Why does my computer battery trick me?

Dependant on what you are doing, different levels of current will be drawn, meaning different “amounts” of power will be used (per unit time), meaning the battery will be drained at different rates.

So if you start playing Dig Dug and watching Dukes of Hazzard and listening to Bon Jovi all at the same time, you may need to get out more.

@The_Idler has it. The PC’s state of charge algorithm calculates how much time you have left based upon what you are doing. Try this experiment. Turn your display way down and see how long it says your battery will last. Now turn it to full brightness and see how long your eyes will last.

I think the trickery also arises from another nuance in way the state of charge algorithm calculates the time left. My guess is that time left is calculated based on the amount of charge lost per second for a completely new, healthy battery. As a battery is used over time, it slowly loses its ability to charge, so the algorithm is fooled. I used to have a Toshiba laptop that said I had 2 hours of battery life remaining even though I knew it would run out in 5 minutes.

Yeppers.

My battery monitor actually has an ammeter, and I can actually see the time drop as the amp draw rises. After a minute, when my hard drive spins down, the amp draw plummets and my time remaining skyrockets. The same thing happens with my CPU load; the more I ask the CPU to think, the more it sucks the battery down.

@camertron That depends. The battery monitor built into Windows does that, but some third party monitors (like my fave, Battery Bar ) have a more sophisticated algorithm and keep track of the actual battery capacity.. How do you think I know my netbbok lost 14% of it’‘s battery over the course of a year without doing any math or measuring?

Doing more CPU intensive things like playing a DVD or a 3D video game drains more battery power. Doing simple word processing with screen on low brightness may last a very long time.

Btw, do you own a Mac? if so, you might want to calibrate your battery every month or so.
Detailed instructions are here: http://support.apple.com/kb/ht1490

Basically you have to make full charge cycles for a Lithium-Ion to stay in “good shape”.
You’ve got to charge till 100% and leave it like that for min.2 hours and then drain the whole battery till sleep-mode and then leave it on that reserverve power for min.5 hours till the sleep light goes out.
Then charge it up again (without use) till 100%.

That way the battery “knows” what its extreme low and high ranges of charge are and you keep all cells in the battery active.
Guess this works for every portable device with a Li-Ion battery.