All Kinds of Wrong?

[RussJeep1]

You attached an appropriately gauged wire from the positive terminal of the small battery to the positive terminal of the large battery.

You repeat for the negative/ground side creating a parallel connection between the two batteries, correct?

Effectively, you bypass the PDS' (Power Distribution System's) isolation of the two upon crank.

Please explain to me all the kinds of wrong such a scenario might create. (There must be some, right?)

As to why: while I certainly don't wish to do this if it damages things or negatively affects proper function, why not have each battery supplement the other at crank and when the engine is on? The batteries are connect in parallel when the rig is off, right?

Thanks.

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[BlackRook]

You're intuition is correct in general; connecting batteries in parallel will effectively pool their power:

https://www.homepower.com/articles/...llation/ask-experts-batteries-series-parallel

But it's possible the jeep doesn't always use the two in parallel. Specific components (likely more than just ESS) probably require specific voltage input ranges. As noted in the above, you're limiting the output of the two batteries to one suboptimal voltage.

Given that modern vehicles are as much computer as car, I'd expect degradation of performance, and reduced lifetimes on electronics, though probably not immediate component failure. It'd also not be particularly good for your warranty; you could remove the wiring harness and lie to the service department, but then they would never identify your gremlins. Or you could tell them and they'll void your warranty. A lose-lose.

It'd be interesting to see exactly what would fail. But I wouldn't want my truck to be the guinea pig...

 

[RussJeep1]

You're intuition is correct in general; connecting batteries in parallel will effectively pool their power:

https://www.homepower.com/articles/...llation/ask-experts-batteries-series-parallel

But it's possible the jeep doesn't always use the two in parallel. Specific components (likely more than just ESS) probably require specific voltage input ranges. As noted in the above, you're limiting the output of the two batteries to one suboptimal voltage.

Given that modern vehicles are as much computer as car, I'd expect degradation of performance, and reduced lifetimes on electronics, though probably not immediate component failure. It'd also not be particularly good for your warranty; you could remove the wiring harness and lie to the service department, but then they would never identify your gremlins. Or you could tell them and they'll void your warranty. A lose-lose.

It'd be interesting to see exactly what would fail. But I wouldn't want my truck to be the guinea pig...

Thanks for your thoughts Ross and I completely get them. I too wouldn't want to be the one that uses my JL as the guinea pig.

And yet, as I understand it, the only time the two batteries aren't in parallel is upon crank, be it user or ESS initiated. And still further I glean that the only reason that this is the case is to keep voltage in the ESS battery, for ESS (and user initiated) crank, as opposed to user crank done by button push. The batteries are connected in parallel when the rig is off, which allows you to "chump start" the rig (the idea of you thinking the wait in jumping starting the rig as per the manual ISN'T actually the process of first trickle charging the ESS battery.) And they're both charged (I don't know if hooked up in parallel) from the engine upon successful crank.

It seems to me that maybe user (as opposed to ESS) crank should have NOT isolated the batteries but I make my claim from much ignorance on FCA's designs and intentions. Time will provide answers here.

If such a hook up ever becomes common place as time and cold weather diminish the ESS battery's ability to deep cycle to supply a very quick but intensive load of an engine crank, you heard it here first.

FCA claims to have thoroughly tested the rig over time and weather conditions. I hope they know what they're doing, despite my extended warranty, which doesn't factor in getting stuck, or time without the rig. In fainess I did though buy with open eyes of its new design and love it.

Let's hope for the best.

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