I'm considering a LiFePO4 to power everything but the starter motor. The starter lead to oem battery would be hooked up normally but the 2nd smaller battery cable that goes to the fuse box would instead be hooked up to the Li battery pack's + terminal.
That cable (and its current) are small enough that I could put a switch in the circuit to power the car off the oem battery if I do run down the Li pack.
Both batteries would be grounded to chassis. IMHO they would be isolated from each other. Correct??
Oem battery would be still connected to the alt via the starter/alt cables and so would be charged for service. I can cut the alt circuit so I won't need to apply ~14V to the oem battery constantly, only for a short while after starting the car, and every now and then to keep the battery topped off.
BATTERY PACK with PCM (Protection Circuit Module):
LiFePO4 Prismatic Battery: 12.8V 40Ah (512Wh, 100A) with Balancing PCM (48.0)
Battery has a 15.2V peak; I think that would be the open circuit voltage and what it provides to the clock & computer etc. at low current draw?? Seems a bit high to me, for use in a car.
Charger, 10A
Smart Charger (10 A) for 12.8V LiFePO4 Battery Pack, Worldwide use, CE listed
Does this combination make sense? Cost would be well over the cost of a "regular" battery but similar to a ~65AH AGM deep cycle Odyssey battery + charger. The disadvantage of the AGM is that to protect battery life you really only want to discharge about 25%, down to approx 75% charge level, on a regular basis. Thus I'm considering a LiFePO4 setup instead.