Auxiliary Battery Wiring for Portable Fridge

Per my original post - I already have an auxiliary outlet wired directly from the battery that is fused 30a and is 10 gauge wire. Also, the 10 gauge ground goes back to the first vehicle battery ground. At worst a 20' run. So, 20' 10 gauge, auxiliary outlet, 3' 12 gauge, auxiliary battery, 6' 18 gauge, fridge.
I have run this fridge with a 12v extension from the dash outlet over several nights before installing this modification.

I think it is a mistake to rely on the factory ciggy plug receptacle wiring. Are you sure the plug is live with the key Not in the ignition?

You don't need to rewire the ciggy plug, just run some new 10awg directly to a new Ciggy receptacle from Aux battery. Were you going to go through the hassle of moving the factory ciggy plug to the Auxiliary battery? This is more labor intensive than just running a new fused wire to a new plug.

My front loading fridge uses about 6 feet of 10awg to the battery. Back when I was modifying the cabinet in which it is housed, I was running the fridge on a DC extension cord. All was fine when the sun was out and my batteries were in the mid 13s+, but when the sun went down, I noticed my condenser fan was kicking on, but not the compressor. After a minor freak out, I realized my battery voltage was 12.6, but the voltage at the end of the extension cord was in the low 11's, and that was running only the condenser fan, not the extra 2.7 amps of the compressor.

The extension cord was pretty extreme though, about 35 feet of 12 and 10 awg, one way, not full circuit length.

Do you want to have to do this job again if your fridge starts acting funny a few hours after engine shut down?

I have to use the factory plug or cobble up a new connection for the fridge. Could do but don't wanna. Factory wire will be shortened somewhat when adapted to battery screw posts, maybe 6'. Aux outlet to battery will be less than 3'. For these wire sizes: With my alternator cranking out 14.2v and charging the battery at 20 amps (more likely less than this) the voltage at the battery would be about 13.6v. If the fridge is running off the aux battery 12.6v, the voltage at the fridge would be 12.3v.

18 awg for 10 amps, But you don't state the distance.

DO not skimp on wire size. you'll always get somebody telling you thin is fine or adequate and stories how it works just fine.

Do it right, once. Run a dedicated circuit for the fridge with 10 awg from the Aux battery.

I know Edgestar just uses a danfoss clone compressor, but Danfoss states to Not share a power source with other wiring, but to tie it directly to the battery over as short a wire as possible with minimal connections, and their recommended wire sizes are much thicker than any voltage drop table you can find that will say 18 awg is all you need.

In terms of charging the Aux battery, the thicker the cabling the more the voltage regulator will be able to 'see' the depleted battery tacked onto the alternator circuit and be less likely to revert to a water saving 13.6 volts prematurely but this will vary widely from vehicle to vehicle and which bean counter and laywer were in the office to overrule the engineer that day.

And a 500 mile drive might not be enough to return an Aux. battery to 100%, sadly. That last 20% takes forever regardless of charging source

landyacht318 - thanks for your comments, but.... Just unplugging it at night without an auxiliary battery will shut down the fridge with no cooling. Not a satisfactory solution even though it should maintain its cool for quite a while. The factory cords are very expensive and proprietary so I hate to buy a second cord (yeah I can afford it but hate to be ripped off). 18 gauge is fine for 10 amps (over double the fridge draw). My concern was adequate voltage to charge the battery - don't see the voltage reaching the fridge as an issue. The Group 34 battery is good for over 50 amp-hours so the draw down in a 24 hour period is not that great although I'm not the type to go a day without driving some. Being in South Central Texas regardless of which way I'm headed, my first day is 400 to 600 miles so even if I don't put it on a charger before hand I would expect to be pretty much fully charged as the trip commences. I believe the Edgestar fridges are designed to shut down on low voltage to conserve the vehicle batteries for starting. Hence the reports.

Old school way of doing it would be not installing another battery, and just unplugging it at night.

These Edgestar Fridges have reports of being very sensitive to voltage drop. Either not working, or just working poorly.

12 awg sounds fine, but use 10awg and make sure the spade terminals on the compressor controller grasp tightly. Ciggy plugs, even the good ones, are not that great of a connection. Most factory installed receptacles use 18 awg, and long lengths of it, and need to be bypassed for powering a compressor fridge

I'd have a non voltage sensing solenoid to isolate a second battery, and no way would I unplug the fridge overnight. I'd have a 10 awg wire with an Anderson connector with as short of a run as possible to the battery, fused at the battery. I'd make another connector with a ciggy plug to plug into the Anderson connector for operating the fridge in a different vehicle.

Even connected to the alternator, the second battery will still need to be fully charged by a different source. Squeezing in that last 20% takes forever, and is required weekly for battery longevity. So have a way to plug into the grid and charge overnight, or resign yourself to replacing the aux battery more often, after it lets you down at the worst possible time, because Murphy was an optimist.

I may run this in two different vehicles. However, since my primary vehicle for this has a 4-way auxiliary distribution block I may just put the whole block on a disconnect if it becomes too much of a bother. (Auxiliary outlets, trailer 12v, brake controller).

I suppose it should work. That is an "old school" way of doing it.
I would install an a voltage sensing relay to connect the aux. batt. to the vehicle when it's running and disconnect it when it's not. I would power the ref. directly from the aux battery. This would be completely automatic.

I plan to go old school and just unplug the power when I shut down for the night.

you need to isolate the batteries, so the diehard is charging when driving, but not connected overnight when parked so the fridge draws power ONLY from the diehard

sounds like your wiring will be heavy enough, and should be NO problem

moving to tech issues