Should I be concerned about switching power supply to a refrigerator compressor with a standard SPDT relay?

Question

I have a 12 VDC refrigerator compressor that is powered by either a battery or an AC-DC converter. Currently, I manually switch power sources whenever I need to. What I would like to do is set it up so that if the power goes out to the converter, the power source is automatically switched to the battery. I understand that I can accomplish this with an SPDT relay.

However, I am concerned that the brief loss of power when switching from the converter to the battery would harm the compressor. Is that short amount of time anything to worry about?

Currently when I manually switch the power sources, I either momentarily turn them both on before turning the other one off, or I switch one off and wait a few minutes before turning the other one on.

Answer 1

Absolutely no problem with the brief loss of power.

All 12VDC compressor fridges I have seen are made for use in vehicles with their less than smooth system voltage. Drops down to 5V (when starting the engine) are common and expected. Jumps up to 16V and even higher transients are common as well. Losy and sparky cigarette socket plugs are everywhere, too.

The compressor is in fact a 2- or most commonly 3-phase synchronous motor controlled by an inverter. The inverter is smart enough not to put the motor in dangerous modes of operation.

This is why a single relay should be absolutely acceptable.

---

What to do if you really want no any loss of power?

The simplest solution: Schottky diodes.

These refrigerators usually accept anything between 10 an 30 volt power (in order to be used in both 12V and 24V vehicles). Check yours first!

Get a 24V adapter that can power the device. Get a separate battery charger adequate for your battery. Connect everything together and separate the battery and its charger from the 24V adapter and the fridge with a single diode.

When there is AC, the 24V adapter will power the refridgerator and the battery charger will charge the battery. The diode will be reverse-biased and will not allow 24V into the battery.

When AC power is lost, the battery will power the refrigerator thru the diode.

Answer 2

Your optimal solution is to keep running off battery with a float charger at its normal resting voltage. Then periodically it will never fail on power outage unless cutout for a long spell.

• if you do not have this type then get one sized to your requirements. smart chargers may or may not have the hysteresis to float CV after decay from charge CV.