Fridge operation on 12 volt

I would not design a system that has fuses at over 200% of calculated load.

I replaced the 12v wires to the fridge with #6 wires going directly to a Battery Isolation Solenoid that is connected to the truck battery and is only on when the key is in the run position. The fridge is never connected to the camper battery.
When we turn the truck off I do have to switch to LP or AC to keep cooling but if I don't I won't end up with any dead batteries just warm food.I still charge the camper battery with the original charge wire thru the 7 pin plug and using the trucks OE battery isolator.Fridge wires run thru a 2 pin plug.

Here is a ready made device that does what you guys seem to want.

Adjustable switch off and switch back on battery guard. You set the trip point low voltage where the device disconnects your fridge from your 12V battery and the device will stay disconnected until the battery voltage climbs 0.8V above the set trip point. Handles 20 amps DC.

There may be many other units, but I found this in 30 seconds.

http://www.ebay.com/itm/KEMO-M148A-Battery-guard-12-V-Made-in-Germany-/370939971852?pt=LH_DefaultDomain_0&hash=item565dbec90c

Forget all the humble jumble of the previous posts!

1. When driving down the road with the truck's engine running to recharge the battery, operate the refrigerator on 12 volts.
2. When parked where you are able to plug the RV into "shore power," run the refrigerator on 120 volts.
3. When parked where no "shore power" is available, run the refrigerator on propane.

No battery will operate the refrigerator on 12 volt very long!

I don't know about now but years ago some 3 ways would auto select 12V if there was no 120V and the DC coming in was high enough. I don't know what the threshold was but it must have been around 14V so the fridge knew the trucks eng was running.
My FIL's didn't work and the RV dealer figured out he had a bad alt. (bad diode).
After replacing the alt. the voltage was back up and the fridge would go to DC automatically.

msiminoff wrote:

The "Auto" setting prioritizes the operation of the fridge.
- Shore Power (if available)
- Propane (if available)
- 12 Volts (until the battery is dead)

This is not correct. On a Dometic fridge, the "Auto" setting prioritizes between AC and LPG only. The Auto setting cannot select DC operation.

I have upgraded the charge wiring between my truck and camper and always run on DC while underway. My Dometic fridge maintains the same temperature regardless of the power source. Of course this does mean that I have to remember to switch the fridge to Auto when I arrive at my destination… to date that has never been an issue. FWIW, the DC heating element in my fridge draws 18.9 A @ 14.7V.

When I am driving, my LPG bottles are OFF.


Cheers,
-Mark

This is exactly what I do. I haven't forgot to switch off 12 volt setting yet. But it would be nice if somehow it could turn off the fridge when on 12 volts when you shut your truck off.

The "Auto" setting prioritizes the operation of the fridge.
- Shore Power (if available)
- Propane (if available)
- 12 Volts (until the battery is dead)

This is not correct. On a Dometic fridge, the "Auto" setting prioritizes between AC and LPG only. The Auto setting cannot select DC operation.

I have upgraded the charge wiring between my truck and camper and always run on DC while underway. My Dometic fridge maintains the same temperature regardless of the power source. Of course this does mean that I have to remember to switch the fridge to Auto when I arrive at my destination… to date that has never been an issue. FWIW, the DC heating element in my fridge draws 18.9 A @ 14.7V.

When I am driving, my LPG bottles are OFF.

Cheers,
-Mark

It would be trivial to make a circuit that cut 12V power to the fridge when the battery voltage dropped below 11.8V. A simple low RDS_on mosfet and a few passives = done. Would cost about $1 in parts. I guess you could even do it with a mechanical relay if you put some hysteresis in there.

Auto, in my experience, is a descending priority deal.

If 120AC is available, it picks that.

If not, it shifts to propane.

A great idea, and very useful, so far. If you need to get off shore power for a few hours, or a few days, it switches. As soon as you are back on the grid, it switches back.

Very useful.

But if you run out of propane, you have a dead fridge AND dead batteries in no time.

Stupid.

Joe

The "Auto" setting prioritizes the operation of the fridge.

1. Shore Power (if available)
   
   
2. Propane (if available)
   
   
3. 12 Volts (until the battery is dead)