[vox-tech] UPS and auto battery

[Jeff Newmiller]

On Tue, 2 Mar 2004, Rod Roark wrote:

> I have had an APC Back-UPS 500 for a few years and it looks
> like the battery is finally dead.  It's a small 12V sealed
> lead-acid battery, about 6" x 2.5" x 3.5".  The back of the
> UPS says it can output 10A of current.  The battery itself
> says "12V, 7AH/20HR".

Pretty common.  Saw a bunch at HSC last weekend.

> One thing I never liked about this UPS is the anemic backup
> power time, about 5-10 minutes (and now zero).
> 
> I guess you know where this is going... am wondering if it
> makes sense to buy a standard (sealed) car battery and wire
> it up to the UPS?  Any EE experts here care to comment?

It is possible that it could work... but then again I can think of several
reasons why it might not.

On the plus side, both automotive and UPS applications require infrequent
discharges.  This is as opposed to forklift or stationary energy storage
applications, which regularly deeply discharge and recharge.

On the minus side, the automotive applications are very high current
applications, which normally leads to some different chemistry choices
when the battery manufacturers make their recommendations.  These can lead
to different charging recommendations... in particular, automotive
batteries may suffer if not charged more vigorously (higher current) than
UPS batteries prefer.  In this case, the UPS "thinks" it is charging a 7AH
battery, and you are thinking of putting a 40-70AH battery on it... so the
UPS may have even more difficulty producing enough current to charge the
bigger battery effectively... particularly after a long discharge. :)

HTH...

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