AUX battery - how is it possible?

[davepl]

I have been ruminating about this off and on for quite some time, and cannot understand it.

When you have the camper battery, as you can see from the schematic, as soon as "IGN UNFUSED" has power the two batteries are connected in parallel. When you turn the key off, they are separated (aux battery positive side isolated from rest of truck).

That's great, but it's a small wire from the aux battery to the junction. So there are two scenarios I can't figure out:

1) When the state of charge is very different. If the AUX battery is depleted and you start the truck, it connects the two batteries together. The fully charged MAIN battery plus the help of the alternator should be pushing a ton of amps to the discharged AUX battery. Yet it's like a 14-gauge connection, so how is that possible?

2) Similarly, if IGN UNFUSED at the fusebox is hot during cranking, then both batteries are again in parallel. I don't know why the AUX battery does not then participate in the cranking and completely fry the 14 gauge wire.

Since a number of people have upgraded to HEI and connected it to IGN UNFUSED I think that -has- to be live during cranking.

Can someone explain it?

[Wrenchbender Ret]

You need to use a bat. isolater. It has diods in it that allow the alt' to charge both bats. but keeps them on separate circuits.

[DeadheadNM]

TP2 schematic from the assembly manual, which differs slightly from above:

[BADAZ chevy guy]

On my '73 Camper Special there was a small (old style) relay on the drivers side inner fender. It was closed when the key was ON, but it was open when the key was in the START position, and also when the key was off. The OEM wiring was a 10 gauge wire and was only for charging the aux battery. Any power for the camper came directly from the aux battery, through a heaver wire, and went through a fuse block inside the camper.

[StingRay]

Originally Posted by BADAZ chevy guy

On my '73 Camper Special there was a small (old style) relay on the drivers side inner fender. It was closed when the key was ON, but it was open when the key was in the START position, and also when the key was off. The OEM wiring was a 10 gauge wire and was only for charging the aux battery. Any power for the camper came directly from the aux battery, through a heaver wire, and went through a fuse block inside the camper.

And consider that the factory alternators were 40 amps. A garden tractor has nearly that now. So there is no Ton of amps to send anywhere.

[davepl]

Originally Posted by StingRay

And consider that the factory alternators were 40 amps. A garden tractor has nearly that now. So there is no Ton of amps to send anywhere.

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!

[jtp67-72]

Dave betcha dinner it's like Badaz state above the unfused lead is only energized in run position...relay open until key in run position....and the 14 gauge is only for charge side ...and hard to believe but it will take that draw...but could always be increased in size if your worried...jim

[BADAZ chevy guy]

Originally Posted by jtp67-72

Dave betcha dinner it's like Badaz state above the unfused lead is only energized in run position...relay open until key in run position....and the 14 gauge is only for charge side ...and hard to believe but it will take that draw...but could always be increased in size if your worried...jim

Something else to remember. Amperage is drawn, NOT PUSHED. Even if an alternator puts out 40 - 60 Amps, A dead battery will only draw 1 - 3 amps. Then as it gets closer to 100% charge the amperage will drop off.

[StingRay]

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!

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