How long will a Dometic RM7401 refrigerator run on 2 shore batteries

[englewoodfl]

We're leaving on a long trip. Our 2006 Free Spirit has 2 six month old full-sized deep-cycle shore batteries. With them fully charged what's a good estimate for the time the the frig will run without damaging the batteries or the fridge?

 

[borabora]

What is the capacity of the batteries in amp-hours?
I can't find average draw numbers for your fridge but based on typical Dometic performance and the size of the fridge it probably averages 2-3 amps per hour. Which means about 50-75 amp-hours per day in normal conditions. Numbers can double when hot.
*If* your battery capacity were 300 amp-hour *and if* you only draw down to 50% *and if * conditions are normal *and if* you use no power for anything else *and if* my power draw estimates are correct then you could run the fridge for 2-3 days.
As you can see there are a lot of ifs when it comes to answering your question. On top of that even 6 months old batteries can have significantly reduced capacity if they have been abused. Your batteries recharge during driving so on a long trip all should be well. Only if parked for days will you run down the batteries.

 

[englewoodfl]

When we bought this months ago the seller said the batteries only charge when it's on shore power. He never used the generator so that was an unknown. We had to get new batteries because with the original inverter there wasn't a trickle function and he left it plugged in for weeks at a time. And then a new inverter likely from the service tech that worked on it a day before we purchased it as the old one was "toast". The new one has a trickle function. I know from cars you can leave a charger on for months but it needs to either be a maintenance charger (I use a Battery Tender brand) or a new electronic charger that has this function.

With my Hall effect meter, use for working on old cars, the large battery cable has .3 amps with the 12V switch on and indicator illuminated. With the fridge on initial start-up the reading is 11.4 amps at an outside temp of 80F. This is of course the worst case scenario. Thus it seems that the 2-3 amps per hour when it's already cool seems like a reasonable because the fridge will cycle.

Back to the recharge while driving. Is this true for an early 2006 T1N - ours is an LTV Free Spirit? I assume that the circuit is isolated and hopefully current limited - good assumption?

 

[Kajtek1]

Millions variables that will affect the answer, but generic will be
SHORTER THAN YOU'D LIKE.
Unless you have big solar panels, don't plan on leaving fridge on batteries in hot weather.

 

[Mr. Bills]

I have a Dometic CFX 95 DZW two compartment fridge/freezer and two four-year-old MK 258 amp hour AGM house batteries, netting about 250 amp hours of usable power before hitting 50%.

The fridge will draw 7 amps on initial start up, but averages about 2-3 amps per hour over a 24 hour period, the same as a typical Engel or ARB fridge. It is helpful in hot weather to adjust the temperature of the freezer up a bit to keep it from running so much, but generally speaking I try not to fixate on battery management at the expense of properly frozen ice cream.

My house batteries typically last 2 1/2 days when parked in the shade in moderate to warm weather (no solar to speak of from my 320 watt panel), sometimes a bit longer, before the voltage reading gets down to 12.2v (50%). That includes using the fridge/freezer, LED lights at night, the water pump as needed, a K-Cup coffee maker 2-3 times each morning, and less than 10 minutes per day of 650 watt microwave use. I think I would get more from fresh batteries and save about 300# in weight, but a switch to LiFePO4 batteries is still below the horizon.

 

[borabora]

When we bought this months ago the seller said the batteries only charge when it's on shore power. He never used the generator so that was an unknown. We had to get new batteries because with the original inverter there wasn't a trickle function and he left it plugged in for weeks at a time. And then a new inverter likely from the service tech that worked on it a day before we purchased it as the old one was "toast". The new one has a trickle function. I know from cars you can leave a charger on for months but it needs to either be a maintenance charger (I use a Battery Tender brand) or a new electronic charger that has this function.

With my Hall effect meter, use for working on old cars, the large battery cable has .3 amps with the 12V switch on and indicator illuminated. With the fridge on initial start-up the reading is 11.4 amps at an outside temp of 80F. This is of course the worst case scenario. Thus it seems that the 2-3 amps per hour when it's already cool seems like a reasonable because the fridge will cycle.

Back to the recharge while driving. Is this true for an early 2006 T1N - ours is an LTV Free Spirit? I assume that the circuit is isolated and hopefully current limited - good assumption?

I don't know if your model RV for some reason omitted alternator based charging but it would be odd. Chances are that it should work but isn't. Basic alternator based charging consists of wires going from your starter battery to your house battery interrupted by an isolation relay. That relay closes when the engine is running and opens when the engine isn't running to protect your starter battery form over-discharging. The isolation relay is usually triggered by the D+ (ignition) signal or in more modern cases it is voltage sensing and closes around 13.5V.
If you drive a lot then repairing, replacing, or adding such a charging circuit would be the easiest fix. It is not current limited. A current limited charger would be a DC-DC charger.
All the correct answers to your charging needs depend on your battery type and size, your consumption and the type of driving/camping you do.

 

[aksotar]

numbers aside, in reality, it really depends on how often you open it and and how long you leave it open.. know what you're getting and don't leave it open and "browse"
it's truly amazing how people don't seem to realize that basic fact...

 

[WinnieView1]

Our Norcold fridge is a 3-way and will run on propane (most often) with a small amount of 12 volt required, AC, and 12 volts when the engine is running. Most of the Norcold's we've had over the years were only 2-way, propane and AC. Doesn't much matter thou since the 12 volts fuse and circuit is burnt on the controller board and we need to fix it someday. The driving part of our trips are usually shorter in duration such as 2 hours so we don't need to switch it to propane or run the generator for AC power but we sometimes do. Fact is it stays closed when we are driving and doesn't change much.
Interesting that yours does not have a propane fuel option if it doesn't.

 

[tinman]

That fridge is a "3-way", so 110v/12v/propane. When you're parked, run it on propane. As others have pointed out, the net power consumption depends on several factors, but it will be higher than you expect and you will not likely get more than a day or so out of your batteries running on 12v. I have run my two house batteries down completely by leaving the fridge on 12v. for an overnight ferry crossing. S.O.P. now is switch the fridge to propane before shutting down the engine. Two other things that might be of interest: The rate of charge from the alternator to the house batteries is surprisingly low, so if they are deeply discharged it takes a VERY long drive to get any appreciable recovery, and your 12volt master needs to be on to run the fridge while you're driving. This situation can be improved by installing a 12v-12v B2B charger in the supply to the house batteries. I used a Renogy 20 amp unit. Second, as I learned on the Forum ("Slaying the propane solenoid dragon"), the propane solenoid valve by itself pulls about 0.6 amps, which translates to around 17 amp hours/day just from having the propane turned on. The tinkerers on that thread determined that the valve will open at around 2.5 volts, and they have measured a reduction in power consumption of around 80% by supplying the power to the solenoid valve through a cheap12v-5v "buck converter". I've bought one and am just waiting for a DPDT switch to install it.

Edit. Your house batteries should be charging with the alternator running. Easy enough to check. Turn on the 12v house master and check the voltage at the battery. Fully charged should be around 12.5v. Fire up the engine and it should increase initially to around 14v+, then settle in around 13.5. The charging circuit is simple. Heavy lead from the chassis battery to an isolation relay under the driver seat, thence a heavy lead across to the house battery under the copilot seat.

 

[englewoodfl]

Ran a quick check:
engine / 12 switch / shore power / one rear batt
off off off 13.11 (had previously been charged)
off on off 13.08 (the panel indicator and maybe parasitics)
idle on off 14.02 (would indicate charging)
off on on 13.57 (with the new inverter)

Based on tinman we'll try running the fridge on propane rather than 12V. Locally it's $3.50 (USD) per pound and I filled up this morning. I'll see how it goes as we'll be using propane for almost 100 hours on this trip unless the results don't permit that. First 2 1/3 days no shore power.

Later I'll use the Hall meter with partially discharged batteries and see what the charge rate is.

 

[WinnieView1]

Most RV fridge's will run from the propane tank for weeks.

 

[borabora]

Most RV fridge's will run from the propane tank for weeks.

I believe, though, that you are not supposed to run them off propane while driving, correct?

 

[WinnieView1]

I believe, though, that you are not supposed to run them off propane while driving, correct?

Incorrect.
Many only run from propane and have no other 'power' source. We've run ours for years on propane while driving, but do turn it off when refueling the RV or propane. There are also couple of tunnels on the east coast that require it to be turned off.

 

[Kajtek1]

When it is legal to drive with propane flame under refrigerator, it is illegal to fuel up with running flame, while lot of owners tend to forget about it (guilty as charged).
The dispute always got pretty heated on RV forum.
Common reply is that there are no records about gas station fires started by RV refrigerators, but as far as I could check, nobody specifically is obligated to report such, so there is margin for such accidents to happen unnoticed.
But lot of areas have tunnels, ferries and sometimes even bridges, who require propane to be sealed at main valve.
Over the years I also found lot of reports that flat tire on motorhome broke inner fender, broke propane line above, rim running on the road start sparking and it totaled motorhome as result.
That is why I always pushed for running everything on electricity while on the road.
I used camper appliances in my Sprinters, but I deleted propane burners and run them on electricity only.
Sleep much better after seeing lot of such pictures.

 

[WinnieView1]

And then there are the 'Nevercold' fridges which could ignite without warning and burn everything down to the metal RV chassis. Just like MB and their never ending recalls and modifications Norcold had a series for their 1200's a few years back and they were annoying to deal with to say the least.

On a side note; the SA called about an hour ago and said ours is ready to pickup again after the new extended warranty replacement of the DEF heater. We will report in soon.

1-Adam-12 clear