Charging using AC power from tow vehicle

[KevK]

I was thinking about methods to charge from the tow vehicle while under way. Instead of charging from a DC-DC charger which would require heavy cables for significant current, why not put a good sized AC inverter on the tow vehicle and run 120 VAC to the trailer and let the converter in the trailer charge the trailer batteries? And added benefit is you can run one outlet into the cab of the tow vehicle for 120V power for use in there. Or if you have a newer truck with 120V in the truck bed already, just run a power cord to the RV? You would have to size the inverter small enough so as not to burn up your alternator or otherwise limit the draw but a benefit is that your fridge would be running on AC while on the road. A downside is conversion losses but you may have enough power from tow vehicle to bring your rv batteries to full by the end of the day.

Thoughts?

[ependydad]

This is actually how we used to boondock. I installed a 1000W inverter and then ran an extension cord from the inverter to the shore power inlet.

I think you're still looking at sizable cables for the battery to inverter wiring. And then the physical size of the inverter itself is going to be an installation issue if you're trying to keep it close to the tow vehicle's battery. I know my 1000W inverter isn't small.

And lastly, you'll have to very carefully manage the load in the trailer to make sure you don't overload it.

I think the theory of it is good but the actual practice is going to be more problematic than you'd expect.

[KevK]

I was thinking in the 500-750 watt range. You are right about overloading the inverter may be an issue. I checked base usage it would only be 325 watts from the fridge when cooling and then only the converter/charger. But the converter/charger is going to want up to 1000 watts and may want to suck up everything which may overload the inverter.

That said, the optional systems in the current Ford trucks may be just the ticket...

https://fordauthority.com/2021/01/ho...d-f-150-video/

[SeaDog]

Why not forgo the idea of the converter doing the charge and use a stand alone charge for the batteries and let the frig run on propane while traveling. just a thought

[KevK]

Whether to run a fridge on propane while on the road is a topic that will never be settled. I confess that I have done it but I can also see the reasons for not doing so. This would give the option to run it on 120 while on the road with an added benefit of charging the house batteries.

[TitanMike]

For my setup the cost of installing a DC-DC charger was:

Charger = $127
Connectors= 2@ $3.13 ea.
Wire= 30ft ---$50
Fuse holder and fuse = $13

Total= $196


An inverter to power up the converter would need to be at least 1,000 watt. To prevent heat issues in the converter it SHOULD be a TSW inverter so figure $200-$300 for a quality unit.

Current draw will be ~ 80 amp max so appropriate size wire requited between battery and Inverter. Now figure the cost of routing the power to the trailer from the Inverter. You certainly don't want to hang the existing power cord on the side of the trailer while driving so something would need to be routed inside to keep it safe.

Now for the fun part. If you connect this new source of 120 volt power directly to the converter or power panel, you need a transfer switch (add ~$30-$50) or the "prongs" at the power inlet socket will be LIVE and when stopped or parked with inverter running will be a shock hazard to anyone who get's curious. Naturally one shouldn't be poking around but unless you lock the cover you can't be sure and the liability will be on your shoulders -- unless you use a transfer switch.

Now add the cost of some heavy gauge wire to connect the Inverter to the transfer switch. 12-3 SOOW cable runs around $1.25 per foot.

Last item and perhaps the most important of all. You will need to have a means of isolating the starting battery from the Inverter, or automatically shutting the inverter off, when engine is not running. If not, remember the potential 80 amp draw? Almost like continually cranking he starter all the time you are fueling, going to the restroom, parked for lunch, etc. A DC-DC charger will do this automatically.

Just my observation but it seems like there's no savings in expense and a lot of issues created just to avoid the hour or two to run the heavier wire to feed a DC-DC charger. Don't forget, you aren't avoiding the need to run wire, just need to run different wire with no significant savings (maybe $0.50 per foot max) as you will need to run MORE of the less expensive wire (from engine compartment all the way to converter instead of engine compartment to near batteries in trailer)

Choice is yours but if you do this, be sure and make it a safe install.

BTW: If running the refrigerator on Propane is an issue a cheaper, 500-600 watt MSW inverter can be purchased for $50-$60 and since the refrigerator heating element is only a resistive device, sine wave purity is a non-issue. Heating elements run around 300 watts so the refer can be powered by a much smaller, less expensive, inverter and will keep refer running when engine is stopped although propane will do the same if you have refer set to "AUTO".

I've run my refer on propane while driving since my first trailer with a real refrigerator back in the 70's. Three trailers later and absolutely NO problems whatever.

[KevK]

Well I guess that kills that idea. Thanks for the input.

[ependydad]

Originally Posted by TitanMike

Now for the fun part. If you connect this new source of 120 volt power directly to the converter or power panel, you need a transfer switch (add ~$30-$50) or the "prongs" at the power inlet socket will be LIVE and when stopped or parked with inverter running will be a shock hazard to anyone who get's curious. Naturally one shouldn't be poking around but unless you lock the cover you can't be sure and the liability will be on your shoulders -- unless you use a transfer switch.

Now you have me curious with my inverter setup if I'm back feeding power to the shore power inlet.

[TitanMike]

Originally Posted by KevK

Well I guess that kills that idea. Thanks for the input.

It's not a terrible idea, it just has things that need to be addressed.