Re: Winch Battery on trailer.... Charge?

Posted by Rick K on April 23, 1999 at 07:15:39:

In Reply to: Winch Battery on trailer.... Charge? posted by Truck on April 22, 1999 at 18:25:42:

I put an auxiliary battery into my E350 cube van. Here is how I did it...
I got a heavy duty relay at the auto parts store. Cost about $8. I connected the relay so that when the contacts close, the auxilary battery(+) was connected across the (+) terminal of the main battery. Then I probed around the ignition switch and found a wire that was energized only when in the run position, not in the aux, off, or start positions. I connected this wire to the coil of the relay. Now, the auxilary battery is only connected to the charging system when the truck is in the run position. When I turn the truck of or put it on Aux, the auxilary baterry is not connected to anything but the accessories that I want to be able to power with it. I must add that I also put in two 50 Amp fuses, one on either end of the charging wire, right at the main and auxilary batteries, in case something goes wrong with this wire. The wire to do the charging is a #6. The auxilary battery (-) terminal is connected to the truck frame with a regular battery cable.
Make sure that you find a wire on the ignition switch that is not energized when you crank the engine, as that would make the auxilary battery help do the starting. At first, that might seem like a good idea, but the 50 amp fuses would probably pop, and #6 wire is kinda on the small side to pump a few hundred amps thru anyway.

As a side note, this isn't actually an original idea. While the parts used are different, Ford actually used basically the same technique on this truck, when they factory installed an auxilary battery on this same chassis for use as a motor home. I guess you could go to a ford dealer and buy their relay - I'm sure it would cost more than the cheapo one that I used. Theirs closely resembles the one that they use as a starter relay, though I suspect that the coil is a little different - a starter relay has to pull in wicked hard, and hold at reduced voltage while cranking. It does not have to stay energized for a long time. If it did, and full uncranking battery voltage was applied, it may get a little hot - so you probably can't use a regular Ford starter relay.

Good luck!

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