2016 Sierra 3500 Main and Aux batteries -

rebrecs · November 27, 2019

Hi,

I need to understand how the main and aux batteries are related. There is only sparse information in the vehicle User Manual. I believe all of my questions would be answered if I could see a wiring diagram. I found a service manual for my make and model on Helm Inc.'s website. It costs $350.00

Before I spend the $350, I thought I would try this forum.

If there is anyone here who knows about the Aux Battery and is willing to chat with me about it - Your help will be greatly appreciated.

I want to know how the Aux Battery is situated in both the cranking circuit (if at all) and how the Aux Battery gets charged (charging circuit)

Additionally, a somewhat related question would be; What exactly is the Battery Gauge on the dash measuring? (main, aux, alternator, all 3, what?)

Thank you in advance for putting up with such a newbie question. Guessing about things makes for bad troubleshooting. Just want to understand what I'm dealing with before I go poking around.

Snowcamo · November 27, 2019

Im currently looking into this for my 1500. From what i understand the batteries are separated by this relay. Do you have a current sensor clamp on both neg battery cables? I am not for sure o. Which of the two the dash reads but I'd assume passenger side battery.

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HeySkippyDog · November 27, 2019

I too am interested, I'd love an auxiliary battery here in FL where the heat (not right now, but in summer) will smoke a battery.

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rebrecs · November 28, 2019

On 11/26/2019 at 9:06 PM, Snowcamo said:

Im currently looking into this for my 1500. From what i understand the batteries are separated by this relay. Do you have a current sensor clamp on both neg battery cables? I am not for sure o. Which of the two the dash reads but I'd assume passenger side battery.

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Thanks Snow, I'm sure I do. This is the first I have seen of the terminology "Aux Battery Relay." So far I have seen automatic charging relay (ACR) and Battery Isolator. Something new to investigate. Finding an explanation has turned in to actual work. I appreciate the info you provided. Its a good lead. (no pun)

rebrecs · November 29, 2019

On 11/27/2019 at 12:54 AM, HeySkippyDog said:

I too am interested, I'd love an auxiliary battery here in FL where the heat (not right now, but in summer) will smoke a battery.

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Here you go ! Everything you need to know.

https://www.youtube.com/watch?v=Z9c1hVXBggk

rebrecs · November 29, 2019

I have 3500 Diesel 6.6 Denali.

There is a battery charging management system built-in from GMC. Based on information in the Owners Manual under the "Battery Gauge" section, there is functionality sounding like what you would expect from a DC-DC Charger w/ Controller. However, the information only discusses the Main battery. There is no mention of how charging is handled for the aux battery anywhere in the Owners Documents. Including the Dura-Max Supplement. There is no charging controller listed on any parts breakdowns either. At this moment I am working from the assumption that either the logic is in the ECM, or the Aux Battery is a well guarded secret.

The cranking battery does have a large plastic box affixed to the top of the battery with a lot of high current wiring coming in and out of it. If there is a charging controller, that would be the place for it, and it would look about like that.

However, there is no mention of that box in any parts breakdowns either.

Do batteries come with big plastic boxes on top these days?

I was going to look up the AC-Delco battery model #, but the mystery box obscures the battery entirely. I can tell that changing that battery would not be a fun thing to attempt.

So far I have not been able to get that box open. I need a better flashlight.

davester · November 29, 2019

29 minutes ago, rebrecs said:

I have 3500 Diesel 6.6 Denali.

There is a battery charging management system built-in from GMC. Based on information in the Owners Manual under the "Battery Gauge" section, there is functionality sounding like what you would expect from a DC-DC Charger w/ Controller. However, the information only discusses the Main battery. There is no mention of how charging is handled for the aux battery anywhere in the Owners Documents. Including the Dura-Max Supplement. There is no charging controller listed on any parts breakdowns either. At this moment I am working from the assumption that either the logic is in the ECM, or the Aux Battery is a well guarded secret.

The cranking battery does have a large plastic box affixed to the top of the battery with a lot of high current wiring coming in and out of it. If there is a charging controller, that would be the place for it, and it would look about like that.

However, there is no mention of that box in any parts breakdowns either.

Do batteries come with big plastic boxes on top these days?

I was going to look up the AC-Delco battery model #, but the mystery box obscures the battery entirely. I can tell that changing that battery would not be a fun thing to attempt.

So far I have not been able to get that box open. I need a better flashlight.

From posts on another forum I frequent, the Duramax's have dual batteries hooked in parallel, so they charge/discharge at the same time. It's not a "main battery/auxiliary battery" situation. They both are the "main battery".

rebrecs · November 29, 2019

4 hours ago, davester said:

From posts on another forum I frequent, the Duramax's have dual batteries hooked in parallel, so they charge/discharge at the same time. It's not a "main battery/auxiliary battery" situation. They both are the "main battery".

I was beginning to suspect that. Now, things are beginning to make sense. Thank you for sending this along.

That sort of puts a new twist on "disconnecting the battery." I'll have think about that.

Snowcamo · November 29, 2019

From posts on another forum I frequent, the Duramax's have dual batteries hooked in parallel, so they charge/discharge at the same time. It's not a "main battery/auxiliary battery" situation. They both are the "main battery".

Anything on the 6.0 gas engine setup? I wonder if they are the same. Its really difficult to find any info on second battery/ alternator setups. I have been wondering if any reprogramming would be needed to to bcm.

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davester · November 29, 2019

No idea on the gas. If they are hooked up in parallel, then there's no need to do anything special for the alternator, as it charges them both at the same time.

Gmman · November 30, 2019

I have put the AUX battery in my gas truck. Really simple. I use it for my camper. I used all factory stuff. I didn't get all the parts from the dealer but they are all GM parts. On the 2500 its plug and play. I don't know about the 1500.

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Snowcamo · November 30, 2019

I have put the AUX battery in my gas truck. Really simple. I use it for my camper. I used all factory stuff. I didn't get all the parts from the dealer but they are all GM parts. On the 2500 its plug and play. I don't know about the 1500.

What's interesting is my 1500 came with a second battery tray from the factory. Do you have the relay in line with the batteries?

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Gmman · November 30, 2019

Yes. I have the one pictured above and the battery harness has a small square one attached to it as well. Mine had what looked like a battery tray as well, but it really isn't. These are the new battery tray parts.

Snowcamo · November 30, 2019

Nice. How did you find the part numbers for the parts?

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Gmman · November 30, 2019

I went to dealer lots and looked at trucks that had snow plow packages and told them I was working on a project and could I look under the hood and got some # that way and the it all started to fall into place.