With many cordless tools and various appliances, overcharging leads to premature battery failure when a high (or "fast") charge rate is used. It's essential to stop the charge when the battery is full, or the temperature will quickly rise, causing extreme internal pressure as the excess current is dissipated as heat. (Don't worry; there is no danger of explosion.) Internal built-in vents do open up to relieve pressure and allow some sealed oxygen to escape; however, what is lost cannot be replaced, and lost oxygen means less power and capacity. To stop depending on vents alone, which can (and do) corrode over time, you can avoid overcharging by plugging cordless batteries needing power into a common household timer set for the prescribed amount of charge time. Then, (as they say on TV) "set it and forget it." Your cordless drill or electric razor will never again be too "powerful!"