New member....dual battery question!

[Matt Buffington]

Hi Everyone!

I have a 1990 Suburban I'm building as an overland/field vehicle. I am going to have a number of lights, and to power them with the car off, I'm installing a second batter under the hood. I have a solenoid isolator splitting the two batteries, and inline fuses between both batteries and the solenoid. With it all wired in, everything seemed fine. When I went to start the truck, the fuse between the solenoid and secondary battery blew! Further, while the solenoid line to the starter battery was still connected, the truck appeared to have no electricity; once the solenoid was disconnected from the starter battery, and factory circuit restored, everything was fine again. I have checked the grounding continuity at the second battery, and all that appears to be very good. All wires are 4 AWG, and I checked that the solenoid appears to be working in acc-on position. What should I try next? Any ideas of what I have done wrong here? Thanks so much for any help you can provide!

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[broketiredugly]

a starter will pull +- 300amps. you have to run primary to starter, and a second leg from primary to secondayr via constant duty solenoid. the idea to isolate is to keep secondary on its own circuit, and when solenoid engaged, then secondary helps primary carry load and receives charge from alternator. no fuse between primary and starter.

 

[Matt Buffington]

A quick update! I just tested the solenoid key on, key off, and I had 12.5 volts through the unit....so it does not appear to be isolating the batteries....including at startup. This may be the problem, since at startup, both batteries are engaged. By the way, secondary battery is a group 24 deep cycle marine battery.

 

[Matt Buffington]

Hi Dennis: I do have a direct 2 gauge cable running from starting battery to starter; line to starting battery cable to solenoid is a separate connection to starting battery. thanks!

 

[animal]

 

[WFO]

Welcome from the Texas Panhandle.

 

[Shorty81]

Welcome from the Buckeye State

 

[1973c10]

Welcome from Oregon

 

[75gmck25]

I have the factory camper dual battery setup in my truck and it is much simpler than the newer solenoid designs.
- 2nd battery is installed, and negative terminal grounded to the truck body and Engine.
- Positive wire (with fusible link) from 2nd battery runs over to a junction point on the fender well (similar to the junction near the brake booster). This junction is used to power any new accessories.
- Between the battery junction and the firewall junction there is a 10 gauge wire with a relay in the middle.
- When the ignition is on, the relay is activated, and the battery is charged using that 10 gauge wire. If ignition is off, relay is deactivated and 2nd battery is isolated.

This scheme isolates the 2nd battery when the truck is off, and charges it when the truck is running, but does not require any large gauge wiring between the two batteries.

Bruce

 

[Matt Buffington]

Hi Bruce,

Thanks for the reply! A couple questions:

1) what line are you using to activate the solenoid?
2) does the solenoid separate the batteries during cranking?
3) is the acc-on circuit ‘off’ during cranking?

For number two, it seems like that’s what happened to me; the batteries were not separated at cranking, ergo both were in the circuit at cranking, and of course the fuse blew between the second battery and the solenoid; starter battery is connected directly to starter, so no fuse blew in that link.

Thanks, Matt

 

[broketiredugly]

Hi Bruce,

Thanks for the reply! A couple questions:

1) what line are you using to activate the solenoid?
2) does the solenoid separate the batteries during cranking?
3) is the acc-on circuit ‘off’ during cranking?

For number two, it seems like that’s what happened to me; the batteries were not separated at cranking, ergo both were in the circuit at cranking, and of course the fuse blew between the second battery and the solenoid; starter battery is connected directly to starter, so no fuse blew in that link.

Thanks, Matt

on his system, the batteries are always seperated. the relay only connects alternator to second battery when engine is running. 2 batteries, 2 systems. primary battery runs truck. secondary runs accessory loads. relay allows recharge via alternator.

 

[Snoots]

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[Matt Buffington]

Okay, looks like the activation comes from a fuse jump, hot on acc or run (comic book window on schematic). I have mine on hot acc running from the ECM adjacent to the TBI (probed this to be hot-acc-key on using my multimeter).

One thing I'll check with the new solenoid is whether it cuts off while cranking; seems it should to avoid both batteries in the same circuit while cranking...

Now I just wait...new solenoid is on its way, as well as new fuses. The time spent waiting has given me the time to think this over before throwing parts at the problem!

 

[Craig 85]

Here's the relay the factory used and a thread from our board.

https://www.gmsquarebody.com/threads/factory-dual-battery-wiring.15855/

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[75gmck25]

The wiring diagram is slightly different for my '75 dual battery option, but functionally the same as the one in Post #12.

I have a brown wire connected to a terminal on fuse block that is shown as "IGN.ACC", so I assume it is active when the ignition is on or it is switched to accessory. The brown wire goes through its own grommet in the firewall and connects to the aux battery solenoid under the hood.

Bruce