How do appliances tell battery charge?

As near as I can tell most battery charge indicators are nothing more than a very simple and cheap voltmeter, which explains the inaccuracies. Even rechargeable batteries will recover some of there charge from chemical reaction after the device they are in is shut off. Specially after heavy use. My cell phone will go from 2 bars when I shut it off in the evening to 3 bars the next morning. The more accurate way to determine the charge state of a battery is with a amp-hour meter which is expensive and I doubt could incorporated into small portable devices such as cell phones, digital cameras, etc. Always have a share battery. Hope this helps.