[brucepick]

Thanks, Ladogaboy. Now of course I have more questions for you...

How do you calc AH for batteries in series? 4 x 3.3V cells in series = 13.2V. If each cell holds 2.5 AH, does the pack have 10 AH capacity at 13.2V? or 2.5 AH at 13.2V?? Or???

Then assuming you want more AH, you could put 16 cells in the pack, series-parallel, to make the same 13.2V. That would give you 4x the AH from the first scenario, right? True that you could merely double or triple the # of cells.

Looks like you're aiming your calcs towards a target of 100AH. Reason? I suspect you have a good one. Can you say what is usually needed to run the car's basics - fuel pump, ignition, computer? I did some calcs for headlights, down below.

I got into this because EOC with headlights on runs down the (lead-acid) battery kinda quickly so I started thinking about a separate deep cycle battery to power the lights. But if Li cells can hold enough AH they might be able to power everything for a day, and then get recharged overnight.

I calc'd 2 headlight bulbs @ 55W each = 110W. I rounded that up to 150 to give some headroom to run some other stuff too.
150 W at 12V = 12.5 Amps
Run for 3 hours = 37.5 AH