> Percentage is pretty darn accurate actually. Mostly because it's based on remaining charge This is incorrect on both counts. It's based on remaining availabl

> Percentage is pretty darn accurate actually. Mostly because it's based on remaining charge This is incorrect on both counts. It's based on remaining available energy, which is actually quite hard to calculate. There are two ways you can estimate the remaining energy in a battery under load, and neither of them are very accurate. You can measure the voltage, the relationship between which and remaining energy depends on a huge number of factors including effective internal resistance (which changes with battery age, temperature, and across batches), particulars of the chosen chemistry, physical profile of the cells, current draw, etc. This is probably what you're talking about when you suggest measuring this "at home". It's not what any decent quality laptops use. It works OK at equilibrium though. The other approach is dead reckoning. This involves estimating the discharge profile of the battery (which isn't as simple as "how much charge" the battery holds) and then integrating a non-linear function of the measured current draw based on the discharge profile. This works OK, but you need to combine it with occasional voltage-based recalibration to avoid integration error drift. That usually happens during charging. Either way, not as accurate as you think, and you have to make assumptions about future current draw. If you give me a AA battery, it's literally impossible for me to tell you how much energy you're going to get out of it. It can easily vary by a factor of 20 depending on how you're going to use it. Once you have a rough estimate of the remaining energy (which, again, is different from remaining charge), you can use that to formulate a percentage. But for human factors reasons, the displayed percentage almost never correspond to the actual remaining safe power. Most products stretch out percentages near 100% and 0% to give an impression of longer battery life and to give the user plenty of warning to find a charger.