Things You'll Need
Replacement battery, if needed
The backup battery in a residential alarm panel provides power in case of an outage. It is located in the main panel box for the system, and is a sealed lead-acid type, most commonly 12 volts. Check your battery regularly and install a replacement if necessary.
If your system is monitored by a central station, be sure to call and put the system on "Test" to avoid police dispatch.
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Locate the alarm panel box (NOT the keypad used to enter your code). The battery will be inside.
Open the lid. Most lids are held with screws; if it has a lock, look for the key on top of the box, or maybe in a "catch-all" drawer in the kitchen. If you can't find the key, call an alarm professional.
Find the battery, usually in the bottom of the panel box. Note the voltage printed on it. The most common is 12 volts. Also notice if there is a date written on it. If it's more than 5 years old, it should be replaced - see Step 8.
Using a voltmeter set to "DC Volts", connect the red probe to the red terminal, and the black probe to the black terminal. The meter will read a little over 13 volts (for a 12-volt battery).
Locate the AC transformer. This is a 2- to 3-inch cube-shaped unit that plugs into an outlet, often near the alarm panel box, or into the garage door opener outlet in the ceiling.
Unplug the transformer, and watch the voltage on the meter. If it drops slowly for several minutes, then settles around 12.5 volts or higher (for a 12-volt cell), then the battery is good. If it drops below 12.5 volts (for a 12-volt unit), then it's bad.
If the battery is good, plug in the transformer and close up the panel to finish. If it's bad, plug in the transformer so the alarm will have power until you get a replacement. Remove the leads from the battery terminals by pulling straight back on the connector, toward the wire. Take the unit to a supplier, to purchase a replacement battery.
Install the new battery. Connect red lead to red terminal, and black lead to black terminal. Write the date on the new unit with a felt-tip, or ink on a piece of tape.
A back-up battery should last 3 to 5 years.
If your system is monitored, be sure to notify the central station first, to avoid a false alarm. Batteries may crack and leak with age. If you see any leakage or corrosion, use gloves, and keep the battery away from clothing or carpeting. Contact an alarm professional if needed.