>> Really? That's interesting. I always thought it was the battery outputs
>> full voltage until just before it actually dies (which is why battery
>> meters are never, ever, under any circumstances, actually accurate).
> 
> Actually, the battery's resistance increases with depletion, which means the
> more current you draw and the lower charge of the battery the lower the voltage
> you get. The idle voltage will almost always be the same, regardless of the
> charge state of the battery.

I have no idea what internal resistance is. All I know is that a battery 
is a chemical reaction that charges two electrodes until they reach a 
potential high enough to halt the reaction. You draw current and the 
reaction starts up again, replacing the charge just as fast as you can 
remove it - up to a point. I presume as the reactants become more 
dilute, that rate of reaction decreases so the maximum current you can 
draw decreases.

But yes, as I understand it, the potential difference of an unloaded 
battery is almost constant until it's completely dead. This (I presume) 
is why my phone tells me it's on maximum charge for months on end, until 
the exact moment when I try to make a phonecall...

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