I've had an idea of using LiFePO4 battery as replacement for car battery, since 4 of those in series make nominal 12.8V which is very close to standard 12V voltage. And depending on the cell we can parallel some to get required starting current. Since LiFePO4 have much longer life then Lead-Acid, such battery should last much longer.
But there comes few problems:
Charging voltage. Since most alternators produce around 14V volts when charging it should be fine with LiFePO4 battery since maximum voltage for those is 3.6V which gives us little headroom up to 14.4V (4*3.6V=14.4V). Individual cell balancing can be done with simple dissipative BMS which dumps excessive charge to resistor. But we don't have possibility to disconnect alternator when battery is charged, so here is first Question: is it OK to hold LiFePO4 battery at floating charge with voltage which is close to maximum battery voltage? Or this will significantly decrease battery life?
Charging current. Some LiFePO4 cells can be configured so they can easily take all charge current produced by alternator. (assuming 70A output and 3 parallel 40152S cells) So second question: Can it overload alternator or modern alternators are smart enough to lower voltage to avoid being overloaded?
No over discharge protection. Currently have no idea how to deal with it besides not letting it to discharge completely.