Mark,

On Mon, 04 Jun 2007 18:06:15 -0400, Mark Hanchey wrote:

> My brother has come into quite a few rack mount ups.
> Some of these have ratings in the 3000-4000 watt range.
> So I started thinking about taking one and instead of running it off 
> the internal batteries , connecting it to 4   12V , 500AH deep cycle 
> marine batteries.
> Internally it is using 8 12V, 10AH batteries wired so the output is 24Volts.
> I already have the marine batteries and they aren't being used.
> 
> 
> I mentioned this to a friend and he mumbled something to the effect 
> that the internal resistance of the
> batteries might pose a problem.
> 
> I have never really worked with batteries much , so can someone give 
> me a quick rundown or link on any issues
> I might see with changing the battery types from the sealed lead acid 
> in the supply to marine batteries ?

I was doing something like this, using some batteries that I got cheaply (of unknown history).  I was using them cyclically each day, and as it turns out 
this was a Bad Thing for these particular batteries.  It would probably have been OK if I'd limited the discharge to about 50%, but I didn't and after a 
couple of months what had been a discharge to about the 15% point gradually fell to complete discharge.  Thereafter the longevity fell by about 3 
minutes each day until I realised what was happening and stopped doing it.

I am using an APC UPS that is designed to have external batteries (has a 50A Anderson SB connector on the back) and recharging and heat of the 
unit weren't problems as it's designed for long-term outages, has a big thermostatic fan and a powerful charger.

I now have some proper cyclic-use batteries, so I'll see how they get on when I get time to rig them up.

One piece of advice: fit an over-current circuit-breaker somewhere in the battery string (I have it doing the "far end" link between two lines of 
batteries) rather than at the output end.  That way if you accidentally short across the terminals while connecting the cables it will trip (or you can 
manually trip it while you work on the cables).  If you have the CB between the UPS cables and the battery bank you can still short the terminals on 
the batteries themselves with no protection, and you *really* don't want to do that!  And in fact thinking about it you may need two of them, one for 
each 24V pair, depending on whether you parallel them just at the ends, or in the middle as well - the following will only work mono-spaced, the 
asterisks represent the circuit breakers:

  + A1 - * + A2 -
 /               \
+                 -
 \               /
  + B1 - * + B2 - 


  + A1 -   + A2 -
 /      \ /      \
+        *        -
 \      / \      /
  + B1 -   + B2 - 


Cheers,


Howard Winter
St.Albans, England


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