bringing back an old thread here, but curious if you (the OP) ever found your answer?
I have been searching to find info on the BMS system for the 2016 LR4/D4 which has lead me to searching forums over seas including this one. I'm in the central US and have the same questions/goals.
On the LR4 the factory aux battery is located under the hood (i wish it was in the boot) and unfortunately the size of the space only allows for a minor size upgrade, but none the less would allow for more capacity. Assuming one goes lifepo4 there may be even more there to work with. Lifepo4's don't like heat but with a proper BMS on the battery you can limit the extremes and whats is a 25-50% reduction in the life of a $400-500 battery that has a 2,000 - 2,500 cycle life. Rarely does one get cycled all the way down anyways.
I have several thoughts on how to approach the power needs I have but finding out how the factory dual battery BMS works has been nearly impossible. Found some info on a german forum about how the BMS works for non start stop equipped units from the 2012-2015 era, but nothing on the late 2015-2016 models (pre capacitor VQMS approach). But my thinking is the same as yours. There is already a DC-DC built in, as well as a battery isolator. The alternator on the LR4/D4 is either 150a or 180a and variable voltage. (thats my logic for thinking there is already a DC-DC) and that the isolator protects the main (start) battery giving you a factory dual battery set up.
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