Batteries always seem to die at the worst possible times. It is especially frustrating when the battery has recently been charged yet runs out unexpectedly early.
One common cause of prematurely dead batteries is degeneration. Normal usage causes the metal plates inside the battery, which produce electricity, to slowly erode and gradually lose their capacity. The amount of time this process takes depends on the recharger and which type of battery is being used. Nickel-cadmium (NiCad) batteries tend to last more than twice as long as the older, lead-acid variety.
Overcharging can also cut a battery's life span short. Putting too much of a charge on a NiCad battery can cause the electrolytes to leak out, which lessens power capacity. This is more prevalent with older rechargers, because the newer ones receive instructions from a computer-chip sensor that detects how much charge the battery needs and can manage.
Battery manufacturers are working to develop new chemical compositions for batteries so they will last longer, be less vulnerable to charge and discharge quirks and be safer for the environment than lead, acid or highly toxic cadmium.