Testing the Power Supply to Your 12v or 24v Fridge

To start a fridge compressor requires a high level of current for a milli second.  Any resistance in the wiring, or undersizing of the cables will cause the voltage to drop below the automatic cut out level when the compressor tries to start.

 To properly test the power supply to a Danfoss or ACC powered 12v or 24v system, the following testing procedure must be carried out. This will confirm if the power supply feeding the system is free of bad, loose and/or high-resistance connections. Reading the voltage on the panel or at the batteries is meaningless, as is the fact that you may have new batteries, your battery size and the capacity of the battery bank is also irrelevant.

The Process

 1.     Turn off the breaker (or remove the fuse) supplying DC power to the system.

 2.     Unplug one of the thermostat leads at the controller (if you can, make up a small circuit to link out the thermostat terminals (C & T).

 3.     Using a multi-meter, read and record the DC voltage at the battery terminals.

 4.     Next, connect the multi-meter reading DC voltage to the power terminals (+ and -) on the compressors controller, so that it can be left connected and monitored.

 5.     Turn on the breaker (or install the fuse) to the system.

 6.     Check that the voltage is the same as the voltage seen at the battery terminals.

 7.     Watching the multi-meter, reconnect the thermostat lead (or your link out circuit) and monitor the voltage continuously before, during, and after the compressor starts or attempts to start.

Interpreting power supply results

·       If the power supply is free of loose, bad, and/or high resistance connections, the voltage reading will stay very stable and only drop slightly when the compressor starts. As a general rule, on a 12v system the reading should not drop below 12v.


 ·       If, when the compressor attempts to start, the voltage reading drops significantly, a bad electrical connection should be suspected. If the voltage drop is sufficient to fall below the 10.5v (23v) cut-off built in to the controller, the compressor will stop. (At this point the voltage may return to it's original reading.) The fan (or in marine applications the pump) will continue to run for approx. 45 seconds and then the compressor will attempt a re-start. If the pre start voltage is above 11.5v (23.5v) the compressor will start or attempt to re-start again.

 WARNING  If the multi-meter you are using is a digital model, the voltage may drop below 10.5v (23v) and then recover too quickly to register on the meter. This means the compressor is attempting to start, then stopping due to low voltage, the voltage returns to its' original value, and there being no significant drop on the meter.

 ·       If the compressor starts and runs OK but stops after a short while, or when the battery charger is turned off, the voltage may be gradually dropping falling below the 10.5v (23v) cut-off point when the compressor tries to restart. This should be identified on your meter.

 ·       If the nature of the fault is such that the voltage reading drops below 10.5v (23v) even before the compressor attempts to start, a very bad electrical connection must be suspected. This is because even the small load of the fan or pump relay, both less than 0.5 amp (0.25 amp), is seemingly sufficient to reduce the voltage considerably.

What to look for

 A loose and/or high-resistance connection can be anywhere in the supply between the batteries and the controller. i.e. a bad breaker or fuse, a loose or corroded screw connection, a poorly made or corroded crimp connection, a damaged section of wire, etc.

12V 3A AC adapter for IP Camera