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Old 05-11-2017, 08:51 PM   #1
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Battery Charging While Running Fridge on 12V

If there are any recent discussions of this, I missed them, so I'll go ahead with my question(s). I'm interested in running our RMD 8555 fridge on 12V while towing....all day, for days in a row, while towing. Is anyone doing this successfully (and arriving each night with a fully charged battery)? If so, how?

The fridge draws something in the neighborhood of 15 amps. I'm well aware of voltage drop and the need for large charge wiring (including ground) to minimize voltage drop. After reading several accounts of folks doing this, it almost seems like this is a losing battle. Even with no 8 AWG wiring, other problems still limit what can be done. Apparently modern charge controllers on the tow vehicle (TV) will limit the current flow from the alternator to whatever is demanded by the nearby TV battery....which normally is not much. The only way to overcome this would be a "sense line" running all the way from the trailer to the charge controller, which I wouldn't want to attempt.

I've read about various schemes to improve the situation, like dual no 8 wires connected through a separate 4-way connector at the bumper. Someone else suggested a voltage booster. One person suggested using one of these to kick the voltage up to 24V at the camper and then feeding this through the solar controller to charge the battery. Novel, if nothing else. Others talked about using an inverter to operate the fridge on 120V while towing. I don't know where they put the inverter. It seems like it would work best connected directly into the TV battery, but that means having a 120V connection to the trailer while towing. That seems pretty sporty. I have to give these folks credit, but I'm not sure I see a sensible way to make this work. The numbers just don't work for me.

I would really appreciate hearing the experiences of others who have operated they're fridges on 12V for extended tows.
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Old 05-11-2017, 09:08 PM   #2
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If there are any recent discussions of this, I missed them, so I'll go ahead with my question(s).
Actually, this is a popular and frequent topic.

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Originally Posted by Parker View Post
Others talked about using an inverter to operate the fridge on 120V while towing. I don't know where they put the inverter. It seems like it would work best connected directly into the TV battery, but that means having a 120V connection to the trailer while towing. That seems pretty sporty. I have to give these folks credit, but I'm not sure I see a sensible way to make this work. The numbers just don't work for me.

I would really appreciate hearing the experiences of others who have operated they're fridges on 12V for extended tows.
I measured over a volt and a half drop from my alternator to my fridge using alternator power only. If I tried this for a long tow with my batteries connected I would end up partially draining my batteries - not charging them.

Yes, in theory you could connect your 110V inverter close to the alternator and run a cord back to the trailer and power the trailer converter to get back to 12V. The point would be to maximize the 110V run and minimize the 12V run.

I'll wait for someone to report how this works...

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Old 05-11-2017, 10:11 PM   #3
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Hi Parker, a Redarc (Australian manufacturer) sells 12vdc 25 and 40 amp chargers using tow vehicle as your generator. Their BCDC1240-LV is a 12V 40A In-vehicle DC to DC charger suitable for vehicles fitted with an ECU-controlled variable voltage alternator, features a MPPT solar regulator. Has tow vehicle battery isolator and can charge AGM, Gel, Calcium content, VRLA and Standard Lead Acid batteries. The 25 amp is $360 USD and the 40 amp $420 USD.

I recommend using 4 AWG welding wire from wireandcableyourway.com as its's a long run. Use tow and trailer frames to carry earth. Anderson AB 120 series connectors with boot will handle the larger wires and make tow/trailer connection easy and solid. PEI Genesis sells these connectors.

Good luck.
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Old 05-12-2017, 06:10 AM   #4
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Good morning. I was hoping for a miracle, and you guys are coming up short! I'm not sure I like the idea of 120V AC trailer connections while driving. I'd certainly want a solenoid switch to turn that thing off with the ignition. Running higher voltage (even DC) does make sense in term of reducing wire size and losses though, with something to reduce it again at the camper. The Redarc setup sound like it really addresses the problem from a technical aspect, but that's a pretty expensive solution. Santiago, when you refer to running welding cable for the charge line (which I have considered) are you doing this with a standard charging setup or have you taken the plunge with the Redarc system?
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Old 05-12-2017, 06:13 AM   #5
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You may be able to run the refer on 12v if you had enough solar and battery storage, at least dual panels and 2x six volts should help during the day driving and remember that 15 a/h draw is only about 50% for when the refer is actually cooling. But that is why propane is so attractive as an alternative. Some new GMC trucks also offer dual alternators, one for tow and one for the rv, so it is possible.
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Old 05-12-2017, 06:35 AM   #6
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Hi Jim. I know that more solar could help take up the slack on sunny days, but our 2013 with two 6V batteries has the smaller panel that is otherwise enough for our needs. If another panel would do it, it might be worth it. I don't think that's a reliable enough solution, though, compared to plugging into the TV, if enough current were available at the camper. I get the separate alternator idea. That makes lot of sense. So far, I'm not hearing (or finding) much that's going to help, short of a hefty investment of some sort. Maybe the solution, if I want to avoid running on propane (and fussing with trying to keep the thing lit) is to run on battery during much of the day, then switch back to propane for a couple of hours before landing to get the batteries topped up for the night.
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Old 05-12-2017, 07:23 AM   #7
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That is what I do sometimes, 12v during day after 9am when there is sun (I have the larger solar panel) and propane on around 3 pm, this combo works.
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Old 05-12-2017, 07:59 AM   #8
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That may have to be my plan also, modified to suit the weather and available solar. We camp mostly in shoulder seasons with shorter days, so that's what got me thinking more about the practicality of 12V. I'm not too worried about running with propane on the road, but I do like the idea of 12V to avoid the flame and also to avoid messing with various screen materials in the vents to help keep it lit.
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Old 05-12-2017, 08:19 AM   #9
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running on 12 volt

We use 12 volt for fridge we have the dual 6 volt batteries and 165 watt solar .Plus we have a 10 awg from battery to tow .We don't seem to have any problems towing for 10-12 hours . the solar seems to keep up . We still need to run our fridge on propane haven't done that yet .camping in the SE there is almost always elec A/C .In the winter or in the PNW it might be harder to do ? Jim
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Old 05-12-2017, 08:30 AM   #10
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We use 12 volt for fridge we have the dual 6 volt batteries and 165 watt solar .Plus we have a 10 awg from battery to tow .We don't seem to have any problems towing for 10-12 hours . the solar seems to keep up . We still need to run our fridge on propane haven't done that yet .camping in the SE there is almost always elec A/C .In the winter or in the PNW it might be harder to do ? Jim
Well, that's encouraging, but I'd appreciate a few more details. Which fridge would be one (indicating the amount of current required). Also, have you measured the battery voltage after towing all day on 12V? I would like to be able to set up in a non-electric site with the batteries closer to 100% charge if possible. So far, propane has been the only way to do that.
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Old 05-12-2017, 08:59 AM   #11
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Dual alternators work well on boats; one for start and one for house batteries but on a trailer and tow vehicle, this might not work so well due to wire size, controlling the charge rate, and installing an extra alternator in the TV.

I had a sailboat and mostly used solar which worked well for me but I had more efficient refrigeration too. If more solar isn't practical then maybe some sort of alternate generator attached to the trailer might work.

One thing that hasn't been mentioned is use of wind generator. Lots of sailboat cruisers used wind generators. I don't know if a small wind generator designed for the purpose being discussed exists or not but it seems that it might work if it isn't too noisy. Maybe even a car alternator with a small propeller instead of a belt pulley and mounted on a bike rack behind the trailer so the alternator is located above the trailer in the slip stream. I wouldn't think a small alternator with small propeller that turns out 5 to 10 amps or so at 60 mph would be too much drag. I don't know if prop might be too big but at 60 mph it probably doesn't need to be very large. I don't know what sort of controller/regulator would work best but it wouldn't need to be very large. Maybe alternator isn't designed for the thrust from propeller.
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Old 05-12-2017, 10:00 AM   #12
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Why not just drive with the fridge operating on propane? Many people (myself included) do so and have great success with that approach.
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Old 05-12-2017, 10:24 AM   #13
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Why not just drive with the fridge operating on propane? Many people (myself included) do so and have great success with that approach.
I've done that for many years with a Scamp, Casita, and now with the Escape. My post was not about the easiest way nor was it intended to open the never ending 12V vs propane question, but whether or not 12V operatuon is really feasible without some extraordinary measures. So far, I'm confirming that it may not be.
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Old 05-12-2017, 10:27 AM   #14
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Parker clearly wants an alternative to running propane while driving.

I would suggest that the cheapest, do-it-once-and-forget-it alternative is have someone run a heavy cable - say 8ga or 6ga - from the alternator to the trailer plug at the back of the tow.
Has to be cheaper than a DC to DC converter, cheaper than a second alternator, and a lot more practical than running a 110v inverter with an extension cord. No moving parts to fail, no expensive electronics, no worries about having 110v in the trailer when you least expect it.

Its what I would do if I wasn't such a cheapskate.

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Old 05-12-2017, 12:57 PM   #15
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Parker clearly wants an alternative to running propane while driving.

I would suggest that the cheapest, do-it-once-and-forget-it alternative is have someone run a heavy cable - say 8ga or 6ga - from the alternator to the trailer plug at the back of the tow.
Has to be cheaper than a DC to DC converter, cheaper than a second alternator, and a lot more practical than running a 110v inverter with an extension cord. No moving parts to fail, no expensive electronics, no worries about having 110v in the trailer when you least expect it.

Its what I would do if I wasn't such a cheapskate.

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My thoughts are that in addition to the heavy ga charge line, you'd need to put the beefiest alternator offered in the tow vehicle. Those two items combined with solar would probably be sufficient.
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Old 05-12-2017, 01:41 PM   #16
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I've done that for many years with a Scamp, Casita, and now with the Escape. My post was not about the easiest way nor was it intended to open the never ending 12V vs propane question, but whether or not 12V operatuon is really feasible without some extraordinary measures. So far, I'm confirming that it may not be.
I guess the obvious solution would be for you to carry a generator in the back of (or on the roof of) your tow vehicle and have it running and charging the trailer batteries while you drive, thus allowing you to operate the fridge on 12V.


Maybe in the same spirit of this guy:


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Old 05-12-2017, 01:51 PM   #17
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Parker clearly wants an alternative to running propane while driving.

I would suggest that the cheapest, do-it-once-and-forget-it alternative is have someone run a heavy cable - say 8ga or 6ga - from the alternator to the trailer plug at the back of the tow.
Has to be cheaper than a DC to DC converter, cheaper than a second alternator, and a lot more practical than running a 110v inverter with an extension cord. No moving parts to fail, no expensive electronics, no worries about having 110v in the trailer when you least expect it.

Its what I would do if I wasn't such a cheapskate.

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I guess that charging the starter battery in the TV and the house battery(s) in the trailer simultaneously off the TV alternator would require some sort of isolation to avoid cross draining one battery to the other since they would be in a parallel circuit. Maybe diodes would need to be placed in the circuits?

Assuming the alternator has built in regulator, wouldn't it be sensing off the start battery which would normally be near fully charged except right after starting the engine? Since it is near full charge most of the time, wouldn't the alternator only be putting out enough voltage and amps to keep the start battery fully charged (float charge).

I think that this lower charging voltage would not allow many amps to go to the trailer battery.

If on the other hand, the sensing was taken from the house battery, then wouldn't the start battery end up being overcharged? I really don't know. Maybe the the alternator (regulator) would sense the entire system load and put out the required voltage and amps and then the batteries would somehow take up just the amps they need. I don't think this is correct but maybe so. I hope someone who understands all this could explain it. It is difficult for me to understand how one alternator can deliver different voltage and therefore different amps to 2 different batteries connected in parallel, even if they are isolated by diodes.
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Old 05-12-2017, 02:20 PM   #18
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21 escape

we have the new 21 ft escape with standard fridge .I was quite suprized that fridge could and does stay cold and batteries are usually only down to 90% . We arrive and plug into a 30 amp service .Since we like being comfortable we use our A/C which needs 110 ac power so boondocking is out as long as we are in SE America in Summer .
Where no A/c is needed then maybe get extra panel 160watt +160 watt . Good Luck in your quest jim
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Old 05-12-2017, 04:07 PM   #19
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I guess that charging the starter battery in the TV and the house battery(s) in the trailer simultaneously off the TV alternator would require some sort of isolation to avoid cross draining one battery to the other since they would be in a parallel circuit. Maybe diodes would need to be placed in the circuits?
Isolation is only required when the engine is not running.

That is normally achieved with a simple relay that cuts off the connection when the key is turned off; that is a normal feature of vehicles which include a trailer power feature in their wiring (but apparently not in a Ram pickup). I added a relay to do this in my Sienna, because Toyota does not provide trailer power wiring (even as an option) in this model, and many people either add this themselves or have a hitch shop do it.

An alternative is an automatic charging relay, which cuts off the connection based on voltage. A poor alternative is a diode-based isolator, which reduces the voltage provided to both the tow vehicle and trailer systems due to the diode voltage drop.

Some people just make sure that they unplug the trailer from the tug when they stop.

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Assuming the alternator has built in regulator, wouldn't it be sensing off the start battery which would normally be near fully charged except right after starting the engine? Since it is near full charge most of the time, wouldn't the alternator only be putting out enough voltage and amps to keep the start battery fully charged (float charge).

I think that this lower charging voltage would not allow many amps to go to the trailer battery.
Yes, that's the problem, although the charging system voltage is often more than most would consider a normal float voltage - 14.2V is normal. Modern vehicles even deliberately lower system voltage under many conditions to save fuel; the "tow/haul" mode on some vehicles can cancel that, but the voltage is still not being sensed in the location which would produce desirable trailer battery charging.

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If on the other hand, the sensing was taken from the house battery, then wouldn't the start battery end up being overcharged? I really don't know. Maybe the the alternator (regulator) would sense the entire system load and put out the required voltage and amps and then the batteries would somehow take up just the amps they need. I don't think this is correct but maybe so. I hope someone who understands all this could explain it. It is difficult for me to understand how one alternator can deliver different voltage and therefore different amps to 2 different batteries connected in parallel, even if they are isolated by diodes.
The voltage at any location is determined by the voltage at the single source, and voltage lost due to resistance and diode drops along the way. The voltage lost to resistance depends on current and wire/connection resistance; that means that if you plug in a trailer with a nearly discharged battery, lots of current flows (big voltage difference), but resistance limits how much, and as the trailer battery charges up less current flows.

The diodes of an isolator just prevent current flow in the undesired direction (from either battery toward the alternator output or the other battery). As long as the alternator is putting out higher voltage than the internal voltage of either battery, the diodes don't isolate the tug and trailer system behaviour any more than any two circuits of the vehicle.
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Old 05-12-2017, 04:13 PM   #20
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Dual alternators work well on boats; one for start and one for house batteries but on a trailer and tow vehicle, this might not work so well due to wire size, controlling the charge rate, and installing an extra alternator in the TV.
I don't see any issue with wiring or control (as long as it had a separate sensing wire), but installing an extra alternator on the engine of a modern vehicle could be a packaging and drive nightmare. That's one reason that the DC-to-DC charger idea is so appealing (but too expensive, because it is not common).
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