Question

how to know whether a battery is full

Answer 1

touch with your tongue or bring a galvanometer near it ....

Answer 2

There are two easy ways to tell when abattery is fully charged: While charging: Measure the current flowing into the battery.If the current is very low, (<0.05C) the batteryis nearly charged. This number can be lower depending on chemistry, but around this ballpark is good enough.