Jump to content
  • Sign up for FREE! Become a GM-Trucks.com Member Today!

    In 20 seconds you can become part of the worlds largest and oldest community discussing General Motors, Chevrolet and GMC branded pickups, crossovers, and SUVs. From buying research to owner support, join 1.5 MILLION GM Truck Enthusiasts every month who use GM-Trucks.com as a daily part of their ownership experience. 

Install a 2nd battery

Recommended Posts

No worries @wxman. Pics are super.  You must be very flexible to get in there.  

your second pic is excellent- I will explore that connection.  It is totally not where I would have checked.  

thank you !!!!



Link to comment
Share on other sites

Working on the “then” right now.  Not looking encouraging on my part.  Just got the loom peeled back.  Was hoping to see a couple (3) wires terminated right there in a small coil.  It is a tough spot.  Will post a couple pics after. 

the white arrow in the 30 amp feed from your Aux battery to the trailer plug. 

if you do not have the Aux Battery the feed comes from the engine compartment fuse block and is “on” all the time.  You could kill your start battery if you leave your trailer plugged in and it has some power consumer on board. 

Link to comment
Share on other sites

Here is the mess - no connection or even the wires according to the drawings.  There is some confusion as the power from the under hood fuse panel (X50A) is from fuse F79UA and the wire is red/white but one drawing has it as red/blue - argh!   Going to tape it back up and will   Find another power source for the isolation relay. 


the next pic is what I have set up with the power as per the other drawings from the upfitters and the parts for the truck:





I still have to hook up the 30 amp fuse but would like to have the battery charging properly first. 

  • Like 1
Link to comment
Share on other sites

So after wasting a lot of time looking for a wire needle in a haystack: 


I took the X50A fuse block all apart (this is the one under the hood).  F79 for the Auxiliary Battery relay is 10 Amp and is connected to nothing.  No wire comes from that terminal out of the fuse block.  There is no pin for the the fuse to connect to and the fuse is just in there holding a spot.  Same with fuse F80.  
I am no longer looking to hook up the aux battery isolation relay control to anything “original” as the truck does not have the wires necessary.   

I am simply going to run the control relay from an “ignition on” 12 volt power source and the aux battery isolation relay from the auxiliary battery itself.  

So for anyone looking to add the Auxiliary Battery in a 6.6 gasser the factory wiring will NOT allow you to connect it up “factory”.  

I think when I am done my install will appear pretty close to a factory install but there will be a few differences with the control wires.  

I checked my neighbours diesel and it has the 2 batteries and just a simple two fuse block on the second battery.  

Link to comment
Share on other sites

Thanks Brent for all your work on this. I spoke with my dealer about installing the second battery and they were ultimately stumped on how to do it since there is no kit to convert it. I thought it would be nice for off grid camping to have a battery on the vehicle and a be able to charge it while driving between fishing or hunting spots. I ultimately just bought a parallel battery for my camper and will use my suitcase generator.

Link to comment
Share on other sites

@Rozz522: for all intentions the second Battery is doable if you can make a few changes to the original trucks charging system:


1.  If for some reason (like mine) you have a 250 amp alternator  fuse connection at the main battery fuse block you can upgrade a 170 amp alternator to the 220 amp.  I did this - it was not a big deal. 

2. You need a second battery that matches the original battery to keep charging reasonably equal 


3. On the Auxiliary Battery you need an isolation relay that cuts out the Aux Batt during starting.  

4.  You cut the 30 amp connection from the original under hood fuse block and rout that to the Aux Battery through a new 30 Amp fuse.  

5.  The auxiliary battery positive connects from the isolation relay, through a 100 amp fuse to the  under hood fuse block where the 175 amp feed comes from the alternator 


I Installed the factory fuse block (X50E) on the auxiliary battery as it looks good and came with the isolation relay, the 100 and 30 amp fuse.

I will post my control connections once I have it installed.  Far better to have it done from the factory (as was my original order they messed up ) - for a new truck (Canadian dollars) it would be 160 (batt) plus 175 (alt) for a total $335. With some salvaged parts and new Alt, Batt and relay box I am at about $550 and I have a spare 170 amp alternator.  


  • Thanks 1
Link to comment
Share on other sites

@BrentH I may decide to make the conversion down the road as my truck already has the 250A alternator since I have the snow plow prep package. It’s more of a nice to have at this point and if I find myself boondocking a lot then I can add it later. 

Link to comment
Share on other sites

For anyone wanting to install the second Battery and has a 6.6 gas 2500HD with a main fuse block (the one on the main battery) that has a 250 amp fuse here is a wire diagram:



this is the connection to the control relay from the BCM (drivers side, left under the dash). Clip wire and solder in a splice that will control the relay (2 in the drawing). This violet/brown wire applied power after the truck is started.  It also runs a relay for the HVAC system.  


the spliced in red wire runs on the firewall at the same place you put in the wires for the UpFitters switches:



this is the auxiliary battery connection with the fuse block and you can see the isolation relay. The yellow connector is pictured below.  Hope this helps out someone. The control relay is the grey/green one with the red wires.  Yours might be black. 



  • Like 1
  • Thanks 1
Link to comment
Share on other sites

@clubba: I was only going by what the original factory installation has listed saying 220 amp alternator with the dual battery setup.  

This could be due to the loading if you have extras like the 120volt outlets or maybe it is linked into the charging management system in the truck?  

I would assume 170 amps would be sufficient? It maybe worth a bit more research .......

Link to comment
Share on other sites

  • 1 month later...
On 3/24/2021 at 3:37 PM, BrentH said:

Is there anyone out there that can take a couple pictures of the second battery installation?  Perhaps the cover off the fuse block and along the wire run?


I ordered my truck to come with the second battery and they did not put this in, I instead got the powered lift and drop tailgate.  This feature is not available (don't think so?) on your typical LT model so it was a bit of a surprise.  The added battery is so I can power things in the back cap while I work out of it when doing on site carpentry type things.


I have been through the GM Upfitters  and the auxiliary battery is not yet a bulletin, also their electrical schematic manual has some mistakes - or there are some differences in my trucks wiring from what they have listed.


The fuse block (X50E) that connects to the positive on the aux battery is about $165.00  the guy at the parts counter doesn't have one in stock and with covid you cannot go and look anyway - I want to know if the isolation relay and control relay are actually included in this part (GM 84663447).  If not, $165.00 is ridiculous for just a piece of ABS plastic.  There is no listing for the relay on the parts page so that is also confusing.


The tray is in place already and the wire harness looks like it is just a short pigtail added to the main connection back to the X50A fuse box - just do not know where the isolation relay power comes from (supposed to be connector X136) so will have to find that.


Any pictures would be great....thanks


@ Brent, just to clarify was the reason you didn't get the second battery because they didn't offer it on a LT?  Was your sales guy able to order it in GM's system?  Was it shown as an option in build and price on the website?  I am about to order a '22 GMC and want/need the extra battery and want to learn from your experience as much as I can.  Thanks!

Link to comment
Share on other sites

  • 2 months later...

@03 Cobra: sorry I have been away for a while.  Hopefully you got your truck ordered ok.  The Package was offered for the LT package and was in the build and price.  My Sales guy must have messed up on the order - or it was dropped by GM due to a lack of wire harness parts (this was mid covid crisis and there were supply issues with electrical parts)


On my truck there are a few odd ends - like the heavy cable coming off the main battery that is not fused and I think is for a cab pre heater if the truck was a diesel.  I also have the remote tailgate lower and Lift function.  The Lift part is not an option for the LT model but my truck has it?  No complaints - works great.  


Sorry I did not get back to you sooner.  you should be able to order the extra battery package.  There may have been changes to the build and price since I bought my truck.  I was essentially  able to build the Z78 truck into an LT for about 10 grand less - the only thing I am missing is a couple cameras and the skid plates as far as I can tell.

Link to comment
Share on other sites

The is for 2022 model:


Standard alternators:

KW7    Alternator, 170 amps
1 -  Requires (L8T) 6.6L V8 gas engine.


KW5         Alternator, 220 amps
1 -  Included with (L5P) Duramax 6.6L Turbo-Diesel V8 engine or (VYU) Snow Plow Prep/Camper Package. Free flow on (L8T) 6.6L V8 gas engine.


Dual alternators:

KHF         Alternators, dual, 220-amps primary, 170-amps auxiliary
1 -  Requires (K4B) auxiliary battery with (L8T) 6.6L V8 gas engine.


(diesel always gets dual batteries)


Gas battery:

    Battery, heavy-duty 720 cold-cranking amps/80 Amp-hr, maintenance-free with rundown protection and retained accessory power
1 -  Included and only available with (L8T) 6.6L V8 gas engine.


Diesel batteries:
    Battery, heavy-duty dual 730 cold-cranking amps/70 Amp-hr, maintenance-free with rundown protection and retained accessory power
1 -  Included and only available with (L5P) Duramax 6.6L Turbo-Diesel V8 engine. 



If dual batteries ordered for the gasser it forces either the 220A alternator or the dual alternators:

K4B         Battery, auxiliary, 730 cold-cranking amps/70 Amp-hr
1 -  Requires (L8T) 6.6L V8 gas engine and either (KW5) 220-amp alternator or (KHF) dual alternators. Not available with (KW7) 170-amp alternator.

Link to comment
Share on other sites

Simplest/cheapest way to do this would be to use an aftermarket ACR with a kit like this:


Silverado/Sierra Dual Battery Kit


Then route the new battery to whatever accessories you need to power.  The GM junction block and cover (item #1 in the hand drawn diagram above) would certainly clean up the install of such a kit nicely.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...

Important Information

By using this site, you agree to our Terms of Use.