Auxiliary Battery Wiring

[w314speed]

Can anyone HELP! with a wiring diagram for the factory auxiliary battery set-up. Also, the availability of the relay? Thanks, Dan

[JimKshortstep4x4]

Here is the wiring layout. Relays are available but I am not sure if the originals are.

Jim

[w314speed]

greatly appreciate this info!!!!

[red71cheyenne]

I just used a regular new style 30 amp relay for my install. Works fine.

[rpmerf]

Oh wow that relay was factory! I was wondering about that. What is the purpose of the relay?

[MagmaJct]

The relay prevents the two batteries from "equalizing" which would reduce the life of one of the batteries. How so?

No two batteries are at the exact same voltage when fully charged, especially when one is older than the other. For example, one battery may be 12.7 volts, but an older would be 12.5 volts. If the two were not isolated by a relay the 12.7 volt battery would discharge into the 12.5 volt batter until both equaled 12.5. This would reduce the life of the 12.7 volt battery.

However, during operation, when the alternator is charging the batteries, the voltage is higher than both the batteries so they should both receive a full charge.

The other reason for the relay has to do with why one would have two batteries in the first place: To use one while the engine wasn't running for whatever reason, yet still have a fully charged battery to start the engine.

Originally Posted by rpmerf

Oh wow that relay was factory! I was wondering about that. What is the purpose of the relay?

[Longhorn Man]

Originally Posted by MagmaJct

The relay prevents the two batteries from "equalizing" which would reduce the life of one of the batteries...
If the two were not isolated by a relay the 12.7 volt battery would discharge into the 12.5 volt batter until both equaled 12.5. This would reduce the life of the 12.7 volt battery...

How do you explain the millions and millions (probably billions and billions) of diesel pickups, vans, straight trucks, semi trucks, off road equipment, millitary equipment that have multiple batteries with no relay or isolator, and run for years with no issues?
My Longhorn for example, I grabbed 2 used batteries back in 05 off a P/30 (bread truck) at work that we were rebuilding. The 2 batteries were actually put into service in 2002 in that truck. GM chassis, 6.5 non turbo truck, wired up as intended by GM and Grummin. When I tossed them in my Longhorn, I run there in a parallel circuit. Positive to positive, neg to neg. I ended up with 12 volts, and twice the amps. Those two batteries are still in service in my truck today. Next year, they will have been in service for a decade.

As you said at the end of your responce, the reason GM wired them this way was so you could charge both batteries while running the engine, but the engine is only putting a load on the man vehicle battery for starting the engine. Upon shutting off the engine, the main battery is issolated... or turned off, so the (presumed) camper can run on the aux battery all night, or untill it goes dead. Turn the key on in the morning, you still have the vehicle battery charged and ready.

[rpmerf]

When I setup my dual battery setup, I plan to have one battery give the main feed to the starter and receive a charge from the voltage regulator through a diode so it will not be able to discharge into the other battery. The other battery will be for the rest of the truck..lights, ignition, radio. I will run a switch so I CAN use starter battery for running the rest of the car if I need to. A diode seems to make much more sense than a relay for this application. I would like to see the logic of this relay, do you have a part number?

EDIT: I think I get it now. It was confusing seeing that relay with 3 pos inputs, but I guess when the ignition is on, the wire that goes in the cab outputs voltage which connects the 2 batteries together. Total genius!

EDIT AGAIN: Just went out to the truck, the in cab wire always sees voltage so ??? maybe something is funky with my wiring?

[rpmerf]

looked at it again, for some reason my IGN UNFUSED line was actually connected to CIG. It doesn't even look like its made to hook into ign unfused?

This is a factory aux batt truck, but it has no aux tray, aux wiring, AND the wire that directly connects the drivers and pass side junction blocks. I think the past couple POs have messed this up a bit.

[MagmaJct]

I can't speak to those specific examples. I have experienced, however, where one battery had a high rate of internal (self) discharge and because the two were connected together the "better" battery voltage was "pulled down" by the one with a high internal discharge.

I would speculate that in the case of commercial vehicles they have a manages maintenance process that requires constant attention to the requirements of that vehicle, including the batteries.

I will acknowledge that my second point is correct to a greater extent. I just though about the second point while writing the first. :-)

--Mike

Originally Posted by Longhorn Man

How do you explain the millions and millions (probably billions and billions) of diesel pickups, vans, straight trucks, semi trucks, off road equipment, millitary equipment that have multiple batteries with no relay or isolator, and run for years with no issues?

[MagmaJct]

I don't recommend a diode because it drops 0.6 volts.

--Mike

[Fitz]

[QUOTE=Longhorn Man;4802071]How do you explain the millions and millions (probably billions and billions) of diesel pickups, vans, straight trucks, semi trucks, off road equipment, millitary equipment that have multiple batteries with no relay or isolator, and run for years with no issues?
QUOTE]

That's the difference between practical experience and reading a book....

[Dunenutt]

I just use these on all of my trucks...

http://www.ase-supply.com/Sure_Power..._p/sp-1302.htm

My main battery is just for starting. The auxillary runs all accessories, including the winch.

Sure power also makes the relays to do what you are asking about.