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What If You ALMOST Need a Generator?

Long-suffering readers know that I like to poke fun -- gently I hope -- at campers who are Gandhi or Thoreau wannabees. They also know that I am not a solar purist. A rational and professional camper uses technology up to the point of diminishing returns. (Or more correctly, the point of diminishing marginal utility.)

And yet there are solar purists who make it work for them. People who have vans or motorhomes probably don't count, since they can always charge their house batteries from their engine battery on a cloudy day. So let's only discuss trailers.

A trailer-puller can connect their tow vehicle to the trailer, and run the engine. But that charges too slowly, perhaps 7 or 8 Amps.

So what do you do when you finally admit that even Arizona is not sunny every day, and that you occasionally park under trees, or near the perpetually cloudy Coast? Buy a windmill? Never heard anything good about them. Besides, you need to supplement a solar system with a secondary system that doesn't depend on the weather.

You could always suck it up and pay to camp at some place with electricity. But let's not give in to defeatism. We will remain loyal to the Cause.

What is so bad about good generators?, such as a quiet-running Honda 1000 watt inverter generator. Well of course, there is the $850 cost.
  1. You must put it inside at night, worry about it walking off, or increase your insurance. Lifting a 30 pound device into a vehicle is asking for a back injury. You must be careful.
  2. Do occasional maintenance, and drag a gasoline can around. Invariably you will forget to fill the gas can when you fill your vehicle.
  3. Find space for it. (probably in a plastic tub, that won't leak oil.)
You must also get a 30 amp, three-stage, battery charger. Of course you need one of them to plug into shore power, although it could cost less.

So I tried an experiment:
  1. Hook an inverter to the battery of the tow vehicle, and run the engine.
  2. Run the output of the inverter to a battery charger for charging the house 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.

So far, so good. Remember, this is only to be used occasionally, as an alternative to buying a generator. (About 20 days per year.)

I did learn not to take the nominal ratings literally [1].  So I downsized the battery charger to a Samlex 30 amp charger [2]. Initially it eats 500 Watts from the inverter. After 15 minutes it has charged four flooded golf cart batteries to the point that they are going into the second stage -- so called "absorption" stage -- of charging, and the power falls gradually. I plan on shutting everything down after 20 minutes total.

You might consider this solution a bit of an extravagance: the pure sine wave inverter did cost $250, after all. Perhaps a modified sine wave inverter would have sufficed, and cost $100 less. But I have had mixed with results with these. 

And the new inverter will be my backup inverter, and perhaps be able to power a 110 VAC tire inflator or powerful tools with it. If it were really necessary, you could get double duty out of your house inverter, if you made it easy to remove.

[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 in Oregon. 


Your system is also illegal to use at some National Parks in the no generator areas.
That makes sense since idling engines can be as loud as a Honda inverter generator. For my part, it won't make any difference because I never go into national parks. Your comment is helpful, though, for those who do.
Wayne Wirs said…
I'm no expert on electronics, but wouldn't a battery isolator on your van with a disconnected "quick connect" second battery cable be more efficient? When you want to charge, just connect the second cable from the battery isolator to the positive terminal on your trailer battery bank.

ie: What you have is Alternator -> Car Battery -> Inverter -> Charger -> House Battery
and I'm suggesting Alternator -> Isolator ->"quick connect" cable -> House Battery

Seems more efficient (but I don't know) but definitely simpler. Maybe an isolator doesn't work unless two batteries are connected???
A battery isolator is worth looking into, Wayne. Surely it costs less than my 1000 Watt inverter.

I am fussy about wanting to charge my four house batteries at a certain rate, 30-40 Amps in my case. It seems like a simple switch (a battery isolator) might charge the house batteries too fast; it would be too much like running jumper cables from your tow engine battery to your house battery.

Presumably the initial charging rate would be 100 or more amps. A car battery can handle that, but a deep cycle golf cart battery has the opposite type of design, and is supposed to be charged more slowly: (1. Bulk. 2. Absorption. 3. Float.)

For instance Interstate recommended a maximum charge rate of 23 amps for a pair of 6 volt batteries.