What could cause repeated battery discharge in an alarm and access control system, even though the battery is connected to the transformer?

Q: If batteries in a hardwired (transformer) access control or alarm system keep discharging for no apparent reason then the chances are that you have a ground fault. This earth-flowing current will discharge batteries quickly, but the presence of a ground fault is easy to detect.

Turn off AC power and then disconnect the positive cable on the positive side of the battery then measure between the battery’s positive terminal and the positive cable. Put negative to the cable end and positive to the terminal after setting the DMM to volts.

If the meter reads zero, then there’s no ground fault problem and the chances are the panel is faulty. If the meter measures any voltage at all then there’s a problem. All ground faults are short circuits caused by things like flawed wiring and terminations, bad switches or by moisture getting where it should not be and causing corrosion.

You’ll need to use your ammeter setting to trace the exact location of the trouble. If you want to further evaluate the ground fault first, connect your ohmmeter between the disconnected positive battery terminal and the negative connection of the battery. What you’re looking for here is the resistance of the fault. The lower the reading the more serious the leak is likely to be.