Passing on some info about second battery option.

3500L8T · February 15, 2025

Thank you, I have been looking for this answer for years. Very helpful

carkhz316 · February 16, 2025

On 2/13/2025 at 5:14 PM, Another JR said:

See my September 20 post up above in this string. Yes there is an isolator. I have a 2021 3500 gas truck with dual batteries and 220 amp alternator.

Can you show where? I never see the relays anymore like they used to have them on the firewall. According to service info, the aux battery relay is on the left battery. I believe this configuration is only if you have the dual battery ordered from factory on the gasser. If upfitting, it doesn't look like it includes a relay.

Edit: it looks like it is indeed only on the factory equipped second battery gas trucks.

Edited February 16, 2025 by carkhz316

Another JR · February 16, 2025

3 hours ago, carkhz316 said:

Can you show where? I never see the relays anymore like they used to have them on the firewall. According to service info, the aux battery relay is on the left battery. I believe this configuration is only if you have the dual battery ordered from factory on the gasser. If upfitting, it doesn't look like it includes a relay.

Edit: it looks like it is indeed only on the factory equipped second battery gas trucks.

My configuration was ordered and received from the factory. On the factory configuration the isolator relay is part of the fuse block assembly mounted directly on top of the aux battery. See the picture I included - the isolator relay is in the center of the picture. Now that I look at it again it appears to be a simple relay controlled by the truck - maybe a simple ignition on command or maybe a command with a short delay to assist start battery recovery. The relay itself is not a voltage sensing isolator. (Note that my fuse block has the fuse kit for the upfitter switch option installed, so it has some additional parts on it.)

I don’t know about the retrofit kits. However, for most people that don’t need dual starting batteries as on a diesel, the whole reason to have a second battery is to have an isolated second battery you can use heavily for accessories without depleted your starting battery. I would have thought a GM retrofit kit for a gas truck would have the same isolator and fuse block assembly.

Edited February 16, 2025 by Another JR
Typo

BCGM · February 16, 2025

On 9/20/2023 at 7:29 PM, Another JR said:

Part of the assembly that comes on top of the second battery in a gas truck is a battery isolator. Loads you connect to the aux battery are isolated from the starting battery when the truck isn’t running so they won’t deplete your starting battery. This is different from the diesel, which has the batteries directly connected at all times because they both are needed for starting.

JR . So you would hook any additional accessories to the aux battery ?

If you were running accessories off the main battery and there was not enough left to start the truck would it switch over and start from the aux .

So if your truck isn't running accessories still run from the starting battery . I'm a little confused . Not much of a mechanic !

Or understand the electrical system that well

Thank you !

carkhz316 · February 16, 2025

5 hours ago, BCGM said:

JR . So you would hook any additional accessories to the aux battery ?

If you were running accessories off the main battery and there was not enough left to start the truck would it switch over and start from the aux .

So if your truck isn't running accessories still run from the starting battery . I'm a little confused . Not much of a mechanic !

Or understand the electrical system that well

Thank you !

I know you didn't ask me, but I do know the answer. If you have the OEM factory equipped aux battery configuration that has the relay like pictured in the above picture then what happens is that the auxiliary battery is during starting and running, but is disconnected when the truck is off. The truck then relies on Key-OFF; Engine OFF power from the original battery. The diesels and upfitter kits DO NOT have the relay and instead have the batteries wired in parallel at all times.

Edited February 17, 2025 by carkhz316

Another JR · February 17, 2025

3 hours ago, BCGM said:

JR . So you would hook any additional accessories to the aux battery ?

If you were running accessories off the main battery and there was not enough left to start the truck would it switch over and start from the aux .

So if your truck isn't running accessories still run from the starting battery . I'm a little confused . Not much of a mechanic !

Or understand the electrical system that well

Thank you !

To answer your question for yourself, you need to have a good understanding of the wiring and the way the isolator works. So here’s some additional clarification.

I created confusion by using the word “accessories” to mean user-added equipment like a winch, compressor, fridge, ham radio, etc. I didn’t mean accessories as used in the basic truck configuration. Sorry for the poor word selection.

I just did some measurements with my voltmeter and verified that the isolator relay is switched just as carkhz316 describes. The relay closes and connects the batteries when the ignition is turned on. The connection is via a 100 amp fuse and a wire that is smaller than a typical start battery cable.

Because it is contributing less than 100 amps due the length and gage of the inteconnect cable, I would not have described the aux battery as “used for starting.” Instead it is simply connected during starting because of the “non-smart” way they chose to control the relay. The ideal way to set up an aux battery isolator is to not connect the aux battery to the start battery and alternator until after cranking and after an initial two or three minute recovery period for the start battery. That would require more complexity and cost, though.

That less-than-100 amp current isn’t needed for starting (as evidenced by all the gas trucks with a single battery), but does help. You would not be able to start the engine via that wiring if the start battery was dead, though. If you want to have the ability to attempt a start from the aux battery, you would need a jumper cable arrangement or to swap the batteries.

You can connect your own user-added accessories to 1) the start battery (hot all the time), 2) to the ignition on bus via the fuse box, or 3) to the accessory bus via the fuse box.

With an aux battery, you have the fourth option of connecting your added accessory to the aux battery fuse block (hot all the time).

If you have the upfitter switches option, you could connect to those switches, which in turn are powered from whichever battery you choose, but the kit is set up for powering them from the start battery.

The advantage of powering your added accessory from the aux battery is that you can use it when the truck is not running, and even use it to the point where you significantly deplete the aux battery if you need to. The start battery will not power the added accessory when the truck is not running, preserving the charge of the start battery for starting. When the truck is running, you can continue to use the accessory and the alternator will recharge the aux battery.

Hopefully your options and the reasons to select a particular option are more clear now. Ask more questions if it’s still not clear.

BCGM · February 17, 2025

22 minutes ago, Another JR said:

To answer your question for yourself, you need to have a good understanding of the wiring and the way the isolator works. So here’s some additional clarification.

I created confusion by using the word “accessories” to mean user-added equipment like a winch, compressor, fridge, ham radio, etc. I didn’t mean accessories as used in the basic truck configuration. Sorry for the poor word selection.

I just did some measurements with my voltmeter and verified that the isolator relay is switched just as carkhz316 describes. The relay closes and connects the batteries when the ignition is turned on. The connection is via a 100 amp fuse and a wire that is smaller than a typical start battery cable.

Because it is contributing less than 100 amps due the length and gage of the inteconnect cable, I would not have described the aux battery as “used for starting.” Instead it is simply connected during starting because of the “non-smart” way they chose to control the relay. The ideal way to set up an aux battery isolator is to not connect the aux battery to the start battery and alternator until after cranking and after an initial two or three minute recovery period for the start battery. That would require more complexity and cost, though.

That less-than-100 amp current isn’t needed for starting (as evidenced by all the gas trucks with a single battery), but does help. You would not be able to start the engine via that wiring if the start battery was dead, though. If you want to have the ability to attempt a start from the aux battery, you would need a jumper cable arrangement or to swap the batteries.

You can connect your own user-added accessories to 1) the start battery (hot all the time), 2) to the ignition on bus via the fuse box, or 3) to the accessory bus via the fuse box.

With an aux battery, you have the fourth option of connecting your added accessory to the aux battery fuse block (hot all the time).

If you have the upfitter switches option, you could connect to those switches, which in turn are powered from whichever battery you choose, but the kit is set up for powering them from the start battery.

The advantage of powering your added accessory from the aux battery is that you can use it when the truck is not running, and even use it to the point where you significantly deplete the aux battery if you need to. The start battery will not power the added accessory when the truck is not running, preserving the charge of the start battery for starting. When the truck is running, you can continue to use the accessory and the alternator will recharge the aux battery.

Hopefully your options and the reasons to select a particular option are more clear now. Ask more questions if it’s still not clear.

Thank you Bruce for taking the time to explain !! I will digest and get back to you

BCGM · February 17, 2025

3 hours ago, carkhz316 said:

I know you didn't ask me, but I do know the answer. If you have the OEM factory equipped aux battery configuration that has the relay like pictured in the above picture then what happens is that the auxiliary battery is used for starting and running, but is disconnected when the truck is off. The truck then relies on Key-OFF; Engine OFF power from the original battery. The diesels and upfitter kits DO NOT have the relay and are instead have the batteries wired in parallel at all times).

Thanks for the reply Carkhz ! Forgive my ignorance I'm having trouble with this . So say I'm running items like the radio maybe charging a device or have a small cooler plugged into the 120 outlet in the dash or truck box . Is that drawing from the original battery and not the aux battery if I have one . So that the original battery wont get drained and will have enough juice to start the truck ?

bruceb58 · February 17, 2025

deleted....duplicate post

Edited February 17, 2025 by bruceb58

bruceb58 · February 17, 2025

1 hour ago, BCGM said:

Thanks for the reply Carkhz ! Forgive my ignorance I'm having trouble with this . So say I'm running items like the radio maybe charging a device or have a small cooler plugged into the 120 outlet in the dash or truck box . Is that drawing from the original battery and not the aux battery if I have one . So that the original battery wont get drained and will have enough juice to start the truck ?

The auxiliary battery on gas trucks(not sure how diesel trucks are) is only used by a trailer or something you specifically hook to it.

It gets charged by the BCM turning on one relay which in turn turns on another relay that allows the 2nd battery to be charged through a 100A fuse from the main battery.

I am thinking this is what people are calling an isolator. It's somewhat smart isolation since the BCM commands the relays to close.

If I was going to use an powered cooler in the bed of the truck, I would use a deep cycle battery as the auxiliary battery and run the cooler off of it. That would include using a stand alone inverter if the thing runs off 120VAC. The truck's inverter runs off the main battery.

From the schematic, you can see the two charging relays as well as the 100A relay which handles the charge from the main battery. In addition, you can see the 30A relay that goes to the trailer and a 175A fuse that isn't used to allow you to hook up to other things you may want.

Edited February 17, 2025 by bruceb58

Another JR · February 17, 2025

39 minutes ago, BCGM said:

Thanks for the reply Carkhz ! Forgive my ignorance I'm having trouble with this . So say I'm running items like the radio maybe charging a device or have a small cooler plugged into the 120 outlet in the dash or truck box . Is that drawing from the original battery and not the aux battery if I have one . So that the original battery wont get drained and will have enough juice to start the truck ?

The radio, the inverter outlets in the dash and the truck bed, and every load in the truck from the factory all draw from the start battery. As said above, only the trailer connector hot 12 volt wire is connected to the aux battery from the factory.

carkhz316 · February 17, 2025

6 minutes ago, bruceb58 said:

The auxiliary battery on gas trucks(not sure how diesel trucks are) is only used by a trailer or something you specifically hook to it.

It gets charged by the BCM turning on one relay which in turn turns on another relay that allows the 2nd battery to be charged through a 100A fuse from the main battery.

I am thinking this is what people are calling an isolator. It's somewhat smart isolation since the BCM commands the relays to close.

If I was going to use an icebox in the bed of the truck, I would use a deep cycle battery as the auxiliary battery and run the cooler off of it. That would include using a stand alone inverter if the thing runs off 120VAC. The truck's inverter runs off the main battery.

From the schematic, you can see the two charging relays as well as the 100A relay which handles the charge from the main battery. In addition, you can see the 30A relay that goes to the trailer and a 175A fuse that isn't used to allow you to hook up to other things you may want.

I should have clarified, but edited my original post. You are absolutely correct that the primary purpose of the auxiliary battery is for towed accessories and the like. When the truck is turned on, it technically can sink power to "help" start the truck, though obviously not much through a 100 amp fuse.

bruceb58 · February 17, 2025

4 minutes ago, carkhz316 said:

I should have clarified, but edited my original post. You are absolutely correct that the primary purpose of the auxiliary battery is for towed accessories and the like. When the truck is turned on, it technically can sink power to "help" start the truck, though obviously not much through a 100 amp fuse.

That shouldn't be able to happen. My understanding is that the BCM waits until the truck starts before it enables the relays for the auxiliary battery. I can't verify that since I don't have a second battery.

EDIT: This is what the Service Manual says:

Edited February 17, 2025 by bruceb58

Another JR · February 17, 2025

1 hour ago, BCGM said:

Thank you Bruce for taking the time to explain !! I will digest and get back to you

I’m Jon. Bruce is bruceb58. Happy to try to help, as are the others I’m sure.

Another JR · February 17, 2025

33 minutes ago, bruceb58 said:

That shouldn't be able to happen. My understanding is that the BCM waits until the truck starts before it enables the relays for the auxiliary battery. I can't verify that since I don't have a second battery.

EDIT: This is what the Service Manual says:

That’s what I would have hoped, because if you have killed the aux battery you don’t want it stealing current from the start battery during cranking. No such luck, though. The service manual just says connected when ignition is on, which really simply means when the truck is turned on.

More importantly, I put a voltmeter on the aux battery and measured what it does during a start. The voltage was at 13.1 before initiating a start. It dropped as soon as I pushed the start button, and dropped more when the engine began to crank. The aux battery was definitely delivering cranking current, dropping the voltage to 12 or so. It was changing fast and my digital meter has some delay/damping, so I couldn’t really tell how low it dipped during cranking. I have no loads wired to the aux battery, so the behavior was not due to other loads.

It’s good to be aware that it works this way, so if I ever really do kill the aux battery I know to disconnect its negative connection before attempting a critical start in a remote location.

Edited February 17, 2025 by Another JR