Quote:
Originally Posted by davepl
Maybe in Saskatoon because it's metric. But in the US the factory alternators put out anywhere from 36 to 60 amps. No way you can run 60 amps through a 14 gauge wire. Not even 36, I doubt. Maybe in a vacuum as a light bulb. But not in your harness.
The only way this can work is if the relay is off during cranking. I could check that, but like the Carnival barker on the Simpsons who doesn't check to see if Hitler is still in the trunk of Hitler's car, "I'm a busy man, ain't got time to look". But I'll test it later.
I checked with an electrical engineer and his take was that the cranking battery should always have higher voltage than the camper battery and so it provides the amperage needed to crank. I'm not sure about that, and there's still the case of someone throwing a charger on the AUX battery such that IT has more voltage.
PS: Just giving you grief about Saskatoon, I'm from Regina. And it never occurred to me that it rhymed with anything until I moved here!
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Have a look at the electrical feeds to the halogen headlight systems of a more modern vehicle. I think it's a stretch to see 16 ga on any of them and they can be pulling 17 amps. I personally find it scary. I've never seen any of these old trucks with more than a 37 amp unit. Did the Aux batt equipped trucks have bigger alternators? For a conductor size reference take a look at the size of the conductor on a 7 pin trailer plug. The constant 12 volt lead that is the charge for trailer batteries is about 12 ga. and it's 25 feet long. On a short under hood conductor that is 5 or 6 feet long it wouldn't surprise me to see 14 ga. I'd never do it myself but I wouldn't be surprised.
The solenoid type isolator is generally set up to only be on when the engine is running. It intentionally does not connect during cranking or a very large conductor would be needed. It is off when not running so the chassis battery does not get drained from RV loads. I used to be in the ambulance industry and we used to build in an emergency boost option that connected the coach battery during cranking through a heavy gauge connector and a solenoid or contactor.
None of it has anything to do with the batteries at different potentials. FYI I'm an engineering tech for an RV manufacturer so this is all in my scope of knowledge. Actually charging systems have become so complex that diode isolators are no longer compatible. We use smart microprocessor processor controlled solenoids now. The solenoid is Back!
Regina! I dated a gal from Balgonie when I was kid going to SIAST. Used to go through Regina all the time.....as fast as I could. LOL