I have noticed sometimes that the "Battery remaing: xx%" tool on my laptop sometimes gives unstable results. For example it might be 100% one minute, drop to 80% and then go back up to 100. When this happens, I think it gets a little warmer than usual too. I know that laptop batteries have a circuit to tell the power supply when to stop charging. I am worried that if there is something wrong with this, my battery might be damaged by overcharging.

 

Is there a way I could diagnose/fix this?

Well, let it charge to 100% (say overnight or so), then turn it on and let the battery run all the way out. That is supposed to calibrate the power circuitry inside those batteries.

 

If it is getting too warm, you may want to blow some compressed air through the fans, to try and clean out the heat sinks. On my laptop, I actually had to open up the access panel to the processor to get the dirt out.