More likely it's the lithium battery pack that's the problem. Lithium powered devices have battery packs that govern the charging cycle and battery cells condition. They monitor the battery cells individually and determine the state of the battery. The plug in charger is a dumb AC or DC supply, does not monitor the battery, just provides raw power, usually 24 to 28 volts DC. End of battery pack life is determined by the software built into the battery monitoring circuitry and is a fixed parameter. The above comments also apply to other lithium powered device, drills etc.