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04 2500HD, 8.1/Alli with dual batteries.


I am a little confused about how the dual batteries must be wired. I ASSUMED that if you always had at least one of the batteries connected that all of the circuits would remain powered. I guess that is not true.


I disconnected the main (front) battery to install a jumper cable harness. I left the second battery (side) connected. No problem installing the jumper cable harness.


When I started the truck I noticed the clock had the wrong time displayed and needed to be reset. It apparently lost power while I had the main battery disconnected. All settings in the DIC and all information - odometer, MPG, HRs, trip info, radio pre sets, etc appears to be correct.


Was I wrong to ASSUME that EVERYTHING will remain powered if only one battery is connected and the other one is disconnected??


Arn't both batteries connected to a common harness? They sure appear to be. Anyone else experienced this???
 

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I’m not positive with your 2004, but I found with the 2002 with the gas motor, that both the Battery-Left and the Battery-Right are NOT always connected together.
The Battery-Left (driver side) positive terminal has two wires, a 2 gauge wire going to the Junction Block-Battery Cable, and then to the Starter Solenoid, and a 4 gauge wire going to the Fuse Block-Underhood.
The Battery-Right (passenger side) has a single 2 gauge wire to the Battery Fuse 125A, then through the Auxiliary Battery Relay contact, then to a Fusible Link, and then to the Starter Solenoid. The Auxiliary Battery Relay is ONLY energized when the ignition is in the Run position. This means the Battery-Right is never used when starting your truck with the gas motor.
With the ignition off, the batteries are isolated from each other. ONLY the Battery-Left is supplying 12V to the Fuse Block-Underhood, when the ignition is NOT in Run. Whenever the ignition is in the Run position, both batteries are supplying power to the vehicle and being charged by the Generator.
The diesels are different. The diesels do not use the Auxiliary Battery Relay. A 2 gauge wire runs directly from the Battery-Right to the Starter Solenoid, which means both batteries are always wired in parallel, and both batteries are ALWAYS used to power the entire vehicle, and be charged by the Generator.

Reference Helms 2002, Auxiliary Battery Schematics page 6-1598.Edited by: modified
 

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Modified - Thanks for the detailed response.


Now that I know BOTH batteries are not supossed to be connected all of the time on the gassers I will see if what you describe for the 02 models is the same as my 04. Don't see why they would be any different.


It makes no sense to me NOT to have them connected all of the time and especially to not use the second battery for starting like the DMax. Cranking an 8.1 at -30F can be a b**ch if you are not using synthetic.


Will be digesting your description this weekend to see if it is feasible to by pass the Auxiliary Battery Relay. Any ideas of do you see any drawbacks???
 

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The Auxiliary Battery most only be intended for additional loads, but looking at the wire size, it’s obvious it was never intended for starting. The factory wiring for your Auxiliary Battery has this 13mm2 (2 gauge) wiring up to the Fusible Link (I don’t know the Amp rating). The outlet wire of the Fusible Link to the Starter Solenoid is only 5 mm2 (10 gauge). This 10 gauge wire is obviously NOT large enough to handle starting current to your Starter. This connection at the Starter is probably only used as a convenient place to connect the Aux Battery to the electrical system.
If you want to permanently wire both batteries to your electrical system, and use both for starting, here’s a possible way to do it. Wire it like the Duramax. Presently, I can’t think of any drawbacks to doing this, but maybe others have ideas. Please verify this idea with others.
Verify if both your Batteries are identical model numbers. Typically you want all batteries in an electrical system to have the same ratings, and age. Also, verify if the 8.1L Starter Solenoid is in a similar location as the Duramax Starter Solenoid. If batteries and starter locations are similar, go to a Dealer and buy the Duramax Battery-Right, Positive Battery Cable (32mm2, same as 2 gauge) . This should be long enough to reach your 8.1L Starter.
Disconnect existing wires, by removing and insulate with electrical tape, the positive wire from your Auxiliary Battery. Now go to your Starter Solenoid, and disconnect the 5 mm2 (10 gauge) wire, which was coming from the fusible link, and insulate it with tape. If your truck has option 5X7, (Emergency Vehicle Roof Lamp), this 5mm2 (10 gauge) wire will be connected at the Starter, and must remain connected. The factory Aux Battery wiring should be totally removed from the electrical system.
Now connect this new Duramax Battery-Right, Positive Battery 32mm2 Cable from the Aux Battery Positive Terminal to your Starter Solenoid. Both batteries are permanently wired to your electrical system, and are large enough to handle the current loads.

Reference Helms 2002, pages 6-1598, 8-312.Edited by: modified
 
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