[Tfug] FTGH: APC Back-UPS Pro 650

[Bexley Hall]

--- Adrian <choprboy at dakotacom.net> wrote:

> On Wednesday 18 April 2007 17:52, Sean Warburton
> wrote:
> [snip]
> > lead acid batteries. There are two of them (in the
> backup, they were
> > togeather...in parallel (I think? where the
> current lasts longer than one
> > battery, as opposed to in series, which yields
> more current/voltage,
> > right?). 
> 
> Not quite... You actually want them in series, not
> parallel, for 2 important 
> reasons (in most applications that is).
> 
> First, P=VI, so power is voltage times current.
> However, power is lost thru 
> resistance in the battery and connecting wires,

But each battery only sees half of the load -- in
theory (given ideal batteries and chemistries).
And, the IR losses in the wiring are minimal.
The bigger issue is the overall *efficiency*
that the switcher can deliver -- see my previous
post.

I had a 3KW UPS that had a 120VDC battery pack.
It was *lethal* to operate it with the covers off!
But, ran quite efficiently since such a small
portion of the input power was "wasted".

> given by V=IR. Reducing 
> factors to find power lost: P(loss)=RI^2. So for a
> given amount of power, if 
> you double the amount of current, you halve voltage,
> but you quaduple the 
> power lost (as heat). If you double the voltage, you
> halve the current 
> needed, and hence cut loses to 1/4.
> 
> Second, batteries are rated in "Amp hours", the
> theoretical value based on a 
> discharge rate over a period of 20 hours (SAE
> standard). So an 8Ah battery 
> theoretically could supply 8amps for 1 hour... in
> reality, it could supply 
> something closer to 0.4amps for 20 hours. At a high
> discharge rate, its 
> capacity is drastically reduced.
> 
> So by putting batteries in series you both decrease
> loss to resistance/heating 
> in the batteries and get a greater capacity out of
> the battery.
> 
> Adrian
> 
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