A customer was writing a program that called
GetSystemPowerStatus and used the
SYSTEM_ value. The customer wanted to know whether a reported battery life percentage of 38% really means that the remaining battery life is between 37.5% and 38.5%.
Although the value is reported to 1% precision, the accuracy in practice is much worse. Similarly, the
BatteryLifeTime is reported in seconds, but if your battery actually lasts exactly the amount of time predicted by that field (and not a second longer or shorter), it's almost certainly a fluke. Even a stopped clock is right twice a day.
These battery levels come from the hardware itself, so you are at the mercy of whoever manufactured your motherboard. Some laptops update the values at 1-second intervals; others can take 5 seconds or more. In practice, these hardware-reported values have been found not to be particularly precise, and in odd cases have occasionally been spotted behaving in strange ways, such as spiking briefly and then returning to a sane value.
But if you use the values (as flaky as they might be), you'll at least be in good company: These are the same values that the Windows Battery Meter displays.
Bonus chatter: There's another interface for retrieving battery life information, and that's WMI
Win32_. The values for this also come from the hardware, but they are more unstable than the values returned by
GetSystemPowerStatus because the estimated run time is an extrapolation based on the current battery load. This makes it more sensitive to short-term fluctuations in energy consumption, creating the paradoxical situation where more accurate information is actually less useful.