Passing on some info about second battery option.

bruceb58 · February 17, 2025

1 minute ago, Another JR said:

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.

I realized that as well when I read the other section of the Service manual that I posted. I use voltage sensitive relays in my boats that have a starter input that prevents the battery from connecting during cranking. Too bad this doesn't do the same.

bruceb58 · February 17, 2025

I am actually kinda curious what people use their second battery for. I chose not to order it since our use case is towing a trailer with a large lithium bank in it with the option to charge it from the truck through a DC to DC converter.

If I camped with the truck alone, I could see it's use but I would likely just mount a lithium battery where the second battery goes and charge it with a DC to DC converter.

I do see a use if I wanted to have a powered cooler in the bed and wanted it powered while the truck was not running.

Another JR · February 17, 2025

2 minutes ago, bruceb58 said:

I realized that as well when I read the other section of the Service manual that I posted. I use voltage sensitive relays in my boats that have a starter input that prevents the battery from connecting during cranking. Too bad this doesn't do the same.

You probably use a similar Blue Sea ACR like originally came in my truck camper before I replaced it with a Victron DC to DC charger to support LiFePO4 batteries. I like how the charger allows me to program in a delay time for start battery recovery before turning on camper battery charging from the truck.

Another JR · February 17, 2025

2 minutes ago, bruceb58 said:

I am actually kinda curious what people use their second battery for. I chose not to order it since our use case is towing a trailer with a large lithium bank in it with the option to charge it from the truck through a DC to DC converter.

If I camped with the truck alone, I could see it's use but I would likely just mount a lithium battery where the second battery goes and charge it with a DC to DC converter.

I do see a use if I wanted to have a powered cooler in the bed and wanted it powered while the truck was not running.

I’m thinking I will use it for a portable air compressor for reinflation after airing down tires, and maybe for a small compressor fridge in the back if I go back to truck topper camping at some point. The option cost was less than I could buy the battery for, so I figured I’d get it rather than later wishing I had ordered it.

Another JR · February 17, 2025

11 minutes ago, bruceb58 said:

I am actually kinda curious what people use their second battery for. I chose not to order it since our use case is towing a trailer with a large lithium bank in it with the option to charge it from the truck through a DC to DC converter.

If I camped with the truck alone, I could see it's use but I would likely just mount a lithium battery where the second battery goes and charge it with a DC to DC converter.

I do see a use if I wanted to have a powered cooler in the bed and wanted it powered while the truck was not running.

I looked at mounting a LiFePO battery there in the future but from what i have read the high temperatures that location can get to after you shut down a warmed up truck would significantly shorten the usable life of the battery.

bruceb58 · February 17, 2025

13 minutes ago, Another JR said:

You probably use a similar Blue Sea ACR like originally came in my truck camper before I replaced it with a Victron DC to DC charger to support LiFePO4 batteries. I like how the charger allows me to program in a delay time for start battery recovery before turning on camper battery charging from the truck.

Yep...BlueSea ACR and I use the Victron 50A Orion XS that they just came out with. I have a 120A Anderson connector at the rear of my truck with 2 AWG wire coming from the battery. The Orion is in the trailer for my 560AH lithium battery bank.

bruceb58 · February 17, 2025

1 minute ago, Another JR said:

I looked at mounting a LiFePO battery there in the future but from what i have read the high temperatures that location can get to after you shut down a warmed up truck would significantly shorten the usable life of the battery.

That's an excellent point. Probably not a good place for a lithium battery. LiFePO4 BMS turns off at 140°F

Edited February 17, 2025 by bruceb58

BCGM · February 17, 2025

16 hours ago, Another JR said:

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

My apoligies !

BCGM · February 17, 2025

Thank you fellas for the replies !

rjgvt · February 17, 2025

I hooked up a 12v socket to the aux battery so it's always on. I ran the wires through the firewall.

I have the 5th wheel/gooseneck option and has the 7 way trailer connection in the truck bed. The 12v is connected to the aux battery. I have a 7 way trailer connector that I use for the 2 LED rope lights for the ARE cap.

garyo · March 30, 2025

I'm looking at ordering a 2500 and looking at the dual alternator and aux battery options. My thoughts are being able to charge and run a DC/DC to charger camper batteries, from the aux battery and 2nd. alternator. Keeping it completely separate from the primary battery and truck electrical system. Will this work?

Thanks,

garyo

Another JR · March 31, 2025

The base alternator for the gas trucks is 170 amps, which likely has quite a bit of excess capacity over the truck loads after initial start recovery. The optional 220 amp alternator gives you 50 amps more in capacity. I would think if you are running a 30 or 50 amp dc to dc charger, or two 30 amp chargers, the single 220 amp is plenty. If you want significantly more than 50 amps for charging a big bank of batteries then the second alternator makes sense. I don’t think it makes sense to order the second alternator unless you really need it, but that’s just an opinion.

Edit of previous edit: Sounds like 64abwagon is aware of something I wasn’t, so I won’t attempt to answer your original question about about about “would this work.”

Edited March 31, 2025 by Another JR

64BAwagon · March 31, 2025

Dont quote me on the GM HD's but on other dual alternator trucks I have worked on the second alternator is "slaved" to the primary and controlled by that regulator. If youre intent is to isolate the second alternator just for the DC/DC setup you will have to do a dive into that arrangement.

bruceb58 · March 31, 2025

46 minutes ago, 64BAwagon said:

Dont quote me on the GM HD's but on other dual alternator trucks I have worked on the second alternator is "slaved" to the primary and controlled by that regulator. If youre intent is to isolate the second alternator just for the DC/DC setup you will have to do a dive into that arrangement.

Of course modern trucks and cars don't really have regulators in the alternators anymore. The ECM/BCM monitors the voltage and send a turn on signal that is a PWM signal that controls the output of the alternator.

The question remains...does the second alternator get the same PWM signal as the first one? Hard to believe it would since there are so many modes the BCM/ECM do regarding load shedding, start up...etc. I do know the second generator gets a separate field duty cycle signal.

Would be easy to verify by measuring the voltage output of the two alternators separately and using a scope to determine the PWM signal duty cycle, I would do it if I had a second alternator on my truck.

Edited March 31, 2025 by bruceb58

Markuscis · March 31, 2025

The diesel trucks have the two batteries connected at all times because of the high compression starting requirements.

The gasser 2nd battery is disconnected with a relay when it is turned off thus running things when the engine is off can't drain the primary battery.

I installed a DCtoDC Victron Orion 50amp charger same as someone else did above using the battery kit that can be ordered from GM. It contains the needed nuts, fuses, etc for auxiliary use cases. Worked out really well for me and looks factory.