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Almost Needing a Generator

This method is meant for tow vehicles pulling trailers with flooded lead acid batteries in the trailer. The intent was to wean myself from the notion of owning a generator, while still charging my trailer batteries safely on an occasional cloudy/tree-shaded day.

Long-suffering readers know that I am neither a solar purist nor a generator-phobe, as long as the generator is one of the modern, quiet ones. I reject the boondocking-ghetto approach of some noisy, yellow, off-brand, P.o.S. generator from China.

There are a few days per year when solar panels just won't do it for you, because of clouds or trees. Since owning a generator is not free or maintenance-free, solar needs to be supplemented. Don't suggest windpower -- the supplementary system must be weather-independent!

Sure, you can supplement solar panels by connecting the tow vehicle to the trailer and idling the engine. But I only get about 7 amps like that. 

(Could be inadequate wiring, but it could also be an electronic circuit that drops the voltage. In either case, a tow vehicle is not always hooked up to its trailer. Sometimes it is parked a long distance away.)

I also reject the idea of connecting jumper cables from an idling tow vehicle engine to the batteries in my trailer, because I want to charge at a current that is safe for the trailer batteries, and doesn't generate hydrogen too fast. (You are only supposed to charge a pair of 6 volt golf cart batteries at the rate of 20 amps.)

I use flooded lead-acid batteries in my trailer, which is well ventilated. Hydrogen has not been a problem, but I insist on charging slowly and in a controlled manner. The only way to do that is to do the following:
  1. Hook an adequate inverter to the battery of the tow vehicle, and idle the engine.
  2. Run the output of the inverter to a battery charger for charging the trailer battery.
Obviously I run the van's engine when the inverter is sending 500 Watts to the battery charger in the trailer. I use a common outdoor 25 foot long extension cord to connect to the charger in the trailer.


The furring strip screwed to the bottom of the plywood keeps the whole thing from sliding off. I added an external fuse to the positive line of the inverter and the charger.

Remember, this is only to be used occasionally, as an alternative to buying a generator. (About 20 days per year.) You wouldn't want to make a daily habit of idling your tow vehicle's engine. I plan on shutting everything down after 30 minutes.

I did learn not to take the nominal ratings of the 1000 Watt inverter literally [1].  So I downsized the battery charger to a 30 amp charger [2]. Initially it eats 500 Watts from the inverter. 

You might consider this solution a bit of an extravagance: the pure sine wave inverter did cost $250, after all. I could have used a cheaper modified-sine-wave inverter, but I have had mixed with results with these. 

I needed a new inverter anyway, for running powerful tools, perhaps even a tire inflater.

This method also needs a $200 battery charger, but this is not really a cost of the project per se, since a battery charger is needed anyway for the times you are plugged into shore power.
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Epilogue: By midyear I have used this arrangement only 3 or 4 times. Perhaps it will get more use if I camp underneath ponderosas for shade, or when the monsoonal rains begin. So far, this is successful because it was meant to be a back-up system for a not-quite-perfect solar charging system. And it is a lot better and less expensive than dragging a generator around. 
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[1] A 45 amp IOTA battery charger was too big for a 1000 Watt pure-sine-wave inverter (Xantrex ProWatt SW 1000), even though the nominal ratings would suggest otherwise.

[2] I always buy electrical controllers, inverters, etc., from DonRowe.com in Oregon. 

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