Ideally you want to use the compatible charger provided with the purchase kit, at all times. The reason for this is if a charger that is not compatible with the device is used, too much current goes into the batteries, meaning they can overheat and eventually stop functioning. Battery capacities vary from each device; they will indicate an mAh, which essentially is the battery capacity. Usually after a 6 months use, the capacity within the battery starts to decrease, making the battery life last less.