Bert the Welder wrote:
use of the second alt ... big ga. wire ... 7-pin
The challenge is battling "voltage drop" over long lengths of charger wires.
Though a typical alternator puts out ~14.4V, that voltage drops to 13.some volts over a typical 7-pin setup where the NEG and POS wires are typically 10-12ga. My 2015 F350 has 10ga for NEG/POS, and be warned some umbilical cords use down to 16ga for POS/NEG. Below 13.6V delivered volts, a LiFePO4 won't really pull current and thus charge. Ideally you want 14.4V+ delivered volts for a LiFePO4 battery to quickly and fully charge, and to enable the LiFePO4 BMS to do cell balancing.
For lead acid batteries, most people report getting 5-10A of anemic charging off the 7-pin, and I believe the 7-pin connectors themselves are only rated for ~30A, and truck manufacturers usually fuse the POS wire at 30-40A.
Some people run LARGE (1/0AWG+) dedicated copper wires thru say Anderson Connectors, but don't under-estimate the weight and EXPENSE of such a setup, and you'll often still be getting suboptimal voltages. At 45' one-way of 1/0AWG wire, 14.4V drops to 13.9V under a hypothetical 60A load.
The increasingly common approach is to install a DC-to-DC charger that can boost the voltage coming off your wiring into what the LiFePO4s want for optimal charging, and it can limit the current draw on your alternator.
Speaking of a second alternator... picking on a 100Ah BattleBorn, the max-max charge rate is 100A, and the recommended max charge rate is 50A, and the longevity of the battery can be further increased by not even pushing 50A.
It begs the question, how big will your battery bank be? Two or eight 100Ah batteries? How fast do you want to charge?
With my 2 100Ah BattleBorns, I went with a 60A DC-to-DC charger so that each gets 30A, and if a battery BMS happens to shut off, dumping all 60A into a single battery won't be fatal. 60A into the batteries translates into a load of 60-80A on the alternator, which is well within the ability of a single and typical 200A truck alternator to deliver. With just 60A of charging for a 200Ah bank, I don't need to drive much to recharge from empty.
--tg