A charging unit was a technological device installed in the Weapons Room of a starship, used to recharge energy weapons. They fed power to storage batteries in the "booster handles", into which the weapons were inserted.
Circa 2359, the charging unit in the Weapons Room of the USS Starbound was sabotaged with an incorrect circuit breaker not rated to handle the power level, whose installation was unlogged. It regularly blew and was reset by unwitting Weapons Room staff, and the repeated drains and recharges damaged the storage batteries, causing the booster handles to discharge and the phasers to deteriorate. (TNG novel: Survivors)