|What is the most effective way to electronically measure the level of charge of a lead acid battery? — RS
The voltage of any battery—the amount of energy it gives to each positive charge that it transfers from its negative terminal to its positive terminal—increases slightly when the battery is fully charged. That's because when the battery is fully charged and its chemicals are highly ordered, the laws of thermodynamics that encourage the development of disorder act to increase the battery's disorder through effects that also increase the battery's voltage. But as the battery discharges, these thermodynamic effects fade and the battery's voltage diminishes slightly. So the easiest way to determine the battery's charging status electronically is to look at the voltage rise across the battery when little or no current is flowing through it. The higher the voltage, the more fully charged the battery is.