The electric scooter's battery never gets full; maybe the charger undercharged it.

undercharged 是动词 undercharge 的过去式，由前缀 under-（表示"不足"）和动词 charge（充电）组成，字面意思是"充电不足"。undercharged 在这里指"充电器没有给电池充入足够的电量"。