed by a short circuit.

Honest question, I thought they were 3.6 volt cells?

Depends on the chemistry. Normal li-ion chemistry is nominal 3.7V, max 4.2V. LiFePO4 chemistry is nominal 3.3V, max 3.6V. Plus there are other variants that have different parameters. I'm not aware of any lithium-based cell that has a nominal voltage of 1.5V, the commenter was probably thinking of alkaline cells.

Your comment about a power shed reminded me of this neat youtube series where someone living off-grid built their own hydro-electric generator to supplement their solar: https://www.youtube.com/watch?v=MUNMjdmGIPI&list=PLEZ2hvCDKU...

I love your comment, but do you mean 50 ft. or do you really mean yards?

Not many people have a yard the size of half a football field, and even if they did, it would probably put the danger close to a neighbors house.

So seriously, what would a reasonably safe distance be? Should this be avoided if you don't live in a really rural area with a giant property?

You could build that in your garage with a concrete block or 3/8 steel box and a fire retardant system. Hell you could probably even just buy a sufficiently large gun safe if you didn't want to build something. Actually even a metal fire cabinet for storing acetone and such chemicals may be sufficient but if someone were interested would need to research this further of course.

Or you could just hire an electrician and buy new batteries so you don't have to worry about burning your house down. Lithium battery packs are perfectly safe to keep in your home when used and maintained properly.

It's not even so much the DIY aspect of this that's an issue, as much as it is people thinking it's OK to use old batteries. It's not. These are not deep cycle lead acid batteries. LiPo cells have a very definite cycle lifetime, beyond which their internal resistance grows to dangerous levels. Using them beyond that is just reckless and irresponsible.

Even new cells of this type have some risks, especially when the charge controller and wiring are all home-brew. Risk mitigation via physical barriers isn't at all a crazy thing to think about.

I just used 50 yards as an overly cautious example.

I haven't done enough research to feel comfortable giving a number, but it should be well under 50ft even.

You do make a good point that this won't be an option for everyone. If you live in a dense city without yards, I guess this is one fun project I'd take a pass on. If you live in a standard suburb you are probably okay at least in terms of space.

Have you done any research into the potential energy release?

Here is a decent primer. At about 26:30 they talk about battery failure testing. They have a 2 ton blast door in the room where tests are conducted.


Since they noted an 1800 degree flame (for what looked like an extended period), I wanted to look up the temperature at which cinder blocks failed, and came across this[1]. Apparently, depending how "cinder" your cinder blocks are, they might actually catch fire. :/

1: http://www.eng-tips.com/viewthread.cfm?qid=94710

Eh. Of course "hacker" has some overlap with "danger", but that doesn't automatically mean "smart" - Radioactive Boy Scout was an essential hacker. (Now get those newfangled JavaScript contraptions off my lawn; kids these days, calling themselves hackers! waves stick)

> The comments here are more evidence the "hacker" part of hacker news really no longer applies :)

More like the same response that an article advising people to create their own crypto library would get. Except creating your own crypto library doesn't have the risk of killing your family, neighbors, and emergency responders.

Presenting this kind of project as a "hack" is really irresponsible given the very serious risk of death, severe physical injury and property damage involved.

> Someone that can build a working DIY powerwall can cheaply build a cinder-block shack and run conduit back to the living quarters

The real issue here is to prevent thermal runaway.

I would go further, and add a thermal cutoff switch to turn off the battery charging if it's too warm outside (We've had summer days here at 115F.) Then maybe a fire suppression system of some sort.

Fire supression should be pretty easy in this configuration. Get a 55 gallon drum of deionised water, put it on top of you battery shack, feed a plastic hose from the bottom of the tank into the shed and seal the hose with a plastic that melts at, say, 150 F.

With all the recommendations of mods, custom PCBs, sheds, cinderblock enclosures, underground bomb shelters, etc - the $5500 Tesla PowerWall doesn't sound so bad at all!

Edit: removed concern about deionized water.

We really need a battery that's high capacity and doesn't explode.

It's quite sad, "don't do this, don't do that, never use the stairs 'cause you can die and insurance won't pay". What's the word for "non-hackers"... "normies"?

While Li-Ion is a different beast, I used to play with car batteries and UPSes, a simple setup could run some lights, fridge, the phones and computers for over a day. It took some time before I wisened up and made a separate metal box for everything. Until then, ran it all from the closed balcony.

"Muggles"? :)

In seriousness - of course putting this in your house is a a pretty risky and likely dumb idea.

Would it be possible to design a minimal, modular metal rack to act as thermal isolation? I could picture a scheme with metal bent into a shallow Z shape with obtuse angles, which would direct the burning debris from a failed battery away from all the others.


In any hacking group there has always been a large contingent who criticize using the worst case scenario. It's pretty par for the course when engaged in any critical thinking.

If I had the space and need to build a powerwall. I probably wouldn't do it with a bunch of 18650s. Calb and Winston cells make a lot more sense.

No, Hacker Rule Zero, "don't be on fire"[1].

[1] https://wiki.london.hackspace.org.uk/view/Rules

Those read more akin to safety rules with "hacker" tacked on for theme.

They are saftey rules, yes, obviously. And they were developed by hackers. It's rule zero for various makerspaces, so I think it's a fairly clear example that caring about fire saftety is compatible with being a "hacker".

Of course safety is compatible with being a hacker. However, safety rules are not unique to hackers nor makers. My issue with the parent is the linking to some makerspace's rules as if they speak for the community at large. Much the same way I find it goofy to use the glider from the Game of Life as the emblem of hackers everywhere.

The U.S. Navy's KISS principle equally applies to the amorphous concept of being a hacker, but you don't see people espousing that as a tenant.

I just guess I'm against people attributing ideas, &c to something that doesn't need them in the first place (and may even be better when left to the abstract). shrug

> The U.S. Navy's KISS principle equally applies to the amorphous concept of being a hacker, but you don't see people espousing that as a tenant.

Well I hear it espoused all the time. I think you do too, otherwise you would not be referencing a 1960s phrase in 2018.

> My issue with the parent is the linking to some makerspace's rules as if they speak for the community at large.

There is no community at large. This site is called hackernews. Paul Graham constantly writes about what a hacker would do in situation X. Literally anybody who feels like it can call themselves a hacker. There is no professional body, w00t!

So if London hackerspace says they are hackers, and they write some rules, ergo => some legit hackers made those rules

> I just guess I'm against people attributing ideas, &c to something that doesn't need them in the first place (and may even be better when left to the abstract)

Well you sound miserable if you are against some people calling themselves hackers.

> Well I hear it espoused all the time. I think you do too, otherwise you would not be referencing a 1960s phrase in 2018.

You misread. Tenant of hacker-dom.

> There is no community at large. This site is called hackernews. Paul Graham constantly writes about what a hacker would do in situation X. Literally anybody who feels like it can call themselves a hacker. There is no professional body, w00t!

Exactly my point.

> Well you sound miserable if you are against some people calling themselves hackers.

Ad hominem isn't necessary.

sorry, bad day.

I touched a six volt battery to the back of my neck and got told by HN I was going to fry my brain. Grossly overestimating risk is a minor league sport around here. Why do you think most of these people haven't actually started their own company or left their supposedly soul-sucking corporate jobs, yet?

This seems like a great way to burn down your house and potentially injure firefighters coming to your rescue. Seriously this is quite a few leagues above normal DIY territory. Proceed with caution.

Exactly right. It will catch on fire, it will burn down the house. I expect the insurance would not cover it (just like it doesn't cover fires caused by storing gas or propane in your garage)

I got a crash course on battery charging subtleties when building battle bots and there are many ways this can so south quickly, not the least of which is a battery in the middle of the pack developing dendrites and deciding its time to burn. The only way to prevent that is to have temperature sensors on every battery and a good enough model of the heat convection between the internal battery and the surface to recognize the temperature rise of a battery on its way to failing. Even with modern "battery monitor" chips like the ones Maxim and others sell there are corners in charge rate/temperature/battery that can get away from the algorithm before it recognizes a battery failure.

The reasons this is "easy" with flooded lead/acid batteries is that, when they fail, the lead doesn't then turn into a torch at 500 - 600 degrees F. So you have a chance to fix failures and move on.

> I expect the insurance would not cover it.

I see this blanket assertion in lots of comment threads, but never with a citation.

> (just like it doesn't cover fires caused by storing gas or propane in your garage)?

This is an assertion I have never seen before. It is extremely common to keep gas cans in the garage around here (IL, USA), where else would you keep them that is safer?

I have discussed it at length with the USAA insurance people, I expect there are policy documents on their web site. The exact discussion was, "If the source of the fire is tracked back to a leaking gas can or a propane tank we won't cover the loss, we recommend you store flammables in a shed away from your house."

My impression from them was they were very dubious about "off book" uses of things as well, which leads to my expectation that they would push back on covering a loss due to a "home made" battery wall. Could I sue them and get them to cover it? I don't know, perhaps. But, as with my propane tanks, I would keep this stuff out away from the house to limit my losses and avoid having to go there.

I googled "insurance fire caused by gasoline in garage" and found https://cpsc.gov/content/consumers-warned-against-storing-ga... as the third result. "CPSC also warns consumers that private storage of more than a limited amount of gasoline (usually five gallons or less) is illegal in many areas, and subsequent fire damage may not be covered by insurance policies."

although IANAL, storing a large quantity of lithium ion batteries seems likely to be considered negligence, which is generally not covered under insurance.

Here is a quote from my insurance policy (Erie Insurance):

> “We” do not pay for loss resulting directly or indirectly from...the discharge, dispersal, release or escape of any solid, liquid, gaseous or thermal irritant, pollutant or contaminant, including smoke, vapors, soot, fumes, acids, alkalis, chemicals and waste

I checked another insurer (State Farm) and they had similar language:

> We do not insure, however, for loss...caused by...Discharge, dispersal, seepage, migration, release or escape of pollutants...Pollutants means any solid, liquid, gaseous or thermal irritant or contaminant, including smoke, vapor, soot, fumes, acids, alkalis, chemicals and waste

Keep reading the Erie policy. Look for phrases like:

"...into or upon the land, the atmosphere or any course of or body of water, whether above or below ground."


"but this exclusion does not apply if such discharge, dispersal, release or escape is sudden and accidental."

I'm pretty confident you will find them, or something similar.

I looked, but I don't see anything like that. The exclusions for smoke and water damage do not apply to "sudden and accidental" losses but the pollution one does. And it does not limit it to contamination of the environment.

My Erie policy was emailed to me, but here is a State Farm policy (not the one I quoted above) that has a similar exclusion:


That said, from googling I gather that there has been a lot of litigation over what counts as a "pollutant," including specifically whether gasoline counts. So I don't think that the presence of this clause means that an insurance company will always try to exclude any losses involving gasoline. I just think maybe they would have an argument if they wanted to.

Look at page 21 of that document.

"This (pollution) exclusion does not apply to bodily injury or property damage:

... (3) caused by common household chemicals used to maintain the residence premises."

Additionally, and perhaps even more importantly, this exclusion is in the "Personal Liability" section of the policy. The coverage/exclusions for the "Dwelling" are on pages 8-12.

Ah, interesting, I didn't read that to include gasoline but I see how one could argue that it does. The Erie policy has no such language.

In any case, thanks for the responses. My gut reaction is to agree with you that a lot of these "homeowners insurance will deny your claim if you do X!" warnings are based on third-hand knowledge or misinformation. I don't have any special knowledge though. (Do you?)

Edit: oops, you're right of course that I was reading the wrong part of the State Farm document! I swear I am not reading the Erie one so carelessly though.

> I see this blanket assertion in lots of comment threads, but never with a citation.

Imagine a company whose only product is one that if anyone uses, they lose profit. It doesn't take a huge leap of imagination to believe insurance companies will blame literally anything to deny coverage on a house fire.

And imagine if you had a house fire and had your coverage denied. Would you be at all shy of posting those details on a public forum? I'm not saying that denials like this can't/haven't happened, just that I've never seen a first-hand account of such an occurrence (it's always somebody's cousin's co-worker's son).

> (just like it doesn't cover fires caused by storing gas or propane in your garage)

What about the gas in your car or lawnmower?

Where do you store your camping stove?

The insurance texts posted downthread don't actually seem that "damning" to me. They talk about chemicals etc in a blanket sense. If insurance companies were worried about the gas can for your lawn mower, they would come right out and say it.

It's not uncommon for cities and homes associations to prohibit sheds anyway. A gas can in the garage seems a lot less volatile than one sitting in the sun that looks like it's ready to burst.

> The insurance texts posted downthread don't actually seem that "damning" to me. They talk about chemicals etc in a blanket sense. If insurance companies were worried about the gas can for your lawn mower, they would come right out and say it.

Why do you think that? I believe the purpose of the exclusions isn't to warn you about what bad things not to do, but rather to allow them to get out of covering certain losses. So broader is better (for them).

The idea the homeowners insurance doesn't cover the homeowner being negligent or stupid seems to betray a misunderstanding of what homeowners insurance is generally used for, namely protecting the mortgage lender from a catastrophic loss of the property's value.

In this case, since I don't have a mortgage, it is really covering my losses. Should my house burn down I don't want to have to go out and pay to rebuild it. I'm willing to take the actions the insurance company requires in order to keep their actuaries happy.

For anyone who isn't aware, California has seen a number of large scale disasters over the last couple of years which have destroyed many homes. From the fires in Santa Rosa to mudslides in Santa Barbara. And I have a number of friends and acquaintances who have been having painful discussions with their insurance companies about what they will and won't cover. I am fortunate that I have not suffered a loss. Seeing that pain though led me to call up and have a long, recorded, and detailed discussion with my insurance company about how they decide what to cover and on what basis they feel entitled to deny a claim. As with people who have actually lost their homes I was surprised at the number of things that they would use as a reason for not paying a claim.

One consistent theme was "unusual hazards that the insurance company was not made aware of." This ranges from people who have turned their garage into a place to do car repairs (adding lifts etc) to people who brew beer under the sink in the kitchen.

On the basis of that conversation, and the assumption that my insurance company was 'typical', I don't expect the battery fire to be covered. :-)

Did you take notes on these conversations? I'm very curious :)

In this particular case I recorded it :-) (with the permission of all involved) so that in the event there was some discrepancy about who said what I could refer to the recording.

Any other specific examples you care to share of things that you were surprised to find would not be covered?

Not really, there was an interesting digression into the question of using/installing "industrial equipment" (I've always wanted a CNC machine center for my garage.) We have an "industrial" washer and dryer pair (but that was ok). No sense turning this battery example into an insurance thread.

> No sense turning this battery example into an insurance thread.


There's a difference between being negligent (leaving the stove on) and performing unlicensed DIY electrical modifications that were not approved by code inspectors and involve storing huge amounts of energy.

Exactly. Insurance will generally cover anything that it doesn't specifically exclude in the insurance documents. If you are walking with a can of paint and drop it and destroy your carpet most homeowners policies will pay to replace the carpet. If you leave the water on in the tub (by accident) and flood your house the policy will most likely pay for it. However it won't cover floods from the outside. But it will cover water that comes in from the roof or through the windows (subject to any exclusion for hurricane potentially).

Likewise if you decide to put together an array of lithium batteries in your bedroom most homeowners policies should cover that. After all those batteries already are in products in the house (as opposed to gasoline which would only exist in a lawn mower in small quantities in the garage).

What is most likely to kill you in most fires is the smoke and that is definitely true about Lithium battery fires.

This what makes them so deadly on a plane: you cannot get away.

Assuming that your smoke detector reacts properly to that kind of smoke, you should put one near your battery array.

In many cases it will only burn down the dedicated battery shed.

Mixed batteries of unknown provenance/capacity without balancing isn't something I'd want in my basement personally.

If you watch the video it isn't quite what the headline suggests. The guy is collecting 18650 cells from old laptop batteries and using them with his own BMS to build a new battery. Not exactly mixing and matching rando used batteries.

"collecting 18650 cells from old laptop batteries" is literally "mixing and matching rando used batteries". well, cells anyway.

Except that the dude is measuring every individual cell (remaining capacity, resistance, ...) and then builds his power packs from the good cells. Just look at his channel, he has lots of videos about the process and multiple thousand cells to choose from.

The dude in the video is using what appears to be the same type of cell.

It's still mixing and matching. Different cells that have been used in different ways have different resistances. That causes each cell to charge and discharge at slightly different rates. Over time, you'll wind up overcharging one of those cells in relation to the others and burn your house down.


most of these I have seen have the charge protection tabs on the top. are the laptop batteries different?

most of my experience is from buying fresh cells to use as caving headlamp batteries back in the day.

> most of these I have seen have the charge protection tabs on the top.

That depends on the cell.

Some cells have charge protection. Others don't. The 18650 cell is standardized only in size-and-shape. The actual protection, chemicals, and other specifications are completely different from cell to cell.

Heck, some 18650s have 3.2V nominal voltage, while others have 3.7V.

IIRC, charge protection causes a voltage drop. So some applications prefer to use unprotected cells (with the overall device implementing charge protection elsewhere). For example, Laptops probably have ONE charge-protection circuit somewhere in their construction, rather than 6-charge protection circuits (each cell). This simple fix will cause 1/6th the voltage drop compared to 6x individual charge protection circuits.

The amount of current a battery can deliver also varies. Flashlight forums where people sometimes want the maximum power output will point you to a couple of specific 18650 parts and a similarly small number of sources where you can be reasonably sure they aren't counterfeits.

(Some) manufacturers provide specs for their batteries, but most of them are coming from no-name factories in China with no reputation at stake, so there's either nothing published or it's exaggerated.

Li-Ion 18650 cells have different specifications.

Just because something is in the shape of the 18650 doesn't mean that the chemicals inside of them are the same. Tesla's 18650s allegedly don't have Cobalt in them for example. Panasonic / Tesla have been working on "special chemical sauce" to make the cells work better for car purposes.

Even WITHIN a company, there are different specifications. Samsung's ICR18650-22F has different specs than Samsung's ICR18650-32A.

Lithium is a potent chemical that is EXPLOSIVE when mishandled. Lithium Fires cannot be put out by a class A, class B, or class C fire extinguisher. Lithium Fires CANNOT be put out by suffocation (they are self-sustaining and do not require oxygen). And sure, the cells are "Lithium-Ion" and not "pure lithium" (Li-Ion is a bit safer), but the potency of the core element here demands caution.

Energy storage works because you're storing energy. And if that energy decides to "stop being stored", you basically got a major, explosive fire on your hands.


For hobby works (especially since I'm not in high-performance hobbies like Battlebots or RC Cars), I prefer the safety of NiMH cells. They're $2 each, sold by Duracell / Energizer / Panasonic / Eneloop.

The "failure" case of NiMH is that the battery vents a little bit of hydrogen gas (okay, also an explosive risk... but its not a lot of hydrogen). So basically, the cells destroy themselves if you abuse them, but otherwise vent relatively safely.

Furthermore, NiMH cells can be overcharged for days and remain fine. They're a lot more durable than Lithium Ion. In fact, a lot of NiMH chargers don't even have timers on them: a common technique with NiMH is to just keep them overcharging.

NiMH is heavier but otherwise nearly as energy dense as Lithium Ion.

Lithium Ion batteries do not contain any lithium metal. They are considered Class B fires. Do not use a Class D extinguisher on them. Water is an acceptable way to extinguish a Li-Ion battery. Most battery factories use water to extinguish Lithium Ion battery fires.

Lithium-METAL batteries on the other hand require a Class D extinguisher. They are not Lithium Ion batteries. They are single use devices and can NOT safely be recharged.

yep. incredibly dangerous. UL would not be impressed. [I have worked in lithium-ion battery design and testing for >10yr]

Do you mind if I pick your brain about something? How feasible is it to homebrew a BMS for a Lithium-Cobalt battery pack that's relatively certain not to burn your house down? I know not going with a BMS at all is a huge nono. Lots of people in the e-bike community have found out the hard way how difficult it is to manage a battery pack without one.

Cobalt cells always have an alarmingly non-zero chance of burning your house down, no matter how good your BMS is. Designing a BMS is a task for an experienced electrical & firmware engineer[ing team].

Caution, yes. But most things in hardware come with varying degrees of risk. If you make a chair it could collapse. Clothes? Toxins. Electric car? Explosive.

Is the upside of "living in the future and building what's missing" not worth it though if you're cautious with the risk?

There is a risk/reward trade-off.

A lot of people build kit airplanes in their garage, but usually get the fuel system done by professionals.

It's not a everything or nothing game. You can still have fun and stay relatively safe.

It is absolutely not worth it. There’s nothing futuristic about this DIY project.

Unknown toxins in clothing is one thing. Building a device attached to your electrical wiring that likely violates the electrical code and puts firefighters as risk is something else entirely.

Not just firefighters but linemen as well.

If those things don't disconnect when the power goes out and have a physical lockout for switch over to battery power they can easily kill someone working on the line.

Fortunately for linemen this risk isn't quite as high as it seems. If you are feeding power back power to the grid when it should be off your neighbors are likely to use enough power as to blow your local circuit breakers.

Do not use the above as an excuse to not have proper backfeed prevention on your system. It is a last resort that might save a life, but there is always the possibility that the isolated section is small enough that you can supply your neighbors without blowing any breakers.

That's assuming that your mains breaker is actually functioning, usually not too many of them get tested at 100-300A since the per-circuit ones trip first.

Last time I read up on backfeeding one of the common refrains was that flipping the main breaker on many homes still left one leg hot back to the transformer. Totally out of spec but common enough to be called out. Hence why a transfer switch is legally required if you're going to run your house from a generator/etc.

It isn't just the mains breaker. It is all the other breakers on your system. Your generators/inverter probably has a breaker someplace as well.

I don't know how common the one hot leg thing is. I know that can buy a lockout device which is just a metal device allowing either my main breaker or one other breaker to be on: thus turning my entire panel into a lockout. That implies the one hot leg thing isn't common but I honestly don't know.

> likely violates the electrical code

I don't think that's likely at all.

This isn't plugging a generator into your house when the power's out. This is a system that's connected 24/7. That's difficult to do, requiring careful synchronization or isolation. Getting it wrong is probably harder than getting it right.

If Tesla assumes liability for a PowerWall disaster than the $3500 price is a bargain.

Yes, obviously do not try this unless you have tremendous electrical engineering knowledge. However, I do give kudos in finding a DIY solution to reuse what would likely just become waste in some landfill.

I would hope most of the lithium in old 18650s is recycled? But maybe that's me being overly optimistic.

Don't even think about building something like that yourself in your home. If one of the cells gets damaged it can set of a chain reaction which will burn down everything around it. A fire like this is quite hard to extinguish, similar to the teslas that burn down sometimes ( https://www.youtube.com/watch?v=vYKTTCNUV3o ). Using lithium iron phosphate batteries might be a safer alternative.

As a firefighter, this is absolutely terrifying. We are not expecting this sort of hazard in a "routine" house fire.

My thought exactly.

Paging /r/osha

Once I volunteered at a large tech non-profit. Between projects, I took over an unused desk in the sysadmin team cave. After lunch one day, I heard a loud noise. I went to the cave door - it looked out over a large cubicle farm.

There were now a dozen fires burning over a forty foot diameter, along with little bits of smoldering cubicle wall.

Someone had apparently removed the battery from a laptop, plugged it into a bench power supply, and then gone off to lunch. It exploded in their absence. It gave me an instant respect for li-ion batteries, plus a small concern every time I fly in a small metal tube full of a couple hundred laptop and phone batteries.

But several hundred used laptop batteries, with no balancing, in my garage, mounted on wood. NO FREAKING WAY EVER.

As a counterpoint to the hysteria expressed in most of these comments, a few years ago I was watching a youtube series by "Rinoa Super Genius" where a similar project was undertaken to create an e-bike battery from disassembled laptop batteries.

The videos are done by an interested amateur with basically a pocket knife and a large soldering iron. I'd highly recommend it if you want practical and confidence building knowledge on the actual behavior and safety of batteries, rather than this comment section which is filled with outrage and "experts" referencing their credentials and years of experience.

I'm an experimenter not an entrepreneur so I'll never claim I can produce battery packs in my basement safe enough to sell to grandma, but I've had no problem ripping apart broken electronics, charging old cells with a DIY dc power supply, and using cells in all kinds of 'dangerous' manners.

Personally I think chance of injury is far higher with dozens of other technologies used every day by non professionals: chainsaws, cars, and many other types of motorized or heavy equipment.

> chainsaws, cars, and many other types of motorized or heavy equipment

You know what a bunch of those have in common? A ton of effing safety measures.

Chainsaw chaps, ROPS, etc all exist exactly because those things are so dangerous. The power density of an e-bike is at least one if not more orders of magnitude less than a 40kWh bank of batteries.

He also has the potential to not only burn down his house but kill a lineman working on the line if the power ever goes out.

All the table saws I've ever used have been super dangerous. Car crashes are a leading cause of death for young people. I could go on, these sorts of things can't be made safe while still being useful. Beyond a point, it's up to the user to understand the danger, learn behavior that minimizes risk, and pay attention when using the tool.

With all of those things, like batteries, it's not good to assume the general public can use them, so they shouldn't be as common as elevators, shopping carts, and other ubiquitous tools/machines. But it's very dis-empowering to claim only elites can use these tools.

Your final comment regarding lineman safety is completely decoupled from DIY battery pack with re-used cells. That would be a failure of the inverter in a grid-tied system. You could have an unsafe grid-tied inverter with a 100% functional battery pack with every certification in the universe and backfeed the utility lines, or I could connect a faulty battery to a proper grid-tied inverter and it would not cause the utility lines to be energized by my house when the utility shuts off the power upstream.

Additionally, you can use a home storage battery without a grid-tie inverter at all.

Your tablesaw example doesn't really disprove anything, that's why they now have riving knives[1], push sticks and lifting guards that make them a heck of a lot safer.

It's not that they can't be made perfectly safe, but that you take reasonable safety measures and mitigate risks appropriately. If he had this off in a shed ~40ft from the house that's a lot different than inside the garage.

The article didn't mention how he's connecting it to the grid so there's a very real possibility that he could have an inverter directly connected which is why we have building codes and inspections to catch things that have the possibility to kill people or do significant property damage.

[1] https://en.wikipedia.org/wiki/Riving_knife

But the dangerous tables saws were still used to build things by lots of people before 2009 when riving knifes were mandated.

I don't see why it's a crime to build a battery pack if you aren't working for Tesla.

I still don't get how you can bring up mis-wired inverter as anything related to a DIY battery pack. Without using any batteries, I can go take a wire strippers and 2 extension cords, make a male-male 120VAC cable, plug it into the gas generator in my shed and plug it into an outlet in my house when the powers out and shock a lineman.

In the country I live in we have building codes that describe how the work must be done. An owner without a license is allowed to perform that work and have it inspected and permitted. Screwdrivers aren't like lightsabers that can only work if you're a jedi (or have a college degree or certification).

I disagree with lumping every form of DIY together. Incompetent DIY projects are dangerous therefore all DIY projects are dangerous?

> Without using any batteries, I can go take a wire strippers and 2 extension cords, make a male-male 120VAC cable, plug it into the gas generator in my shed and plug it into an outlet in my house when the powers out and shock a lineman.

Yup, and that's highly illegal because it can kill someone. If you want to do that you're required by code + law to have a mains transfer switch.

I don't think anyone is saying you can't play with battery packs, it's just that this guy is a freaking idiot for doing it in his house where any miscalculation can cause a chain reaction that may kill him and his family.

It's about orders of magnitude and safety mitigations.

Look at blackpowder/low power explosive regulations. You can keep a small quantity in your house to reload/etc. However if you want to store more you need a separate shed + BATF is allowed to show up at your house and inspect it any time. Back when I was involved with amateur rocketry all of the our vendors went through this process. It's a standard thing when you're involved with elements that have a high potential energy and can go wrong if not treated with the right respect.

> Yup, and that's highly illegal

Which part? Powering your house w/out transfer switch (when disconnected to the utility), or actually back-feeding power to the utility? I would be surprised if the former was uniformly illegal across the US, but the latter certainly is.

Table saws are dangerous, yes. It is important to be careful with them.

But it's a whole different kind of stupid to recommend building your own table saw with salvaged parts and zero safety features.

"In contrast to what all the expert statisticians here have to say, I watched a guy on Youtube play Russian roulette and he was fine. I've played these kinds of "dangerous" games myself and it's always been fine. Ignore the experts, what could go wrong?"

There are all kinds of dangerous things in the world, and many can injure or kill you if you don't treat them carefully. I don't understand the callous disregard for safety concerns, nor the attacks on people with expertise and credentials in these areas, all in favor of some random guy you saw on Youtube who happened to survive and your own limited experience.

If that's your interpretation then we fundamentally disagree.

"Random guy on Youtube" happened to be an intelligent and capable human being who learned all the relevant techniques and principles from available information without paying for "credentials".

Go watch the technique used for testing, evaluating, sorting, and re-using batteries used and tell me specifically what is unsafe rather than trying to spread fear because there's no college degree/certification/other form of economic gate keeping.

I'm not attacking people with credentials, only the type of people who vaguely threaten people not to twist 120VAC wires together in a wirenut unless they understand Maxwell's Equations.

People, in general, are terrible at effectively evaluating risks.

It's my impression that those that DO understand Maxwell's equations (or any other similar field-specific factors) often overlook just how fundamentally bad others are at factoring risks. The stupidity of those assuming "it won't happen to me" is likely nearly as great as the field experts expectations that "it most definitely WILL happen to YOU".

Hmmmm, wonder if there's a maxim or law for that?

Hopefully at least some of those dabbling at the fringes will at least become better educated about the risks when reading a wider range of perspectives on it.

Anyway, quite often the idiots double-down on their beliefs and the experts fail to realize they're beating a dead horse. Welcome to human discourse.

I think the main issues it that it's always "on" and sourced with an unknown usage history. And it seems like it's placed in a location where if things were to go wrong, without proper mitigation, it could cause serious harm.

The chance of injury is probably higher with Chainsaws and cars as you say, but they are not in constant use, and usually have safeguards in place by a regulatory board.

This is the important point. A whole lot of things are dangerous when you're using them. But when you're not they're largely safe. Various saws, tools, cans of gasoline, propane, butane, etc. They're all unlikely to be used on a 'set and forget' kind of basis.

Whereas a battery bank is VERY likely to be used that way. Set it up and get on with other activities, likely including leaving the building entirely. Meanwhile the unsupervised device has an exponentially greater risk of become unstable and causing tremendous damage.

It seems like a clever idea but how much risk it poses is debatable. One would hope anyone attempting to do this is going to be dissuaded from pursuing it on a grand scale because of the various problems they're going to have along the way. But some 'mad scientist' types are never going to be dissuaded. With luck the rest of us will at least get an entertaining video clip of their disaster. Hopefully without anyone else suffering from the ignorance.

18650 cells are great!

But be sure to protect them, and be aware that most commercial li-ion chargers and off-the-shelf protection circuits are designed to maximize short-term battery life. They'll charge to 4.2V, and many won't cut off until 2.5V.

Also know what to do if they short and catch fire; have an appropriate extinguisher and a plan for quickly getting failed cells outside and onto an inert surface like concrete.

If you want to make a 'powerwall' whose batteries could last you for a significant amount of time (e.g. decades), you'll also probably want to design your own charging and undervoltage protection circuits.

There's some good information here: http://batteryuniversity.com/learn/article/how_to_prolong_li...

> have an appropriate extinguisher and a plan for quickly getting failed cells outside and onto an inert surface like concrete.

There are two problems:

a) There is no appropriate extinguisher when you have a thermal runaway on 20 kWh of chemical energy.

b) This isn't some shielded, extinguisher-nearby, one-time experiment, when you, for example, solder something off and then while yelling "see, I'm still alive!" run outside and throw it out. This is something that is intended to be running 24/7 at your home (and maybe mounted on wooden wall like the pictures in the link...). Unless you post guard duty shifts around the clock, there simply may be no person to perform the plan of getting failed cells onto concrete.

I see this as super-dangerous, especially when dealing with cells from many different vendors scavenged from thrown-out laptops.

That's fair, and you'd definitely want to balance each cell and think hard about where to put it, but these batteries are popular for a reason; they're a potentially versatile and effective power source with charge density that is currently very hard to beat, and I'm happy to see people trying to make information about doing it safely more widely available.

And the extinguisher would be for things around where the batteries used to be, in a catastrophic failure.

>There is no appropriate extinguisher when you have a thermal runaway on 20 kWh of chemical energy.

Yes there is, it's called a hose. But you're right, that 20 kWh isn't just going to up and disappear. I think it's realistically possible to do this with a modicum of safety even for a hobbyist but it needs to be passively safe. It can't rely on cells being inherently balanced with respect to one another, it needs to have some serious thought about what to do with that 20 kWh if the worst occurs but that's not an insurmountable task. Sprinkler heads are cheap and even just a garden hose will put out 25 gallons a minute. A very slight risk of water damage can be acceptable IMHO, I wouldn't have trouble sleeping with something cobbled together sitting in my garage so long as I planned for the worst along the way when piecing it together.

On A) a diyer could solve this the "nuclear" way by storing the batteries underground or submerged in a deep tank of fluid.

Of course, then you run into cooling and maintenance access issues.

That's quite an understatement. Venting superheated steam is no walk in the park, either.

Well it certainly wouldn't be superheated steam and it's not like it would be a ton of steam in the grand scheme of things. It takes about 2500 kJ of energy to boil a liter of room temperature water. Someone else mentioned 20 kWh of batteries which in a catastrophic failure I'm assuming the cells might release a little more than that amount of energy but for the sake of argument since I don't know exactly what percent to tack onto that we'll just look at the nominal capacity of the cells. 20 kWh is 72 MJ so that's only enough to boil a bit less than 30 liters of water. Submerging a burning lithium ion pack in something like a bathtub full of water is a great way to absorb all of that energy.

Depending on chemistry, 18650 cells are quite dangerous. Tesla at least puts them in a metal box and will warn you when they’re about to catch fire.

re: Tesla - and they have individual fuses on every cell. And they have liquid cooling for every cell. And they have a rugged enclosure that has been designed to fail in a somewhat safe way while (briefly) containing the fire.

Not arguing that you shouldn't pay close attention to safety considerations, but many of those optimizations are necessary for Tesla because they have constraints that would be easy to relax in a DIY scenario: very low volume/high density, the possibility of a collision, consistent proximity to humans, etc. I don't think it's fair to suggest that you need to be individually liquid cooling every cell to ensure safety.

Have you seen docs about their batteries ?

not a doc but a good writeup with pictures of failure testing the 'powerpack' product: https://electrek.co/2016/12/19/tesla-fire-powerpack-test-saf...

I sure as shit would not want 1/10th of that in my garage ever.


yes, they made all their patents public awhile back. also: https://www.google.com/search?q=tesla+battery+teardown&oq=te...

aren't all patents required to be public?

derp, I meant [quasi-]"public domain" and therefore, in some legal sense, safe for engineers to read without fear of getting sued. https://www.tesla.com/blog/all-our-patent-are-belong-you

Somehow I never considered batteries as part of their IP :)


Well sure, something like a portable phone power bank is probably a better place to start than a battery for your house, but they're very useful things to learn about.

in middle school for a science project I converted non-rechargable batteries into rechargables. They all worked, they all held 80-90% of their original capacity.

They all leaked/exploded 6 months later while they were idle in flashlights and rc cars. The chemistry is different but that experience taught me to be more careful with 'outside spec' electronics.

I see lots of people doing this in Puerto Rico, but mainly they are installing them in outbuildings, such that even a complete runaway fire will probably not really damage anything else. In that case, worth the risk I think.

Hell, I'm planning to buy a commercial solution, and I am still going to put it into an outbuilding. It's not an option for everyone, but when you start planning for failure, there's a lot of sense in not putting anything into your house itself that you don't have to.

Yup, that's a fair risk mitigation.

Inside the house/garage? Hell no.

> While there are some risks to building a powerwall if you don’t have the proper tools and resources

understatement of the century there. DIY electricity is risky. DIY li-ion at scale with used batteries is suicidal.

Jehu Garcia [1] does a bunch of youtube videos on old batteries and different DIY powerwall designs. I'd say to always double check his work, because I've seen some stuff that looked a little dangerous, but for the most part what I've seen is good.

[1] https://www.youtube.com/channel/UCcMfCkN1juSa49DJFYltOTw

I recently bought an APC UPC to protect my DSL modem and phone when the power goes out. I have enough battery powered devices that I can be in communication as much as I need.

Those two devices draw about 8 watts, but the UPC I bought can run for only about 10 hours on that load. It could run a lot longer except that the inverter is scaled to handle a larger load so efficiency is bad. And why the hell is it converting AC to DC, then converting to AC, then converting it to DC in some awful wall-warts.

Since you don't run the inverter for really long times, there is a pressure to put in a cheap, inefficient inverter. However, if you took out the inefficiencies, you could afford to use lithium batteries and produce a much better product for this market. Hell, maybe the thing supplies power to the phone and modem through USB-C and it is just a super-powerbank.

Basically a "micro-powerwall". Safety issues still concern me but smaller scale reduces the odds.

Something like this 50Ah battery pack?


Google around for external laptop batteries, they make some really big ones.

That is close. The single 12V port packs enough amperage that I could wire the two devices in parallel. I'd still have to make cables to connect to the devices. (The phone in question is a cordless phone connected to a landline. It probably should take usb-c but it does not.)

Mike Gyver sells some interesting batteries: http://mikegyver.com/

If you only need to backup power 12/24/48V devices, then you can skip the mains voltage inverter and use a battery-backed power supply. MeanWell comes to mind — they make these power supplies for security and telecom systems.

It's further down my list, but I am starting to investigate adding DC circuits to my house. Between solar and battery backups it would make sense to run as much as possible off of 12V DC.

What I've learned powering an offgrid cabin…

Modern inverters and efficient walwarts (thank you EU) will be more efficient than 12v DC because of the power lost to the cables for anything but trivial loads.

The problem is some squared math functions which come and bite you. Your 12V DC system will pull 10 times the current as a 120V AC system. That's going to mean bigger wire. But then the kicker comes in, each volt you drop means 10 times the energy loss because 11/12 is a lot worse than 119/120, so you'll have even bigger wire. My 24V system with the batteries in a shed has $900 of copper wire between there and the house.

Then consider that all your loads probably already have a voltage converter in them, either an AC-DC external converter or something internal. Most of them won't take 11-14VDC inputs with spikes on it, so you'll need DC-DC converters on them anyway, so you aren't saving the conversion losses on the device end.

In summary, with modern high efficiency inverters you will probably find that your dollar is better spent on solar panels and batteries than masses of copper wire. If I were starting in 2018 I would not have run the DC lines.

From your experience it sounds like I should do the math carefully before spending any money.

Have you had to replace your batteries yet? I’d assume batteries are a recurring (if with a long period) expense, while wire is a one-time cost. Does the downward trend in battery prices still kill any consideration in that direction?

I’m on my third set of lead acid batteries. I got 8 years out of the last ones and 7 from the first. This is on the long end of lead acid battery life. Capacity was down at the end for both, but I limped along. These are my last lead acid batteries. In 6 years I’m sure I will use something lithium based. The location is remote, requires small boats and hand carrying and the batteries are about 1000 pounds. A lithium replacement system will weigh a fraction of that, last longer, and won’t rupture and spill acid all over the place when I accidentally freeze them in the winter while the cabin is inaccessible and in telemetry only mode. (I’m two for two on this, both sets I’ve misjudged just how small their capacity had become and not gotten into emergency power conservation mode fast enough.)

DC-DC converters don't have the hellacious design constraints that AC inverters have.

If an AC inverter doesn't provide a sine wave, some equipment will fail. Making the sine wave is a serious burden, drives up the cost, and doesn't make efficiency any easier. In my case the DC cables are a few feet long.

Have you considered using aluminium wire instead of copper?

It might have been cheaper, but my conduit is full and won't handle the extra size. I trenched and buried that in the rock years before I learned to always oversize any conduit, especially ones that I have to trench and bury.

I had the pleasure of working with one of the best electrical (motor) companies in the states, and decided to sit in on the journeyman electrician course (because sysadmins want to know everything), and I was surprised to find out that DC is what is used for the large high-power long distance transmission lines. I think there are probably quite a few applications where DC might be more appropriate than AC that haven't been realized yet.

DC works for transmission because it operates at incredibly high voltages. Power is voltage * current, but resistive losses are proportional to current only. So the theory that makes it work is since P=I*V, energize the line at incredibly high voltages and get very little actual current flowing to minimize resistive losses for a given power rating. UHVDC is something like 800kV.

DC is much less workable at low voltages like 12v or 5v common in household application -- the P=IV equation is more dominated by current, so with more current the resistive losses add up quickly even in a run across your house (it'll be down to 10 or 8v by the time the wire reaches the far end of your house). Not to say some people haven't suggested wiring homes with 12v lines... data centers have implemented (or at least experimented with) DC distribution due to the density of DC devices.

That's all true for AC as well though.

The benefit of HVDC transmission lines is that you don't have the skin effect at all which makes a difference even at 60 Hz when the cable is as thick as your arm. If you've ever seen one of those large transmission lines where each phase has three conductors all very close to each other but kept slightly apart that's done because of the skin effect.

Also there's always capacitance which will cause some loss for AC but of course capacitance won't affect a DC transmission line. HVDC also makes combining different electrical grids extremely simple as there's no phase difference or frequency difference to worry about. DC also requires lower peak voltages as AC is going to be sqrt(2) times higher than the rms voltage.

AC powerlines start suffering unacceptable dielectric losses above 750kV. DC power lines can go to about twice that. I don't want anything near 750kV anywhere near me.

DC power lines are also better underground or undersea, they don't suffer from the capacitance to the surrounding material.

The AC-DC conversion equipment at each end has a cost, but for 300 mile aerial lines or 30 mile underground lines it is currently cheaper to do DC.

I'm little enough of a hardware person that I'm just interested for the reasons the OP mentioned; constantly switching between AC and DC and AC and DC is obviously inefficient, and batteries are expensive enough that it's probably cheaper to add a DC circuit than to double or triple the battery capacity.

Tesla's powerwall claims 90% round-trip efficiency (AC in vs. AC out) and most SMPS power supplies are also 90%+efficient as well so you are losing <20% capacity (not 100-200%).

90% is going to be in ideal conditions; probably not quite at full load, but not much below.

If you are using it to power a very light load, the efficiency will be much worse, as it takes a minimum amount of energy to run the inverter.

Numerically, I'm worried about ~50% power loss. If you are converting your solar from DC to AC there's another loss.

There still might not be enough power loss to matter, but even with 70-80% efficiency you're still talking an extra 25-50% batteries over the ideal.

You can buy 'super-powerbanks' for a few hundred dollars. Depending on the voltage and power requirement of the DSL modem you could probably run it, and your phone, right from there.

People aren't making hacked versions of Tesla's powerwall. Tesla is making cooperate versions of preexisting diy power storage solutions.

There's a startup incubator West of Lansing, Michigan in the little town of Grand Ledge called The Fledge http://grandledgefledge.com/ It's really a cross between a makerspace and an incubator.

I toured there and among the projects I saw was someone who had made a Tesla Powerwall from laptop batteries. Another was a solar array on a trailer which they explained could be shuttled between your house as well as a cottage up North.

Course building a project is less than half the journey to successfully commercializing it however it was inspiring to see what people were creating.

This is a tangent^: How many people dump a laptop with a usable battery. My experience has always been that the battery is first to fail/suck.

^to the main point, this is a cool project. go DIYers. try not to get killed

Typically what happens is only some proportion of the cells are bad, but that makes the whole battery unusable. Scavenging these batteries requires testing each cell individually to find which are suitable for reuse.


This isn't news. People have been doing this before the Powerwall has come out (except jehu has started branding them this way after it did). Newer Li-on batteries aren't all that dangerous, especially the round cells. Even when overcharged absurdly they don't combust nowadays.

The real downside is that according to my calculation, at least from German E-bay, DIYing gets about the same price per capacity without all the added technology and convenience of buying from someone.

It's not much cheaper but you can get good deals on 18650 cells. It's not unheard of to find deals on genuine cells for as low as $1 a cell.


Then there's the option of buying lightly used battery packs like a single module from a Tesla.


That's $690 for 3kWh that is all assembled, tested, cooling is taken care of, BMS included, individually fused cells as well as a 100A fuse on the entire pack. That's not too shabby, slap an inverter and a charger on that pack and you've got yourself a somewhat decent diy powerwall for like $1100 bucks.

Seems like a really stupid thing to DIY to save money, and a great way to burn your house down.

You can unplug from the electrical grid with one of these. If you don't have 5 grand to plunk down on a giant battery I'd say it's a pretty effective way to save money.

You don't have to keep it in your house. If you have a small section of land which would otherwise normally house a shed, you can build a fireproof enclosure outside to house the pack. If it burns down, your house doesn't. And if you're building it from scratch, you can include enough sensors for it to auto-shut down when an unsafe voltage or temperature is detected. (the linked video mentions all this)

When people build their own car, they're also taking their life in their hands. But if you learn the right way to do it, you won't blow yourself up.

If I were to do this, I would go the old-school lead-acid deep-discharge marine battery route. They are safe enough to put in the basement or garage. How to do this is well understood and doable by hackers.

Lithium batteries are pretty scary. For example, https://en.wikipedia.org/wiki/Boeing_787_Dreamliner_battery_... is pretty frightening.

To at least satisfy all of the "fire" comments, what if you designed a Powerwall with many interchangeable batteries? Like a street light, instead of having a massive bulb that is expensive to repair/replace, hundreds of tiny little lights that are cheap and the repair/replace of a single failed unit is not as detrimental.

Could the home battery then be formed in a similar manner to this article, without the scary "fire" danger?

That's how a power wall works. They are made up of thousands of 18650 cells. The problem is charge balancing. When you have a pack with thousands of different cells from different origins with different internal resistances, keeping the charges balanced becomes a really tricky thing to do.

TIL you can get 18650 batteries from old laptops, which is something I use in my mechanical vape..


Yeah true I use these for vaping, although regulated mod not mechanical mod.

I'm personally using old laptop fan and hard drive motors to build my own version of Tesla's Roadster.

Dont forget to use the wifi adapter to ward off evil spirits

To people coming into this thread, let me sum it up "don't do this it's dangerous let tesla handle it"

There will be fires.

My paranoid side wonders if this is more clever marketing/pr by or for Tesla. Why headline this 'powerwall' rather than 'home battery packs' or 'rechargeable lithium-ion battery home energy storage'?

'Powerwall' is like a prelaunch version of 'Hoover' for vacuum cleaners, a brand name used to describe a generic. People have been experimenting with capturing energy from solar panels for years. We know it is fraught with risks.

Powerwalls are really expensive, barely launched and not a proven household name with reputation like Hoover. Is this ultimately a 'better choose safety and choose the known brand' gambit?

Eh, a powerwall is a well defined product with well-defined uses. “Home battery pack” is nearly meaningless—are you talking about a UPS unit, or something that backs your home power, or is it something that feeds back into the grid?

Not saying you’re wrong, but I think the name is undeniably useful at quickly describing a set of functionality. This could also be a case of poor evangelism on behalf of people who care/competitors.

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