There is some easily available information on finding the status of a battery, or weather it's charging or not. (GetSystemPowerStatus API or System.Windows.Forms.SystemInformation.PowerStatus).
I want to be able to stop a battery from charging based on some criteria, e.g. battery power > 20%.
Is there an API to do this?
I think it's impossible, because you have need some API for battery or battery charger.
And this API can provide to you manufacturer of notebook and battery or battery charger support this.
I think MiCTech is right. I dont think it is possible but here are some resources which you might useful.
I honestly don't know, but I'd have a look at the APM or ACPI APIs.
Other than that, the only option I can think of right now is a USB controlled robotic arm that ejects the battery when you need to stop charging, but that's probably not what you are looking for, and borders on the complicator's glove in terms of level of over-engineering. :)
But for example Vista won't charge battery if it is unplugged and then plugged while it's capacity is above 96% so it means that in can be accessed and manipulated somehow.
One of the options is to get hold of the device(I) for battery (Microsoft ACPI-Compliant Control Method Battery). Listen for PowerNotification events forever. On each notification check the PowerStatus of the battery. There are APIs for all of the above purposes in .Net and win32
Keep the device(I) disabled as long as the powerstatus is >threshold. Enable it as soon as goes below that or when you are not on AC power (i.e. before removing AC power, your continuously monitoring software should enable that battery device - or you manually enable it).
hmm,...this is a very buggy solution, but it can achieve what you want, although you have to be very careful.
I use Sony Vaio. It has a utility called Battery Care Function which allows you to control the charging level.
I would just get a UPS and programatically tell it to cut all power... most should have an interface for doing this. Otherwise, as someone already said - a computer-controlled power strip would do it ^^
I've actually played with this idea when I was testing/writing about way too many new laptop models a while ago and the battery testing was annoying to set up, monitor and analyze.
I wrote an app that would do exactly everything (setup, listening, measuring, reporting) except unplugging the power and then replugging it and starting the computer again...
I think it's possible, because it was patented by IBM - patent no. 7570015.
Not sure if there's a Windows API available for that.
You can swap for my battery if you like. It used to last 5 hours, now it only lasts for 1, so it never charges beyond 20% of its original capacity :-)