Separate battery locations in parallel - temperature sensing?

[windpipe]

My boat was factory outfitted with two separate sealed, vented battery compartments - one in the salon, one under the V-berth. Each holds 2 series-wired 6v golf cart batteries. My first thought was that the V-berth batteries were dedicated for the thruster and/or windlass, but no, they are wired back to the main bus bars, which are in the salon. So, we are talking about 2 batteries in parallel placed in different locations in the boat.

Referencing Calder's Boatowner's Mechanical and Electrical Manual 4th Ed., page 25: "It is not acceptable to expand battery banks by wiring in additional batteries in a different physical location.... different ambient temperatures... prematurely age... thermal runaway... explosion..."

So, this is obviously not ideal engineering. That said, I am not a livaboard. I hope to do some overnight sails and gunkholing this summer, but I resort to marinas with shorepower sometimes, also. I am going to be lucky if I have 20 days of heavy battery use in a season. So, I don't like it, but I am not overly concerned about the wear on the batteries.

Regarding managing the risk of overheating and thermal runaway: If I am using smart charging with a shore power to battery charger and a alternator to battery charger; which bank should I place temperature sensing on? The salon bank has cable runs of ~1 ft. The V berth roughly 12 ft. My assumption is that the salon batteries with shorter cable runs and less resistance will take more wear (preferentially charging and discharging) such that I should place the temp sensors there? The battery compartments are similar as far as size and ventilation.

They are very nice battery compartments, with good quality wiring, so it is not practical to totally re-engineer the system (I couldn't afford to replace things such that I would gain a net improvement). The other solution that comes to mind it that I could change to lithium and get rid of the V-berth batteries all together. But again, expense... If I am not going to use 100+ cycles per year I don't see how the $ per usable Ah would be justifiable, let alone the effort required to set everything up to handle LFP (my marina has no experience with LFP).