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Dual battery TP2 and Stud 1 Fuse 68 question


intheburbs

Question

I've been reading up on the fuse in question, and it's brought me to a question that I'm hoping to get an answer on.

 

I've seen this:

 

"If a fuse is installed in this position, the accessories will discharge the primary battery in addition to the auxiliary battery."

 

So, the auxiliary battery will power trailer accessories, and the primary battery powers vehicle accessories? One of the main reasons I installed a second battery is to prevent battery rundown, and to make sure I'm able to start the truck after hours or days. I was under the (mistaken?) impression that the aux battery is isolated when the ignition is off.

 

Here's my question: If I'm boondocking with my travel trailer in the middle of nowhere, is the trailer draining the aux battery? Do I need to disconnect the trailer plug to make sure the aux battery is completely isolated?

 

Or should I remove the inline 40-amp fuse in the wire that goes from the aux positive post to stud 1 in front of the load center?

 

If this is the case, it kind of seems silly, as it negates one of the benefits to having an aux battery.

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6 answers to this question

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If you pull the fuse then your second battery will only power the 7 pin power plug. This effectively "isolates" your second battery as long as your trailer is not plugged into your truck. With the fuse installed, you essentially have both batteries in the truck "paralleled". My truck came with the TP2 option from the factory and I have test this circuitry to confirm. So if you don't want your starting battery to be drained while trailer is plugged in, you must remove the Stud 1 fuse in question.

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In that case it would be easier to just unplug the trailer - that won't cost you a dime. Does seem like bad engineering to have the trailer pull down the truck's battery along with it's own. No idea why they'd set it up that way .... but I find myself questioning GM engineers on a daily basis lately ...

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Sorry to dredge this up again, but I still haven't found an answer.

 

In both my GMT900 trucks I've installed a second battery and all of the related wiring to mimic the TP2 option. I bought all original GM parts - the battery cables, isolator, connectors, megafuse holder, etc. Everything is wired up per factory specs - the isolator relay on studs A5 and B4 under the load center. The isolator relay connects the batteries in the run position, and opens in any other position, isolating the second battery. Anyone looking under the hood would think the trucks came from the factory with two batteries installed.

 

So - back to my original question(s):

 

If a fuse is installed in position Stud 1/68, will both batteries be drained by 1) vehicle accessories, and/or 2) my trailer?

 

I did this under the assumption that whether I've got my trailer or not, I can run accessories off the main battery as much as I want, and the second battery will remain isolated and ready to start the vehicle. Is this not correct?

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I don't think the setup has changed from my 2000. The second battery is only being used when the truck is running. It is for high electrical loads like snow plowing. Any draw from the trailer with the key off, will come from the starting battery.

 

Mark

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The power circuit in the 7 pin connector is primarily designed for the trailer brake battery. It is not there for running two batteries under the hood. That 40 amp fuse is how the trailer brake battery gets charged, and also will supply 12 volts to any accessories you have running in the trailer. The trailer electric brake battery is mounted on the trailer.

 

If you are mounting a second battery under the hood, there is a secondary battery wiriing kit you need to get in order to get isolation.

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