[vox-tech] UPS and auto battery

[Ted Deppner]

On Tue, Mar 02, 2004 at 03:13:07PM -0800, Rod Roark wrote:
> Thanks to everyone for the informative replies.  Probably
> what I'll do is look for a smallish battery in an auto parts
> store to experiment with, and post here again when and if
> anything comes of it.

Lots of good stuff already said, but figure I'll briefly chime in with
some of my experiences.

Those sealed lead acid batteries in good UPSs have a life of 2 or 3
years on the outside.  They're not really good like a good marine
battery for deep cycling, but being a lead acid they can take some abuse
(very similar to a car battery).

The issues with using a wildly higher current battery are fairly obvious.
A 7Ah batter versus a "650 cold cranking amp" battery than can easily
sustain 20+Amps for a good long time (10s of minutes).  A 7Ah battery
would only look like a dead short to the charger for minutes... a dead
car battery could look like a dead short for an hour.

Dangers... Only the cheap car cells still give off hydrogen (under
normal conditions anyway), you can get sealed car batteries.  I've seen
4 or 5 year old APC units burn up their circuit boards when new
batteries where put in.  The low end units with < 10Ah batteries
typically have a charge controller that's on a PC board.  Your typical
car charge controller is an ABS plastic thing (or older bakalite).  Big
difference in current capacity, heat load, etc.

If you can handle the possibility of fire and things exploding, have
some fun.  The current is enough to burn you and cause fires, but you
can be safe about experimenting, and it's always fun.  (You might be
able to isolate the inverter from the charge controller and then
integrate your own charge controller for instance.)  In my estimation a
small UPS is good for one or maybe two battery change outs, ie a battery
change every 1 to 2 years, and the unit over all having a useful life of
3 to 5 years.

All that said, investing in a heavy duty DC power supply, a car battery,
and a DC-AC inverter would probably be a safer and ultimately less
expensive and more useful endevor.  DC-AC inverter would be less than
$100, battery $50 on sale for a decent one.  Not sure on the DC power
supply, it would need to be able to sustane 25 Amps continuously, as
you'd be actually running off the inverter the whole time and not be
"switched from AC to DC on power failure".  

A simple car battery charger won't do for this (I've tried)... most of
them have 'auto shut off' modes where they pulse on and off, while your
battery drains current but the voltage stays high enough to shut off the
charge sequence.  You can run like this for several hours though in a
pinch.

A good link for this is http://www.dansdata.com/diyups.htm which showed
up on Slash dot a few years ago I believe.

-- 
Ted Deppner
http://www.deppner.us/