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Install a 2nd battery

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I'm sure this is posted somewhere in here i just can't seem to find it. I'm wanting to run a 2nd battery in 2020 2500HD and was hoping someone would have some insight into what was the best way to go about doing this. 

If there is already a thread then just point me at that and I will read through that, otherwise thanks for the help.

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  • 2 months later...

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

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I don't think the factory auxiliary battery for the 2500 gas includes an isolation system. 


afaik they are wired up same as on duramax, so they operate same as one big battery and the option requires either the larger alternator of the dual alternators


standard setup on LT8 (gasser) 

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. 


Dmax set-up

    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.


Extra battery setup for gasser

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. 



The snowplow package and dmax both include the bigger alternator -- but it can be ordered without snowplow prep

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. 




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So here is the crazy thing:


On my truck: 2021 2500HD 6.6 gas LT I requested the 220 amp alternator and aux battery - It didn't come that way - no aux battery and a 170 alternator 


BUT:  My fuse panel on top of the regular battery is NOT the correct one for the set up as fitted (according to GM parts)  my fuse block is part number 84646133   this is a fuse block from a 1500 diesel with a 220 amp alternator.  So my fuse block (X50D) has the following F1 - 250 Amp connected to 170 Amp ALT, F2 - 175 Amp connected to Fuse Bock X50A  F3 - 400 Amp connected to Starter Motor  The next 2 fuses are 60 amp and run to the interior fuse panels and then I have a 1 more wire in the supposedly F4  position that is not fused and is a decent size wire.  No idea where this runs to and does not jive with any wiring diagrams on the GMUpfitters.


From the wire diagrams it "should" be easy enough to install the GM Fuse Block Auxiliary Battery 84663447.  The wire from F2 175 amp  would run to the 100 amp fuse in the X50E aux battery  block and then back to the X50A fuse panel.  The Auxiliary Fuse Block may already have the battery isolation relay  (KR79) that is supposedly controlled with a simple connection to the body control module. (connector x136).  This would be a very simple install and follows the jist of the parts diagrams for the auxiliary installation.


The GMUpfitters electrical seems to be riddled with mistakes or errors , probably created by the lack of parts during covid - my truck is supposed to have the main battery fuse block X50D as part number 84646132, which I suspect has a lower rated 200 Amp F1 Fuse, not the 250 - it will make a 220 alt upgrade a lot easier though.


I have an after market isolator, but for simplicity and looks under the hood it would be just as easy to purchase the required X50E OEM part.  


I tried to load the wire diagram and pics but exceeded the limit and then it won't let me try again...






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What I think is correct the the 2500HD LT for the Aux Battery circuit:




On this drawing though, F79UA 10A is misnamed and actually called the AUX BATT/SCRPM in other locations of the electrical schematics

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  • 2 weeks later...

Hey @Rozz522,


I ordered part number 84663447 - J Block Auxiliary Battery,   84471670 Junction Block Cover


I still have to get under the hood (I am away till next week) and clarify a few things:  "Supposedly" under the battery mount could be the connector X136 which would then allow you to run  to the relay KR81 (Auxiliary Battery Relay 1)  from this relay you go to the KR79 (Auxiliary Battery Relay 2) that supposedly comes included in the J Block Auxiliary Battery?  I ordered the 220 amp alternator so I have to wait for parts to arrive.


I have some questions still to answer about the fuse block X50D as I have a wire coming off it that is not identified in the Upfitter wire diagrams.


I plan to make the battery hold down parts as they are expensive for what they are.  A new positive cable just has a pig tail off the end connection so 

I will also make that verses buying a new cable - same with the ground.


I downloaded a pic from this site where someone was discussing the connections.  This is what it would/should end up like:




Trying to figure out where the unfused terminal above the blue connection goes to?  It is not on any drawings other than the Diesel engine where it would go to an auxiliary heater?  I have seen YOUTUBE videos and it is not on some of the trucks??  Maybe it runs to the Invertor for the 120 outlets??



From the Upfitter site material the Auxiliary battery will charge when you are in tow/haul mode and pulling a trailer, or I assume in that mode.  Still figuring out if that is the whole gist of it?






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My son just checked the truck and the battery connection to the lug I was asking about in the previous post-it is dead - no power going to it.  So Hmmmmm....What is somewhere in the truck that has no power and then why is it there?

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  • 2 weeks later...



So i have everything installed except for the one control relay that is supposed to come off the body control module to allow charging to take place after the engine is running.  


A wire from the fuse panel  is for the power to close the isolation relay.  Some where in the wire harness should be a plugged 4 conductor with three feeds -  I cannot find it anywhere - "should be" under the auxiliary battery tray.....frustrated.......


I installed the upfitter switches - that job was a couple hours, everything clicked easy.


If anyone has the auxiliary battery fitted in gasser 6.6 can you post a pic or two of the wire harness tie in after the first small relay?  

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I might not be the best to try to help, but I do have a gasser with the factory Aux batt.

Pics should be in order, from top of battery with cover removed and then just follow the wire loom.  Last pic is where it connects to fuse block with that cover removed.  If you let me know where you need more detail I might be able to get it.


FWIW, as far as I know the aux batt is connected to the 12V of the camper/trailer plugs.  I have not added any accessories.  It is isolated from the primary battery in every 'key' position except run. Connected in run to charge.  Doesn't matter if tow/haul, etc.


Good luck




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More pics.


Next to last is a more close-up of the engine end , pos end of the aux batt, sort of repeat of pic two

Again, if you point out on pics where you need more I might be able to help.





Edited by Wxman
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@Wxman You are awesome!   so the grey relay in your second last pic and also in the second pic has 3 wires coming into it : red/white, black and a violet/brown


The violet/brown comes from the BCM and energises the grey relay once the truck is started.  The red/white comes from the fuse block right there beside the battery (X50A) and once the truck is running  the grey relay closes and White/Red powers the isolation relay in picture .  The black is just ground.  There are a pile of errors in the GMUpfitters electrical drawings.  Your assembly contradicts the wire harness from the parts options - no issues, just adds more confusion.

Looks like I will need to splice into a few connections.  My truck has the "auxiliary Battery" 10 amp fuse in the engine compartment fuse block (X50A) (red/white) and has a violet/brown coming off the body control ....where did they run it??    I also have an electrical heater in the cabin for a diesel engine that is not connected....WTF


Here is your pic with a few questions.  If you can locate the end of the connector spice, I "should" be able to chop open the wire harness and possibly find 2 ends with a seal on them.IMG_1086.JPG.de2f50a0f2bd8a94126b225eb573c4f4.thumb.jpg.afb4c6b53062db351f732e8a5a222528.jpg 


Your pictures give me hope! thank you for response.  If I can get this mod nailed I will post how to do it.  Terrible access to some of the stuff, had the wheel well protectors out 5 times now.....alas





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Not easy, sorry.

The first pic shows how that loom 'terminates' into a very large bundle below the aux batter tray.  That large bundle runs up behind the power steering reservoir. There it most of it turns and goes back toward the firewall, but it has a couple sprouts right behind the reservoir.  One of them goes into some kind of plug right there; the other looks to go and follow along the valve cover on the passenger side.  The third pic shows the large bundle going the other direction.  It goes into a black hole behind part of the support for the aux battery tray.  Sorry it isn't obvious to me.




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