Charging Trailer Batteries with VAN Solar System - Fiberglass RV


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Old 07-11-2021, 12:49 PM   #1
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Charging Trailer Batteries with VAN Solar System

The title doesn't really accurately describing my question.

I have 2 70ah batteries on my trailer. I have 2 100 w solar panels which charge the batteries. When I tried to run my 3-way refrigerator off the batteries, the battery alarm went off about 3 am.

I use a van as my tow and it has 600w of solar, 2 100ah lithium batteries, and an inverter. What I would like to be able to do is charge the trailer batteries at night using the lithium batteries from the van. This is not a permanent connection but a quick connect/disconnect.

What are my options?
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Old 07-11-2021, 01:04 PM   #2
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In 12v mode the fridge is using 10 amps of current. I suppose you could try using the tow vehicle lithium batteries via the inverter to charge the trailer batteries at a 10 amp rate or higher.

Or try this:

In 110vac mode, the fridge uses 1 amp of current, so you could just run a 110vac extension cord from the tow vehicle inverter to the fridge 110vac plug.
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Old 07-11-2021, 01:10 PM   #3
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You can certainly charge the trailer batteries using the tow vehicle batteries. The tow vehicle batteries together with the large solar array (and possibly a dc-dc charger from the alternator?) provide PLENTY of power to do that.

The problem is with running the fridge in dc mode, which draws 10 amps at 12vdc.

Another option would be to run the fridge in propane mode.
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Old 07-11-2021, 01:54 PM   #4
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So using van battery bank to charge trailer battery bank to power fridge (or anything else)..

Why not just use the van battery bank directly to power what you want? It makes little sense to charge a battery with a battery (with the associated power loss) when the first battery is the appropriate voltage and sufficient capacity. You need appropriate wiring of course and battery isolation switch I would think.

Either way you will want to take steps to not kill the van battery and leave you stranded (unless they are separate from the starting battery).

And if you are running an absorption fridge on 12 vdc then yes, thats going to be a challenge. Propane is better if its an option. A Danfoss / Secop compressor fridge if propane is not an option.
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Old 07-11-2021, 02:55 PM   #5
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So using van battery bank to charge trailer battery bank to power fridge (or anything else)..

Why not just use the van battery bank directly to power what you want? It makes little sense to charge a battery with a battery (with the associated power loss) when the first battery is the appropriate voltage and sufficient capacity. You need appropriate wiring of course and battery isolation switch I would think.

Either way you will want to take steps to not kill the van battery and leave you stranded (unless they are separate from the starting battery).

And if you are running an absorption fridge on 12 vdc then yes, thats going to be a challenge. Propane is better if its an option. A Danfoss / Secop compressor fridge if propane is not an option.
The solar system and lithium batteries are completely separate from the van's battery.

I have several reasons I don't want to use propane except for emergency, especially when I have plenty of solar power.
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Old 07-11-2021, 03:06 PM   #6
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Could I plug the trailer's power cable that is used to plug into shore power into my inverter?
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Old 07-11-2021, 03:19 PM   #7
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Originally Posted by Huck View Post
Could I plug the trailer's power cable that is used to plug into shore power into my inverter?
This is probably not a good idea. Your shore power connection is rated for 30 amps. You could use a 30 amp to 15 amp adapter, but again, probably not a good idea.

This really gets down to details:

1. how much dc and ac power do you want to be able to use in the trailer?
2. same question for the van
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Old 07-11-2021, 03:30 PM   #8
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This is probably not a good idea. Your shore power connection is rated for 30 amps. You could use a 30 amp to 15 amp adapter, but again, probably not a good idea.
Isn't that what we do when we plug into an extension cord at home?
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Old 07-11-2021, 03:31 PM   #9
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There's actually nothing wrong with charging the trailer batteries from the tow vehicle "house" batteries. The scenario is as follows. Drive the van around during the day while charging two 100 ah lithium batteries using 600 watts of solar (also could use a dc-dc charger off the alternator). Then upon returning to the trailer, charge the trailer batteries using the tow vehicle house batteries. Really not a big deal.

Sure you could also just run a power cable from the tow vehicle house batteries to the trailer to run all trailer loads. That would require cutting off the trailer batteries.
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Old 07-11-2021, 03:32 PM   #10
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I agree Huck, but still goes back to the details.

1. how much dc and ac power do you want to be able to use in the trailer?
2. same question for the van
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Old 07-11-2021, 03:38 PM   #11
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Other problem with just plugging the trailer into the inverter is the point that I think Gordon made. The inverter even assuming its capacity is large enough is only 80-90% efficient. Better to run 12 vdc using thick copper wires and andersen connectors.
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Old 07-11-2021, 03:38 PM   #12
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Could I plug the trailer's power cable that is used to plug into shore power into my inverter?
Yes. Unless you're trying to run the trailer's air conditioner or a microwave oven, no problem. I successfully run my fridge on 110AC from the car's inverter while traveling all the time.
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Old 07-11-2021, 03:45 PM   #13
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Certainly agree with what Lynn is saying. Fine to run the fridge off the inverter, not the AC or microwave. Other 110 vac items that would likely be a problem: hair dryer, toaster, coffee pot ... You would have to consider what your inverter is designed to handle.
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Old 07-11-2021, 03:46 PM   #14
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Why not use the refrigerator the way it was intended on LP gas?
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Old 07-11-2021, 03:59 PM   #15
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Why not use the refrigerator the way it was intended on LP gas?
That wasn't my question.
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Old 07-11-2021, 04:20 PM   #16
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Originally Posted by redbarron55 View Post
Why not use the refrigerator the way it was intended on LP gas?
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Originally Posted by Huck View Post
That wasn't my question.
Well it sorta was.....

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... What are my options?
The problem is that the people on this forum see all sorts of Rube Goldberg contraptions and feel a need to help fellow forum members to adopt better methods. But, as it is in this case, it is often not clear what the objectives or particulars are, so the advice ends up being based on too many assumptions.
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Old 07-11-2021, 06:06 PM   #17
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That wasn't my question.
But it was my question.
Efficiency (overall) would be served.
If you wanted all electric (which I have with a swing compressor refrigerator) they the very high power draw of the absorption refrigeration on 12 volt power would be avoided.
It seems to be a problem looking for a problem to go with it.

If the goal is to avoid LP gas (which my wife insists be kept out of the camper (thus a tankless water heater on the tongue) I understand.

I fail to understand why recharge a battery from charged batteries when a better or at least an easier method is already present.
Each charge cycle costs something even with solar as the life of the batteries is measured in cycles and depth of those cycles.

So why not LP? still a valid question.

Perhaps not the answer you are looking for, but with a better understanding of the reasoning perhaps someone could give you a better answer.
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Old 07-11-2021, 06:21 PM   #18
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Basically absorption refrigerators use heat to operate and the easiest and cheapest source of heat is burn something like LP gas, which is easily obtained and easily replenished and relatively inexpensive.
Using electrical power to generate heat is expensive and in this case heat is generated at several steps reducing the actual transfer of electrical energy into heat for the refrigeration system.
I believe the higher energy of the electrical power could be put to a better use than the charge two batteries with two different charging systems.
There would have to be a pretty good reason to go to so much trouble.
Why not use the solar power during the drive to directly provide the power and charging (if in excess) for the refrigerator?
Actually most refrigerators on 12 volts don't particularly cycle on and off and are pretty much a constant current draw.
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Old 07-11-2021, 06:37 PM   #19
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Quote:
Originally Posted by John in Michigan View Post
In 12v mode the fridge is using 10 amps of current. I suppose you could try using the tow vehicle lithium batteries via the inverter to charge the trailer batteries at a 10 amp rate or higher.

Or try this:

In 110vac mode, the fridge uses 1 amp of current, so you could just run a 110vac extension cord from the tow vehicle inverter to the fridge 110vac plug.
I believe that using the inverter to convert the DC to AC would actually use more power than just running it directly from DC. Although only 1 amp at 120 volts, that still is 120 watts, and to get 120 watts from the batteries will still call for 10 amps, plus inverter losses.

As long as heavy duty wire is used between the van and the trailer, borrowing power from the van via DC shouldn't be a problem as long as the van batteries also get enough charge during the day. As the days get shorter, and the sun gets lower, the solar charging decreases.

I don't know what fridge the Parkliner uses, so I don't have any idea what the actual power usage is while running on DC, or if it cuts on and off based on temperature, or runs full time. I know the heater in our 5 CF Dometic fridge in our Escape uses 175 watts on either AC or DC, so that would be about 15 amp hours on DC.
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Old 07-11-2021, 06:46 PM   #20
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Again, consider how much dc and ac power (amp hours) you want to be able to use both in the trailer and in the van.

In the best case scenario, at the end of the day the two 100 ah lithium batteries are fully charged. Assuming they can both be discharged to 20%, that gives you 160 amp hours at 12vdc. Now assume the trailer is plugged into the inverter. If the inverter is 90% efficient, then reduce what power can be used in the trailer (until solar power generation resumes) to about 16 amp hours at 110vac. The fridge will consume 1 amp at 110vac while running. What other loads are expected?
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