A lead-acid battery is a type of battery called a "secondary cell." A secondary cell may be discharged and recharged multiple times. Many lead-acid batteries sold at automobile parts stores are sealed-cell (also known as "maintenance-free) batteries. Since a "maintenance-free" battery does not require periodic changes to the acid electrolyte solution, it may be difficult to gauge how to keep such a battery properly charged.
One solution for keeping a "maintenance-free" (or any other secondary cell) battery charged is trickle-charging. This charging method uses a small current to keep the battery charged and is generally regarded as a safe charging method.
Cut the tip off the power adaptor and separate the wires approximately 6 inches from each other. Strip the wires of ½ inch of insulation.
Turn on the digital voltmeter. Attach the red (positive) probe from the digital voltmeter to one of the exposed wires on the power adaptor. Attach the black (negative) probe to the other exposed wire. Plug the power adaptor into an electrical socket. If the voltmeter registers 12 volts, attach a piece of electrical tape to mark the wire connected to the red probe. If the voltmeter registers -12 volts, mark the wire connected to the black probe.
Unplug the power adaptor from the wall outlet and disconnect the probes.
Attach the marked wire to the red battery clamp and solder this connection. Attach the unmarked wire to the black battery clamp and solder this connection. Connect the red battery clamp to the positive battery terminal. Connect the black battery clamp to the negative battery terminal. Plug the power adaptor into the electrical socket.
Make sure the proper battery clamp is connected to the proper battery terminal. Otherwise, the battery will overheat and possibly explode. Depending on how much charge the battery has, trickle-charging may take several hours to bring the battery to full charge. Do not trickle-charge a fully charged battery. This will reduce battery life.