

Build your own UPS at home, for cheap - profquail
http://www.instructables.com/id/Rework-a-UPS-with-Massive-Capacity/

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dmfdmf
A note of caution. The power output from a UPS while running on battery is not
a pure sine wave like line AC. Its an approximate sine wave consisting of a
collection of square waves. This output is generally very noisy (i.e. has a
lot of high frequency elements). The output is "close enough" to a sine wave
for most electronics to run but the high frequency noise is damaging to the
input circuits because they generate a lot of heat. This problem is worse on
the low-end UPS's because one way to cut the costs is to save money on the DC-
to-AC converter and accept more noise on the output line when running on
battery. The higher-end UPS's (particularly models that are designed to run
long term) have higher quality DC-to-AC converters which is why they are more
expensive, not due to just larger batteries.

One final note, this project seems like a really uneconomical use of time and
money.

~~~
billswift
One thing you could do to smooth the waveform, since you're building the
battery pack anyway, wire it for half or 2x the voltage (with corresponding
decrease or increase in amperage) and run it through a transformer to the
correct voltage. You will lose some power in the transformer, but the waveform
will be much smoother.

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KirinDave
This is easily the most dangerous post I've ever seen on Hacker News. I really
hope no one gets hurt trying this. A total lack of safety advice in the
section involving connecting the batteries in series is particularly
disturbing.

~~~
profquail
My apologies for not including the safety information. There is another very
similar article that includes some safety tips:

<http://www.instructables.com/id/SYB2G6JFMZAFYV2/>

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ggrot
Title is a little misleading. More like, "replace your UPS battery at home,
for cheap"

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bcl
I would be more concerned with the charging circuitry in the UPS. They are
designed for use with the batteries they install at the factory, not large
capacity batteries. You may run into problems with it not being able to detect
battery capacity and as an extreme you may unduly stress the charging circuit
with the longer charge cycles needed for the big batteries.

~~~
Kadin
Assuming it's a current-limited "float charger," which is I think what almost
all UPSes have (they just float some voltage, like 13.8V, over the batteries
with some current limiting circuitry so they don't charge them too fast), how
would you do that?

If anything, by using larger batteries for the same length of outage that
you'd use small ones for, the batteries wouldn't be run down as far. That
means while charging that they wouldn't draw as much current initially
(although they'd draw it for longer), which might make them easier for the UPS
to charge.

What I'd imagine you might want to do is charge the batteries initially with a
fast/slow charger, and then hook them up to the UPS to keep them topped off.

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hapless
Auto batteries make _terrible_ UPS cells. They're designed to survive very
shallow cycling at very high currents, whereas the cells in your UPS will be
very deeply cycled at very low currents.

I don't know about the marine batteries he's using. I expect they're still
designed for high current output, which is a needless cost.

You will probably be better-served to find a local supplier that specializes
in battery supply, not an auto parts warehouse or a costco. Big lead-acids can
be pretty cheap, if you don't buy them from UPS vendors.

Secondly, don't use a consumer piece of trash as your "scrap" unit. Large,
commercial-grade UPSes are cheap as dirt on eBay, because their batteries are
invariably shot. Just persuade the seller not to mail you the batteries and
shipping becomes reasonable.

~~~
tricky
Wow. You admit you don't know anything about marine batteries and then you go
on about how horrible of an idea this is? Do some research before you pull the
trigger there, tiger.

Marine batteries work well in this application. I've been using them for
years.

~~~
hapless
I just noticed the "Costco" label on his batteries, and that raised my
suspicion. For me, specialized suppliers have been cheaper for this purpose.

The important bit in my comment is unfortunately in the second part: Don't use
consumer trash when you can get the really good equipment for cheap.

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sown
I'm going to feel real dumb explaining this to the firemen.

~~~
pavel_lishin
Not as dumb as your loved ones explaining this to the hospital.

