How do laptop battery voltages affect runtime?

Question

I ordered a new battery for my faithful XPS M1710. I'm not sure of the voltage of the battery I have now, but the new one that the Dell rep got me (after 3-4 times confirming my phone number and laptop model number) is 14.8v. I was a bit concerned about potential incompatibilities (as most of the other compatible batteries listed were 11.1v), but I figure that there's no way that Dell would "recommend" batteries that wouldn't work or fry your system.

Now, my question is, how does voltage affect battery life? If we assume the needed power draw to be constant, a higher voltage would indicate less amperage needed, therefore the battery would last longer before running out, yes? Or am I missing something?

For reference: P=I*V

P = power

I = current

V = voltage (duh)

Answer 1

A quick look at the Dell web site shows the replacement battery for that model is 11.1 volts (3 stacked Li-ion cells). That is the voltage your systems voltage regulator will expect. Any additional voltage (as supplied by a fourth cell in series as found in the 14.8 volt part) would have to be dropped across the voltage regulator. That increased drop would be converted into heat energy (and wasted).

The single-charge battery life is entirely determined by its charge capacity, measured in amp-hours, and the current draw on it by the load. So you should really be comparing the mA-H rating of the batteries (old and new) to determine its life difference. The voltage doesn't really make that much difference, and any excess is mostly wasted.

Answer 2

The higher voltage means that the battery can maintain the minimum voltage required to operate the computer for a longer period of time, hence extending battery life. The actual voltage used by the computer is electronically regulated, so this cannot cause any damage to the computer.