[Tfug] FTGH: APC Back-UPS Pro 650

[Adrian]

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, 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