Some safe ways for charging a battery include using a specified charger for the unit or having the charge done at a service center. Store-bought chargers have limiters that shut the device off in order to avoid overcharging the battery. Professional shops use commercial-grade machines that ensure the battery will not overcharge, and this switches the liability from the owner to the technician.
Before charging, remove the battery and clean its terminals. If the battery uses cells, fill them up with the appropriate amount of water. Follow the directions on the charger, and make sure the battery and well ventilated as its being charged.
Batteries can also be taken to auto part stores for free charging. Just bring the battery inside and request a test, and the technician will handle the rest.