DIY Alarm System Battery Replacement
How to Test an Alarm System Battery
Open the alarm system panel box, and locate the battery. Most panels just have the battery sitting on the bottom of the “can”. If there is a date written on the battery, and it’s at least 5 years old, replace the battery.
If there’s no date written on it, continue with testing. Position the alarm system battery so that you have access to the terminals on the top. This usually means laying the battery on its side.
Disconnect AC power by unplugging the low voltage transformer, if possible. If you can’t easily reach the transformer (or if you don’t know where it is), you’ll need to disconnect one of its wires inside the panel.
The circuit board will have at least one row of screw terminals. At one end or the other, look for 2 screws marked “AC”, “Trans(former)”, or “XFMR”. Loosen one of these 2 screws, and pull the wire free. Be careful not to let the wire touch anything, or you could fry the transformer. By the way, don’t be concerned about electric shock, since the transformer only puts out around 18 volts.
Set your DMM to measure “DC Volts”, within the range of 12 volts. Touch a probe to each battery terminal, and check the voltage reading. You’ll notice the voltage dropping, hopefully not too rapidly. A good battery will drop steadily for a few minutes, then level off and even begin climbing slightly. If yours behaves this way, and stays above 12 volts, the battery is good.
If after 5 minutes or so the voltage continues to drop, or if it ever goes below 12 volts, the battery should be replaced.
How to Replace an Alarm System Battery
If the battery fails the test, reconnect transformer power so the system will run when you take out the battery.
Remove the connectors from the battery terminals. They are the quick-connect spade type, and simply pull-off and push-on. Take the unit to a battery store, where they will sell you a replacement battery of the same size.
Note that the new alarm system battery must have the same rated voltage as the old one. The amp-hour rating should be the same or higher.
Label the new battery with the date, for future reference. If you use a black Sharpie, it won’t show up well if the battery casing is also black. In that case, add a short piece of masking tape, and write on that.
Install the new alarm system battery, making very sure to match the positive and negative terminals to the correct wires: red to red, and black to black.
Using your multimeter, check the battery voltage. It should climb slowly but steadily, as the alarm panel’s charging circuit does its job. If the voltage doesn’t increase after 5 minutes or so, check the AC power connections again. If they are good, you may have a blown transformer.
Allow 24 hours or so for the replacement alarm battery to reach full charge. During this time, the keypad may still show a trouble light or low battery indication, but this will reset when the battery reaches a full charge.
To check the plug-in transformer, set your DMM to the “AC Volts” position, and touch a probe to each AC terminal on the circuit board. You should see 16-18 volts or so. If not, you’ll need a new transformer. These are available online, or you can get a comparable unit from Radio Shack or other electronics supplier.
Replacement is done by simply swapping out the old transformer for the new one. Unlike the battery, AC connections aren’t polarity-sensitive. There is no “positive” or “negative”, as the leads are interchangeable.
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