# How do laptop battery voltages affect runtime?

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)

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.

• With modern voltage regulators the "excess" voltage is not simply "burned" in resistors, but rather the current is turned on and off rapidly at high frequency, so relatively little energy is wasted. Commented Jan 14, 2014 at 18:51
• @DanielRHicks, yes... so assuming the regulator can handle the higher voltage without burning out, and the amp-hour rating of the battery is the same, the 4 cell pack will run longer. Commented Jan 14, 2014 at 19:03
• @psusi - That would be the assumption. Of course, batteries can have weird properties at times. Commented Jan 14, 2014 at 19:05

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.

• What do you mean by "minimum voltage required"? Does this refer to the fact that an 11.1v battery would have to upconvert to 12v somewhere along the line and hence use more power? Commented May 13, 2011 at 22:24
• No. The computer likely needs only about 9 to 11 volts (depending on the model; some models may require higher or lower voltage) to operate. The voltage regulation electronics ensure that the voltage reaching the computer parts is correct. Commented May 13, 2011 at 22:26
• This assumes that the two batteries have the same amp-hour rating. Commented Jan 14, 2014 at 19:07

Laptop contains couple of switch mode regulators. It does not matter what voltage is available from battery, high voltage and low current equals low voltage and high current. Power in watts is still same. Battery A: 3600mAh * 14.8V = 53Wh
Battery B: 4800mAh * 11.1V = 53Wh
If your laptop consumes 30W it will work 1 hour and 46 minutes with battery A or B.
The higher voltage DOES NOT mean longer period of operating time, laptop shuts off when any single cell in the battery reaches the low voltage limit. It is actually more likely to have one weak cell if there are 4 cells in series than 3 cells. The low voltage limit is always same. Full 14.8V Li ion battery is 16.8V and empty 11.1V battery is 9V. Laptop charger usually provides 19V. If the 30W consuming laptop used a 9V voltage regulator it would use 63W at 19V. It would be insane to waste 11V and 33W as heat.

• If the higher voltage battery has the same mAh rating though, then it holds more power, so will last longer. Commented Jan 14, 2014 at 19:05