REFRIGERATION

tidetim

I have a GE over-under refrigerator on my boat (48' trawler) that obviously uses AC current to operate. When off the dock, that power comes via our 3000 watt inverter/charger and 900 Ah battery bank. When the fridge goes into defrost mode (about 6x/day), power use skyrockets from approx 13 amp to over 26! I'm hoping someone has installed a manual defrost switch in place of the automatic/timer currently on the unit. That way I can choose when to defrost, like when I'm running my generator or main engine. Any details as to how to do this would be appreciated. Thanks. Tim

bill von novak

tidetim said:

I have a GE over-under refrigerator on my boat (48' trawler) that obviously uses AC current to operate. When off the dock, that power comes via our 3000 watt inverter/charger and 900 Ah battery bank. When the fridge goes into defrost mode (about 6x/day), power use skyrockets from approx 13 amp to over 26! I'm hoping someone has installed a manual defrost switch in place of the automatic/timer currently on the unit. That way I can choose when to defrost, like when I'm running my generator or main engine. Any details as to how to do this would be appreciated. Thanks. Tim

Find the element, cut the wires, add a timer switch that turns on the element for X minutes.  (It should also turn off the compressor.)  Beware - the refrig will freeze up fast; auto-defrost refrigerators are not designed with large clearances in the evaporator.

BB.

Do you have a link to the fridge model?

Generally, a home sized fridge takes around 120 Watts running and about 600-1,000 watts (really VA) starting.

The defrost heater takes around 600 Watts.

P=V×I

120 Watts / 120 Volts = 1 amp.

However, if you are talking about 12 VDC amps, then:

120 Watts VAC * 1/0.85 a.c. inverter efficiency * 1/12 vac = 11.8 amps @ 12 vac

Pretty close to 13 amps running if 12 vac battery bus.

- Bill

mvas

If your Defrost happens every 4 hours, then most likely you have Mechanical Defrost Timer.
You can simply add on On/Off switch in series with the Defrost Heater.
Turn the switch OFF when running from batteries.
Turn the switch ON when running from the generator.
Get the model # & then download the schematic.
You may not have [easy] access to the needed wires, from the outside, in a Top-Freezer.
GE Defrost Timers are usually inside a panel in the "ceiling" of the refrigerator compartment.
Add a mostly plastic switch near the Defrost Timer?

My GE fridge has an Adaptive Defrost Computer Board (located on the back)
If I don't open the doors on my GE fridge, then it will defrost only once every 60 hours.
If I open the fridge and/or freezer doors too much, then it will defrost 3 times per day (max). 

The Adaptive Defrost Computer Board uses a temperature sensor on the evaporator coil
to know when the ice has melted, which indicates it is time to exit Defrost Mode.
You can't just turn off the Defrost Heater with in an Adaptive Defrost Computer.
It will wait a very long time for the Evaporator Coil to heat up, but it never will.
And then it will schedule the next Defrost Cycle even sooner, since the previous defrost cycle failed.
This can allow the food in the freezer to melt 
So, what you need to do is ...
"trick" an Adaptive Defrost Computer Board into thinking the doors are never opened.
My Defrost Cycles are very infrequent and are not an energy issue.

Mechanical Defrost Timers are extremely inefficient.
Adaptive Defrost is extremely efficient. 

jonr

So for dumb, fixed time defrost modes, just interrupt power to the heater (I wouldn't worry about the compressor being off during this short period).

For smart ones, find the door contact and somehow override it (wiring, magnet?)

Maybe someday refrigerators will have a way to be told about power cost.     For example, in a solar power situation, it's wasteful (of battery life) for a refrigerator to defrost or even run 1 hour before sunlight is available.