You could do it. But it's probably wise to use as short a run as possible.
I think you are correct after considering your input. I appreciate it a lot.
And, I believe the trailer harness wires are each fused at 30 amps or less.
The science for not paralleling non-identical batteries is based upon charging such a system. Due to AGM engineering and lead-acid chemistry, prompt and accurate charging (along with discharge limits) is essential for maintaining battery bank performance over time. Charging is governed by Ohm's Law, which explains why non-identical batteries in a single bank system will experience unnecessary electro-chemical damage (likely non-identically).
So I'm clear, and probably wasn't, I'm suggesting that this below the cabin route might also be one, albeit for considerably thicker (lower gauge) cable that might connect a larger Aux battery in the cargo area with that which goes on under the hood.
I got the sense from the OP Paul, maybe wrong, that this larger Aux battery would be one that might serve appliances as a disconnected one (unlike factory setup) from the main battery when the engine is off and such power is desired, with its role as an ESS battery being only, at best, some other need the battery might serve.
The desire to remove the auxiliary battery comes from people's desire to not run the ESS system, resulting in little upside to keeping the ESS/Aux battery within the electrical schematic of the vehicle: a battery that for more than a fair number of owners has proved problematic and cannibalistic on the main battery in part due to the very size difference it bears with the main battery, which it is connected to in parallel 99.9% of the time in the factory based wiring.
This explanation is not wrong per se, but is in my view incomplete in the real-world context of Wranglers and this thread.
There isn’t really a fundamental problem with parallel batteries. If there was a good reason then they wouldn’t do it. The only slight disadvantage is that over the lifetime of the battery, the smaller battery will not get the amount of cycles as the larger battery, it will cycle more. This is due to the imbalance of how each battery shares or divides the current either during charging or discharging. When there are two batteries in parallel, current will slosh around between them for a little bit until they balance. This action will shorten the life but not by much. If the batteries were the same size and in parallel, from the same manufacturer, the balancing would be better but there are variations from battery to battery. The balancing is worse when the battery sizes are different but like it said it doesn’t affect the life that dramatically.
Hey Thaw, you asked "What are the physics of dissimilar size batteries causing battery wear?" The question was reasonably related to the thread's topic and discussion flow, and it seemed to be asked in good faith and not as some sort of rhetoric or sarcasm. The answer--the science behind maintaining AGM performance and longevity--is not context specific.
