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pedwards2932 View Drop Down
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Posts: 333 		Direct Link To This Post Topic: Charging while towing
    Posted: 25 Oct 2020 at 8:20am
Not sure if this would work but I was thinking of a way to avoid trying to run really large wiring from vehicle battery to TT.  You can get 300 watt inverters fairly cheap that will run off a cigerette lighter outlet.  The NOCO GENIUS10 puts out 10 amps and requires 150 watts.  Seems like this may be enough to keep the battery charged while towing.  I know you can get dc to dc chargers but they require you run really large wires.  What do you think?
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StephenH View Drop Down
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Posts: 6417 		Direct Link To This Post Posted: 25 Oct 2020 at 9:07am
I'm unsure of your question. I ran wires to run a DC to DC charger for an LiFePO4 battery. If you are only running Pb-Acid, you may not need to do anything if your tow vehicle already has sufficient wiring to charge it. If your tow vehicle's wiring for charging is at least 10 AWG, you should be okay. Otherwise, run the wire. Running an inverter and then a charger would add significant efficiency losses. Since your TV should be able to charge the battery, why add these in? Now if it is for a DC to DC charger because you installed an LiFePO4 battery, it is a different matter. The TV won't charge it properly and you need to run the charger. See my mods for what I did.
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pedwards2932 View Drop Down
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Posts: 333 		Direct Link To This Post Posted: 25 Oct 2020 at 9:25am
The LifePo's are a bit expensive for my budget.  The TV will only charge based on the starter battery so it doesn't charge much more than 80%.  The Ford Escape is very difficult to run wiring and I just had it wired for the trailer and they used 12 gauge so I would have to pay to get them to run larger wire.  The DC to DC wiring would require large wire as well.  I like to run my refrig on 12 volt while towing which adds about a 6 amp draw so on a long haul the battery would be down pretty good if I do a pull off stop with no hook up.  Just thought this would be a cheap way to boost the charging circuit.
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offgrid View Drop Down
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Posts: 5290 		Direct Link To This Post Posted: 25 Oct 2020 at 10:14am
Not sure I understand, are proposing to install a 300watt inverter in the Escape, then run an ac line to the trailer, and connect it to the existing WFCO charger? Or to a separate charger maybe? Some other configuration?

You would be fighting basic physics the whole way, because each of those devices is only about 80-85% efficient. So that's an overall efficiency of about 60-70%. The rest is lost to heat. And you would need to run a new set of conductors for the ac circuit anyway. IIRC, the fridge takes about 11-12 A on DC, so you would need to supply the inverter with  12/0.6 = 20A or so to make up for the losses. That will just put more load on your TV charging circuit. Like everything in the real world, TANF (there ain't no free lunch) applies. 


IIRC, the fridge in DC mode consumes about 11-12A. So, while its unfortunate that they ran 12 awg wire, it has adequate ampacity for the application. You will lose about 1 volt to wire resistance. So you're right that you won't be able to maintain full charge on your trailer battery. 

I suppose you could add a dc/dc boost converter on the 12V line, that would be better than an inverter plus battery charger.  Or,  How about just running the fridge on propane?
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pedwards2932 View Drop Down
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Posts: 333 		Direct Link To This Post Posted: 25 Oct 2020 at 10:45am
No I was saying use the inverter to supply AC to the 10 amp NOCO GENIUS10 that requires 150 watts so you could use a 300 watt inverter.  Noco puts out 10 amps of charge to battery and rolls back to 3 when charged.  So I would clip it to the trailer battery and plug it into the inverter hooked up to the  vehicle 12 volt.  Not sure the 85% efficiency would apply.  My thought is between the alternator and the charger that would keep the charge up.  The refrig doesn't run constantly does it?  I could be wrong I am just thinking outside the box.
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mcarter View Drop Down
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Posts: 3419 		Direct Link To This Post Posted: 25 Oct 2020 at 10:50am
They do make battery to battery charging systems that feed off the tow vehicle, charge a remote battery(RV/boat/etc). They do not interfere with the vehicle battery charge. Google "battery to battery charge" or "charging a remote battery". A lot of big motorhomes and RVs have them. They are not cheap at least for good systems. It also may give you ideas with your proposed solution. The problem with most vehicle systems is they are designed to take care of the vehicle battery. They monitor and provide based upon vehicle need, not a remote battery. If I travel with reefer on DC my TT battery will be somewhat discharged upon arrival and that is with a 12V battery source thru the 7 pin. If you really want to charge a remote battery with a vehicle system, you need to look at battery to battery systems that isolate your vehicle charge and have proper components to charge a trailer battery.
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pedwards2932 View Drop Down
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Posts: 333 		Direct Link To This Post Posted: 25 Oct 2020 at 11:10am
The charger looked like a pretty full featured charger and fairly small so it would be easy to mount at the battery.  Here is the link https://no.co/genius10
By specs it takes 150 watts to run it which is a pretty small inverter (300 watt)  
I looked at DC to DC chargers and again ther require large wire to implement.....the inverter would not. I may try it just to see if it will work....at worst i would end up with an 300 watt inverter and a nice charger.
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offgrid View Drop Down
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Posts: 5290 		Direct Link To This Post Posted: 25 Oct 2020 at 12:00pm
Originally posted by pedwards2932

No I was saying use the inverter to supply AC to the 10 amp NOCO GENIUS10 that requires 150 watts so you could use a 300 watt inverter.  Noco puts out 10 amps of charge to battery and rolls back to 3 when charged.  So I would clip it to the trailer battery and plug it into the inverter hooked up to the  vehicle 12 volt.  Not sure the 85% efficiency would apply.  My thought is between the alternator and the charger that would keep the charge up.  The refrig doesn't run constantly does it?  I could be wrong I am just thinking outside the box.

Yes the 11-12A 12V fridge current is continuous. And yes two 80-85% efficiency multipliers would apply. How could they not, you are taking your TV alternator/12V battery output through the inverter and then through the battery charger. 

Where are you planning to connect the inverter to the TV 12V supply?  You would be doing this while stopped using a temporary connection? If so just use propane. If you are doing this while driving you would need to run a separate 120V line between the inverter ac output and the battery charger. Better to just run a heavier 12v dc line. 

Or are you thinking to mount the inverter in the trailer and connect it to the 12V supply from the TV there? If you do that you will have even more voltage drop in the 12V feed from the TV, because of the efficiency losses. You would also have to disconnect the 12V feed from the trailer battery, or you would be trying to charge the trailer battery from itself, losing energy in the process. That would be a form of perpetual motion. 

Like I said, TANF. 

I do think a dc/dc converter would work. You would have one efficiency loss (dc/dc's are around 90%), which the 12avg wire could handle and still supply 12A to the fridge.  The dc/dc would boost the supply voltage from the TV battery/alternator which would compensate for the voltage drop in the 12awg wire. You would disconnect the 12V TV supply to the trailer battery and run it through the dc/dc instead, just like StephenH has done with his Li battery installation. 
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pedwards2932 View Drop Down
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Posts: 333 		Direct Link To This Post Posted: 25 Oct 2020 at 1:10pm
I must not be explaining this very well.  Inverter is powered by vehicle battery thru a cigarette lighter plug.  Charger plugs into inverter.  Charger connected to trailer battery.  I would run the AC cord from inverter thru back door.  This would be temporary setup to see how it works.  I am not sure how you are getting the inefficiency as I know the inverter is not efficient but if it supplies 300 watts at 120 volts the charger should be as efficient as if it were plugged into a house outlet?
I didn't realize the refrig draws constantly I thought with the thermostat it would shut on and off.
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offgrid View Drop Down
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Posts: 5290 		Direct Link To This Post Posted: 25 Oct 2020 at 3:37pm
Ok, I think I get it now. 

Ultimately you would need to run a permanent 120Vac line to the trailer. So, if that is the plan, why not just run a heavier gauge 12Vdc line now and be done with it? 10 gauge is what I and I think most of us have and that works fine to run the fridge while on the road. The voltage drop from that is only about 0.5V at 12A. Since you already have 12 awg running you could also just add a second 12awg conductor in parallel with the one you have and that would even be a little better than one 10 gauge.

The absorption fridges require a continuous heat source so the 12Vdc element just stays on all the time. 

Re efficiency, if you run straight dc from the TV battery to the trailer battery, the only loss is the voltage drop in the wire.  If you proivide energy via an inverter and charger then the losses would be the voltage drop from the TV battery to the inverter (significant), the efficiency loss in the inverter converting 12Vdc to 120Vac (about 0.85), drop in the ac cable to the trailer from the inverter (insignificant), the efficiency loss in the charger converting 120Vac back to 12Vdc (about 0.85), and the wire loss between the charger and the trailer battery. You will wind up needing about 20A out of your alternator/TV battery to get 12A to the fridge.  Also, you will end up with no current flowing through the existing 12awg dc circuit because the charger would be trying to maintain the trailer battery at a higher  voltage so current would not end up flowing through the 12awg connector. 
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