Dometic Fridge on 12 Volt while towing

[Joe MacDonald]

I always ran my fridge on 12v when towing with the envoy, never had a problem with the trailer battery , that was an inmont RM33, just unplugged if we were going to be more than an hour
My new fridge is a monster norcold n3150, haven’t tried on electric on a trip yet, did try propane and it worked, but I didn’t feel safe with The propane running, next trip will be with electric
On propane, set at 1 out of 5 it kept ice cream from melting, just a bit soft. The controls used about 1% of my battery capacity overnight according to my power meter and I recharged by solar in the daytime

[RogerDat]

Someone here finally got my brain wrapped around the charging system and multiple battery question. The vehicle voltage regulator is going to deliver alternator charge until circuit voltage reaches the desired voltage of fully charged.

The circuit voltage can't reach that voltage if any battery on the circuit has a lower voltage than that. One battery or two, or even three. All on same circuit so voltage won't be raised until all are raised.

The other point that matters is the current is going to flow into the battery based on the difference between line voltage and battery voltage. The greater the voltage in the line the more will flow into the battery or batteries. Like an air tank filling a tire if the tire is at 20 psi and the air supply is at 100 psi the air flows quickly if the air supply is at 32 psi the flow rate will be slower to get to full tire at 32 psi. Closer the two are to the same the slower the change will be.

Low battery is going to have more difference between line voltage from alternator than the mostly charged battery so the flow rate will be greater to that battery until something approaching equality is reached.

There is a lot of discussion on need for heavier wire going to house battery to avoid voltage drop. There may be a little but even 12 gauge shouldn't have a huge drop. However if that battery is getting a lot of current going to it heat might become an issue. You could smell the hot insulation from a dead battery on the circuit trying to be charged, dead as in well below 50% from leaving a device on that drained the battery. That was with 10 gauge wire. With 12 gauge it might have been worse.

[Thomcat316]

Originally Posted by RogerDat

There is a lot of discussion on need for heavier wire going to house battery to avoid voltage drop. There may be a little but even 12 gauge shouldn't have a huge drop.

You'd be surprised. See this table.

You get to the end of the charge curve and the demand is so low that the loss is negligible, and the battery charges up to capacity.

We ran 10-2 boat cable from the truck battery to the rear bumper, and 10ga. power and ground from there to the trailer battery. The roughly fifty feet (round trip) of 10ga. wire acts as a resistor, limiting the current flow between the batteries until the battery voltage climbs a bit. This makes the alternator load palatable, and avoids presenting too high a current to the trailer battery.

This rig, which has a 30A self-resetting breaker and a TV protection relay, has reliably kept a 55Ah trailer battery well fed and happy, with a Dometic CF40 cheerfully running, for weeks at a stretch.

[stevebaz]

remember when you do the running voltage test turn on everything in your car, headlights, wipers air conditioner and flashers. test at idle and slightly above idle where the engine runs most of the time. alternators are pretty poor power generators at idle speeds. so don't be suprized by the test results.

[Thomcat316]

When in transit my truck runs around 2000 RPM, which is in the "full production" range of the stock alternator.

[RogerDat]

Originally Posted by Thomcat316

You'd be surprised. See this table.

You get to the end of the charge curve and the demand is so low that the loss is negligible, and the battery charges up to capacity.

We ran 10-2 boat cable from the truck battery to the rear bumper, and 10ga. power and ground from there to the trailer battery. The roughly fifty feet (round trip) of 10ga. wire acts as a resistor, limiting the current flow between the batteries until the battery voltage climbs a bit. This makes the alternator load palatable, and avoids presenting too high a current to the trailer battery.

This rig, which has a 30A self-resetting breaker and a TV protection relay, has reliably kept a 55Ah trailer battery well fed and happy, with a Dometic CF40 cheerfully running, for weeks at a stretch.

I think the important piece there is it is a 30 amp circuit. Wire and circuit protection have to be compatible. A 30 amp fuse with 12 gauge wire would I think be out of whack. I personally think 10 gauge is about right but people have insisted 8 is enough better to warrant using it. With suggestions for using arc welding cable having shown up. I mean if you have it I guess why not but... not going out and buy some.

I don't think the voltage drop is enough to prevent charging since the alternator voltage should be more than 3% over voltage required to charge a battery. I would have to look up alternator voltage but I think even 10% leaves one with 12.9 volts which is fully charged battery voltage. Just barely.

I run 10 front of Escape to hitch. Hitch plug is whatever is standard with pigtail for 7 pin plug. Group 27 battery seems to stay charged, I don't tend to run a lot of electric so not much draw down of battery when parked.

If one was going to try and run a lot of electrical draw so battery was drawn down each night or over a few nights of camping to be charged while driving that might also be a situation where wire gauge becomes more critical.

Running an evaporative fridge on alternator is I think dubious with the stock alternator in most vehicles. That situation would warrant as much wire size as you can provide because the load is going to be pretty high.

[Thomcat316]

We actually used the run of 10ga. wire as an intentional resistor. It drops the voltage enough (ca. 1.5V drop at 30A) that the initial current is low. We didn't want to stress the TV battery or alternator too heavily, but still wanted to be able to top up the trailer battery.

By the time the trailer battery is close to being fully charged, the voltage drop is under a tenth of a volt. If you go larger on the charge cable you could end up with significantly higher initial current.

[Joe MacDonald]

My stock alternator is 140 amps, adding all vehicle loads at idle doesn’t come close, and unless stuck in traffic you aren’t often at idle, on newer vehicles with led factory lights that total drops even more,
having said all that, my new norcold fridge actually has a sensing wire to tell if the vehicle is running and will not pull from battery power unless that is connected. I plan on putting a separate two conductor #8 harness with separate plugs and a sensing wire from the fusebox to the fridge connector
Will post more when I get it done