‘97 C2500 Dual Battery Set Up

Seymorey · Sep 10, 2024

Hey guys,

My ‘97 C2500 came from the factory with a dual battery set up to power accessories. The previous owner bought new batteries, started experiencing electrical issues right afterwards and proceeded to delete the secondary battery in a futile attempt to stop the issues he was experiencing from a bad-from-the-factory battery cell.

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This is what I’m working with. I’m an electrical idiot but would like to make this truck a hauler again. I believe most of it is still left, except for the battery cable. The trailer power and electronic break wires are still there and, I believe, intact and run to the rear of the truck still.

Anyone with more of an electrical aptitude care to shed some light on what it is I am working with here? I can’t find any other pictures of this particular set up so I have no idea where I’m supposed to start here.

RichLo · Sep 10, 2024

What are you hauling and what are you trying to accomplish with two batteries?

If you want camper power without effecting the starting battery then you will need an isolator to allow it to discharge on its own and charge when the engine is running.

Otherwise if you just want another battery in parallel, why? I have two batteries and two alternators in the plow truck in my avatar but that is because it has an extremely high electrical demand at low RPM in the coldest weather of the year.

Dual batteries in parallel sounds great but often leads to premature battery failure if they are just connected together. You still need to isolate the two when the engine isnt running otherwise if one is 12.6v static and one is 12.55v static, the 12.55v will pull from the 12.6 until it is discharged to 12.55, but by then that one will be so dead that it will drop even more and start pulling from the 12.55 untli they are both gone.

Seymorey · Sep 10, 2024

RichLo said:
What are you hauling and what are you trying to accomplish with two batteries?

If you want camper power without effecting the starting battery then you will need an isolator to allow it to discharge on its own and charge when the engine is running.

Otherwise if you just want another battery in parallel, why? I have two batteries and two alternators in the plow truck in my avatar but that is because it has an extremely high electrical demand at low RPM in the coldest weather of the year.

Dual batteries in parallel sounds great but often leads to premature battery failure if they are just connected together. You still need to isolate the two when the engine isnt running otherwise if one is 12.6v static and one is 12.55v static, the 12.55v will pull from the 12.6 until it is discharged to 12.55, but by then that one will be so dead that it will drop even more and start pulling from the 12.55 untli they are both gone.

I believe the truck has the original solenoid to accomplish this. If you look at the picture I’ve posted, right above the brake fluid canister is a thingamabob mounted to the firewall. I’ve been told that is responsible for isolating the batteries and controlling what one is used for starting and what one is used for accessories.

Cadillacmak · Sep 10, 2024

88-98 Service Manuals

Some heroes don't wear capes. Thanks everyone.

www.gmt400.com

Post 92. Download the combined volume 1&2 and go to page 3831 in the PDF.

Seymorey · Wednesday at 12:14 PM

Cadillacmak said:
88-98 Service Manuals

Some heroes don't wear capes. Thanks everyone.

www.gmt400.com

Post 92. Download the combined volume 1&2 and go to page 3831 in the PDF.

Thanks. That’s close, but not exactly what my truck has. I don’t see the thingamabob in the diagram, nor where to attach what wire to what thingamabob appendage.

I cannot find an exact match for the auxiliary battery setup in my truck anywhere. A picture of an intact identical setup would be more valuable as I cannot read electrical diagrams without a visual reference.

Cadillacmak · Wednesday at 12:20 PM

Seymorey said:
Thanks. That’s close, but not exactly what my truck has. I don’t see the thingamabob in the diagram, nor where to attach what wire to what thingamabob appendage.

I cannot find an exact match for the auxiliary battery setup in my truck anywhere. A picture of an intact identical setup would be more valuable as I cannot read electrical diagrams without a visual reference.

I will dig up some more info for you, I am an electrician so that diagram make perfect sense to me, let me see if I can make it easier without having to learning a new language.

RichLo · Wednesday at 12:28 PM

So you are trying to wire it as an auxiliary battery for camper power or similar?

Cadillacmak · Wednesday at 1:03 PM

Seymorey said:
Thanks. That’s close, but not exactly what my truck has. I don’t see the thingamabob in the diagram, nor where to attach what wire to what thingamabob appendage.

I cannot find an exact match for the auxiliary battery setup in my truck anywhere. A picture of an intact identical setup would be more valuable as I cannot read electrical diagrams without a visual reference.

How about this? See attached. #2 and #3 are interchangeable as it is just a switch and does not care. Most people call that relay a solenoid. You are missing the wire that goes to #1, so I am hoping it just sitting there somewhere and the PO didn't cut it out, but if they did just take it to any key on hot, it will not take much at all for power.

Seymorey · Wednesday at 1:31 PM

Cadillacmak said:
How about this? See attached. #2 and #3 are interchangeable as it is just a switch and does not care. Most people call that relay a solenoid. You are missing the wire that goes to #1, so I am hoping it just sitting there somewhere and the PO didn't cut it out, but if they did just take it to any key on hot, it will not take much at all for power.

Okay, thank you. You didn’t have to do that and that was a huge help.

There are a couple more wires floating around:

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My understanding is that the orange wire provides power to the trailer wiring harness as the wire goes all the way to the back of the truck’s current 4 pin trailer connector, along with a blue wire not visible in the photos. The blue one is supposed to pass through the fire wall to a trailer brake controller, if I’m not mistaken.

There are also 2 studs for the remote battery, A and B:

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I am not sure what the diagram’s “HOT IN RUN” is referring to, either.

Cadillacmak · Wednesday at 2:00 PM

Seymorey said:
Okay, thank you. You didn’t have to do that and that was a huge help.

There are a couple more wires floating around:

My understanding is that the orange wire provides power to the trailer wiring harness as the wire goes all the way to the back of the truck’s current 4 pin trailer connector, along with a blue wire not visible in the photos. The blue one is supposed to pass through the fire wall to a trailer brake controller, if I’m not mistaken.

There are also 2 studs for the remote battery, A and B:

I am not sure what the diagram’s “HOT IN RUN” is referring to, either.

Aux B stud is trailer wiring from factory, Aux A stud is for what ever you want. Its not clear on the Aux Batt post so I am assuming it goes on the normal batt post on the side. The Hot run is just a brown wire coming off fuse #24 inside fuse box via the ignition. When ignition is on its hot. Hopefully you can find that brown wire under the hood and just plug back in.

‘97 C2500 Dual Battery Set Up

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88-98 Service Manuals

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