
Lithium Battery Pack Design for Boosted's Electric Skateboard - skdoo
http://boostedboards.posthaven.com/technical-progress-part-iii-lithium-battery
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atourgates
I'd be concerned about buying a $1,300 battery-powered skateboard without the
ability to replace the battery.

I'm not exactly sure how much it costs them to produce the battery, but it
looks like single cells of similar LiFePO4 batteries are about $20/ea. With 12
of them, that's $240. Add maybe $50 for the BMS and $10 for the enclosure, and
the whole battery pack is maybe $300?

It'd be a shame to have to replace an entire $1,300 skateboard after a few
years when your battery capacity is reduced, when all you needed was a $300
battery pack. Hopefully "non-swappable" means something besides "non-
replaceable".

~~~
skdoo
Yes, we mean "non-user-replaceable". It is easily swappable at our facility.

~~~
baddox
Just like Macbooks. Definitely not a deal breaker.

~~~
hengheng
Except today's Macbooks will be outdated and superseded in 5-10 years, whereas
a skateboard won't.

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TaylorAlexander
I'm imagining:

BMS PCB has high current input rings and a blade connector for balancing (like
removable laptop batteries).

Bottom of case has screw lugs going up through high current input rings in
PCB. Battery has ring terminals that are crimped & soldered to high current
wires. The battery is in a protective casing that holds mating balance
connector and injection molded features keep the high current ring lugs in
place.

Battery drops into board, high current rings go over lugs and balance
connector engages.

Rigid plastic goes over the top of all this (bottom when in riding position)
with silicone seal around the perimeter. High current lugs go through the
cover. Special ratcheting thumb screws tighten down the cover using the
battery lugs as a mechanical connection while also pressing the ring
connectors into the BMS PCB. Conductive grease on the PCB helps keep out
water.

Battery would be 100% rigid, no dangling wires. Installation is two lugs,
drops in, can be done by hand. Battery should come with a protective pouch or
have integrated flip-over cover so it can be dropped in a messenger bag etc.

Solves I think all the issues, including avoiding costly or underperforming
connectors, environmental sealing, ease of use, and the expensive BMS does not
need to be attached.

I emailed them something to this effect, since they asked for input, though it
sounds like they've purchased battery packs already.

Plus they may have considered this, it sounds good but I know production high
current battery stuff is really tricky, I've looked at the area quite a bit.

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baddox
It sounds like their decision was justified, but it's too bad they settled on
a non-removable battery pack. It would be nice to carry a spare battery with
you if you were planning a longer ride.

~~~
uniclaude
My thoughts exactly. I wouldn't mind carrying a spare in my backpack for
longer rides with friends or SO on weekends. In this state, the board is
limited to be a commuting tool for me.

For the days I don't feel like riding a motorbike, I guess I'll keep using my
regular $100 skateboard + train combo.

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chamakits
I hadn't heard of this, but it is very interesting to me. I couldn't skate to
save my life, but for my capstone course, in our brainstorming sessions I had
the idea of doing an all terrain electric powered board on wheels.

My initial intent was actually for carrying stuff for me. The idea came one of
the many days that I was carrying a huge laundry bag to the laundry store a
couple of blocks away. If only I had something I could just place the bag on
top of with wheels. Then I thought it could also carry my backpack to school
everyday, groceries, etc. Maybe I could tie a surfboard-like leash to it, and
it would have a sensor attached that upon pulling it would know which
direction to follow me on. The possibilities for my laziness are endless!
Anyways, we ended up going with another idea, probably for the best, since the
kinda board I was thinking off was probably out of the scope and time we had
for the course.

Looking forward to how this goes. Best of luck to all involved.

~~~
xal
Longboarding isn't like skateboarding very much. I never skated before but
earlier this year I bought a $60 longboard of Craigslist and used it to get to
work. It was a bit weird at first but I got there about 25% faster than
walking. On the way back it felt pretty OK and next day it felt natural. I
skated every day where it was dry outside since. It's exhilarating.

If there was one thing about it that had a learning curve then it was pushing.
Knowing where to put the food and how to evenly push off takes some getting
used to but that's not even something you have to do with the boosted board.

It's weird that skateboards became popular and longboards took much longer to
go mainstream. Skateboards are roughly like BMX trick bikes, cool but limited
use and hard to learn, longboards are the road bike equivalents.

~~~
nikatwork
You mean shortboards. Longboards and shortboards are both types of skateboards
- despite what some overzealous shortboarders will tell you. Hey at least
you're not a fruit booter (I kid! I kid!)

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Samuel_Michon
I wonder why they used cylindrical battery cells. It looks like the battery
pack on this skateboard could have been a lot smaller if they had used
rectangular cells.

[http://batteryuniversity.com/learn/article/types_of_battery_...](http://batteryuniversity.com/learn/article/types_of_battery_cells)

~~~
lgeek
Since they're low volume, off-the-shelf availability is bound to be an
important factor. I've only seen LiFePO4 cells in either soft pouch packages
or hard cylindrical packages, and in this case the choice seems obvious.

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sfjailbird
Having ridden an electric skateboard a fair amount, I am excited for Boosted's
play. They seem to have good solutions to two of my main gripes, which are
weight (crucial for turning on the spot) and weight distribution (most boards
have the battery pack in the middle where it gets in the way if you want to
climb a curb). The battery pack still looks bulky, though.

One thing I am missing is the ability to disengage the electric motor from the
wheels - as it is, you can't just free roll on the board if you want to save
power or the battery is depleted; the motor is engaged to the wheels at all
times. I guess it is a difficult problem since no boards seem to be able to do
this.

(By the way, I used to ride five-six kilometers through downtown Copenhagen to
work, charge at the office and do the same trip back at the end of the day.)

~~~
Gravityloss
I wonder if you just opened the circuit, wouldn't the motor then turn at
pretty much no resistance?

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Tossrock
Obligatory xkcd: [http://xkcd.com/139/](http://xkcd.com/139/)

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mahyarm
Is the board waterproof with this design? If it rains, can I still use it?
Most electric boards I've found are not water proof and it's going to suck
dragging a 15 pound lode stone when it starts raining.

~~~
skdoo
A light splash won't hurt, but rain, puddles, and long rides over wet pavement
will probably damage it. Wet roads will also be dangerous with the slick
longboard tires and will corrode the bearings and other mechanical parts on
the skateboard.

~~~
triplesec
that rules out San Francisco northwards, then, and certainly much of Western
Europe, surely?

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skdoo
Even in San Francisco, we can't use it everyday. We have ridden it in Seattle
and thoroughly enjoyed it.

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shurcooL
I'm really curious to evaluate the following:

Instead of a remote control with buttons to control the speed/braking of the
board...

What if it had a pressure pad on top so that it would accelerate when you put
more weight on the front [1], and brake when you put weight on the rear. How
would that control scheme compare (after getting used to it)?

[1] - The opposite arrangement wouldn't work, because it would create a
feedback loop that'd drop you out of control. As you put more weight back, it
accelerates, shifting your weight more back, accelerating even harder.

~~~
sfjailbird
That would probably not work, since braking puts all your weight to the front
of the board (which would then make it accelerate). Remote controls works very
well in my experience.

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bestham
Then explain why the Segway works.

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rektide
A123 cells. 70A continuous, 120A burst, will stay at 3.4V until really tired.
The bottom, with the array of holes, and the dead on power delivery numbers
both give it away.

B456 now.

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mrampton
The batteries sound impressive. But I think statement

"...the battery needs to deliver thousands of watts of electrical power"

should read "hundreds of watts" \-- the watt has a unit of J(oules)/s.
Thousands of watts then refers to thousands of Joules per second. They later
clarify actual power output as being a sustained 230W (or 384W for a 10sec
burst).

~~~
TaylorAlexander
That was the power rating for a single cell - the pack has lots of cells in
it.

If you take hobby remote control cars as an example, I just bought an average
lithium powered one that can do 45A at 11.7v, or roughly 500w to drive a <5lb
toy car (very quickly). A 12lb skateboard and a 180lb rider will need several
thousand watts peak I would imagine, at least.

~~~
mrampton
Ahh -- makes sense. I haven't played much with RCs or batteries in general and
typically think of power in terms of cycling. 1.3kW sustained for a skateboard
is rather impressive. I'm comfortable doing 20 minutes at ~310W on the bike
(more like 1.24kW once you account for cycling being roughly 25% energy
efficient) and it's enough wattage to get you moving pretty fast.

~~~
msandford
Yeah 310 watts for 20 min on the bike is impressive, but accelerating at 310
watts isn't terribly so. Also remember that you've got gears to achieve better
acceleration but that the electric motor has none.

If you're sprinting or accelerating hard you could easily be putting down
800-1000 watts or more.

~~~
gridspy
Gears are only required because human muscles only work efficiently at pedal
RPM. In cars, they're only required because the engine is only efficient at
certian RPM.

Electric motors have a far greater range of RPM at which they can deliver a
sufficient power.

A motor that delivers 700W attached to any gearbox will deliver 700W after the
gearbox (minus losses). The difference is that shaft will rotate at different
speeds and with a different number of Newton / Metres. A slow shaft attached
to a 700W motor (at full power) will have more torque than a fast one.

~~~
phaedrus
It sounds like you're saying "Electric motors are less dependent on gearing
because torque goes up as shaft RPM drops, so you still get 700W of usable
power at the output shaft regardless."

If that's what you're saying, you're making several false assumptions and your
statement is incorrect (because electric motors _DO_ have a medium high RPM
range where they are much more efficient than when near stall). Although it is
true that ( _for some but not all types_ ) of motors the stall torque is very
high, that uptick in torque does not and cannot rise high/fast enough to
balance the loss of work output caused by choosing a gearing that results in a
slow motor shaft RPM. Recall that work is force over distance. A completely
stalled motor is _doing no work_ yet consuming prodigious amounts of power
(which all goes to waste heat). In order to produce "700W of mechanical power
into the gear train", a motor in a stall or very near stall would have to
produce infinite torque, which is clearly impossible. Furthermore even if you
consider, not a stalled motor shaft, but one turning below optimal RPM, you
don't always get twice the torque when you halve the speed, you get maybe a
fraction of that. So there's still a power/efficiency curve. Finally, a motor
that is turning at an RPM significantly slower than its optimal rate is
usually drawing much more power than it will for the same input voltage if
running at higher RPM, so looking just at motor RPM vs torque at the shaft as
a measure of output power ignores the important fact that you're putting a lot
more electrical energy into the motor to get that output.

The only way in which your comment makes any sense would be to talk only about
torque and say that for most types of motors, if the motor has enough torque
to turn the drivetrain at operating RPM then it will almost certain have
enough torque to _start_ the drivetrain from a dead stop. So the motor doesn't
necessarily need _changeable_ gear ratios in order to get going _if_ you
ignore or amortize the energy wasted getting up to most efficient operating
speed.

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gridspy
I need to spend as much time writing articles about the tech behind GridSpy as
Boosted boards write about their tech.

Not only is it fascinating, it's great branding and marketing collateral. I
wasn't sure I wanted a boosted board when they ran their KickStarter but I
sure am ready to buy now.

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ChuckMcM
That was an awesome write-up. Incidentally, these guys have designed the
ultimate BattleBot battery pack. Send the enclosure specs to a Ti shop to get
a Titanium holder made and we're talking serious durability. Almost perfect
sweet spot of power capacity too.

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beambot
Very sexy build photos!! I asked on their post, but I'll ask here too: "How
about LiFePO4 batteries instead?" They're slightly lower energy density, but
much safer that LiPoly. It's the type of batteries we preferred for big
robots...

~~~
Groxx
I think you didn't read too closely - that's what they chose.

> _Cells

After realizing the inherent risks in some lithium chemistries and finding it
difficult to source low volumes of high-power cells, we decided to use a
different lithium-ion chemistry known as lithium iron phosphate (LFP or
LiFePO4)._

~~~
beambot
Ah, missed that. Thanks!

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stathack
I love the effort that they are putting into the battery, but wouldn't it be
better to cut costs and bring the price down to compete with zboard?

[http://www.zboardshop.com/](http://www.zboardshop.com/)

~~~
nikatwork
I love the effort that Porsche is putting into their engines, but wouldn't it
be better to cut costs and bring the price down to compete with Hyundai? :)

Not hating on the zboard, they're just different markets.

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nextstep
Unrelated to the battery issue: how does the rider control the throttle and
braking?

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TaylorAlexander
Wireless remote they designed in house. Check out their promo video, just
google for boosted boards.

~~~
lloeki
Although not a close up, it's actually visible in the rider's hand on the
article's 4th photo.

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KentLatricia
This is pretty sweet. Remote is Bluetooth Low Energy?

Would be amazing to control with the Myo. :D

([https://www.thalmic.com/en/myo/](https://www.thalmic.com/en/myo/))

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sliverstorm
I'm surprised & impressed by the power goals. That board, at "thousands of
watts", delivers at least 3hp peak. I've ridden old heavy motorcycles with
less.

~~~
TaylorAlexander
Yeah, electric is really amazing for high power stuff, I'm realizing. It's
hard to do, but the simplicity in the end of battery + motor with just
electronics doing the rest means we can get some pretty amazing form factors
out of electric things.

Since I was young I have dreamed of a "boosted" snowboard, with a snowmobile-
like tread that came down the middle and would accelerate me in slow areas.
The weight and noise of an internal combustion engine would have been
prohibitive, but clearly we're getting to the point where electric isn't so
bad.

If I'm doing snowboard runs all day though, I want to show up with a handful
of battery packs and run through a few in a day.

The same would be fun with Boosted. Not being able to replace it means I can't
buy a handful of batteries and take them somewhere for a day of uninterrupted
riding up and down my favorite hills.

Charge dead packs in the car while you ride and with a few packs you can
manage a continuous day of fun. That's not possible with non-removable
batteries, and customers willing to pay for all that are good customers to
have and indulge.

If I have a fully charged board and 5 friends, I feel like "OMG look at how
cool my $1300 board is" is going to turn into "oh it died, sorry guys, more
fun tomorrow".

I think that even if people don't buy the spare battery, knowing it's an
option will help them feel more comfortable with the $1300 purchase. If the
current range falls just short of what you need, an inability to simply buy a
second battery to keep in your pack is going to be pretty upsetting. And
always being able to keep one topped off at home means that after your commute
you can still go out and enjoy your board.

~~~
sliverstorm
An electric snowboard would probably be less compelling. Batteries do _not_
like freezing cold temperatures, and a little 2-stroke could fit in a very
small weight profile.

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nsomething
If RC Monster trucks can have replaceable battery packs, I imagine board
vibration more than moisture has to be pretty severe to warrant permanent
connections.

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gridspy
RC Monster trucks don't have people standing on them. Thus the force exerted
through their wheels is far smaller.

Also they don't need to maintain ground clearance.

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rzt
I think I read about this in Snow Crash.

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coin
The "power" claims for AA and 9V don't seem right. A 9V has the same current
draw as as AA?

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TaylorAlexander
Sounds right, 9v batteries aren't about current, never were. They are about
voltage, that's even why they have the voltage in the name unlike basically
every other kind of battery. Inside though, they're just coin cells in rolls.
Cells are 1.5v each, so you need a stack of them to get to 9v. You wouldn't
expect a stack of coin cells to put out a lot of current would you?

I think a lot of people think voltage means power, so the 9 volt battery
probably sounds much more powerful than a 1.5v AA, but they don't realize the
9v has a bunch of smaller cells and the internal resistance is a lot higher,
reducing the current output capabilities. I had a guy tell me one time I
should hop up the strength of my walking robot by replacing my 6 AA cells with
a 9v. :)

~~~
sliverstorm
And you could have, if the AA's were in parallel!

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ck2
Electric bikes have been using lifepo4 for years now.

My pack is nearly 5 years old.

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auctiontheory
Let's hope they don't run over any "large metallic objects."

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toomuchtodo
I know you're joking, but keep in mind that the metallic object that
"polevaulted" into the Model S battery pack did so with ~25 tons of force. On
this board, you're just going to get thrown off.

