Second battery wiring?

BillArnett

New member
My understanding is that the JL with the 3.6L engine and ESS has a second battery, presumably to insure that the engine can always be started. My question is what the wiring diagram looks like. Is the second battery simply wired in parallel with the main one? If so, it would seem to be kind of silly; a larger main battery would be cheaper. Or are some of the electrical loads connected to one of the batteries and the rest to the other? Sounds complicated. Or is there a relay that connects the second battery only when starting? Or...?
 

13_gecko_rubi

New member
My understanding is that the JL with the 3.6L engine and ESS has a second battery, presumably to insure that the engine can always be started. My question is what the wiring diagram looks like. Is the second battery simply wired in parallel with the main one? If so, it would seem to be kind of silly; a larger main battery would be cheaper. Or are some of the electrical loads connected to one of the batteries and the rest to the other? Sounds complicated. Or is there a relay that connects the second battery only when starting? Or...?

They are not simply wired in parallel. The negatives are both common but the positives go to two different posts in the power distribution center under the hood. That module controls when to use it and when not to via relays.


Sent from my iPad using JL Wrangler Jeep Forum mobile app
 

NFRs2000NYC

Member
They are not simply wired in parallel. The negatives are both common but the positives go to two different posts in the power distribution center under the hood. That module controls when to use it and when not to via relays.


Sent from my iPad using JL Wrangler Jeep Forum mobile app

So you're saying at on the 3.6, you can kill your main battery camping and it will still start because there is a secondary smaller battery specifically there for starting?
 

nvabill

New member
So you're saying at on the 3.6, you can kill your main battery camping and it will still start because there is a secondary smaller battery specifically there for starting?
I disconnected the main battery in my JL today and was able to start the vehicle with the aux. battery with no issues whatsoever.
 

NFRs2000NYC

Member
Interesting, so it seems the jeep uses the second battery to start itself? I guess someone just needs to come out with an isolator kit, and you have yourself a perfect dual battery system with minimal hassle.
 

NFRs2000NYC

Member
Interesting. So then the opposite question arises: if the engine in not running and you’re running electrical loads is the secondary battery disconnected? IE can I “safely” run down the main battery and still start the Jeep?

For my JK, I have a little lithium battery for emergencies:

https://www.amazon.com/Starter-GOOLOO-18000mAh-Portable-Battery/dp/B0748D8KT6

Amazingly for such a small device it really can start the Jeep.

It seems that may be the case, unless the Jeep doesn't know to try the secondary battery if the main one is dead. Seems if you disconnect the dead one, the Jeep will use the secondary to start, but not sure how it will react with a low voltage situation. What Im curious about is working on the jeep...as stated, the Jeep will start with the main battery disconnected, so do you have to disconnect BOTH batteries to kill live wires so you can work on them?
 

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