Generally a charger passes a constant volt or current until it is switched off. If you fail to switch of the charger at the appropriate time you will either have a battery that is overcharged or a battery with hardly enough charge. Overcharging does more damage to the battery than under-charging. When a battery is overcharged, it gets rid of this extra charge by heating itself up. Now this heating results in, building a pressure inside which can at times make it rupture and result in leakage of gas, chemicals and also explosion. Battery chargers work by passing electric current through batteries for a period of time. The cells inside the battery hold the energy passing through them. This is quintessentially how a charger works.
