Did some testing with a potential replacement for lead-acid 12V starter batteries, using lithium iron phosphate (Li-Fe-Po4) cells from batteryspace. Charged and discharged a string of 4 18650 LiFe cells as a lead-acid cell, and got this from CBA (btw, I made a data entry error in CBA chart says 1.4Ah@1.30A rate, but the capacity of the cells is actually rated at 1.3Ah):
This is the same test done on an Odyssey PC680 lead acid battery:
It seems like the lithium phosphates can be used as lead-acid cells! The LiFePo4 cells don't fully charge, and don't fully discharge when treated as a lead-acid, but at a loss of under 150mAh (on a 1300mAh cell). It leaves about 90mAh capacity left, and less than 50mAh or so undercharged, based on how much capacity I was able to get from these cells (about 1200mAh or so) using the LiFe charging/discharging profile. I think the amount of undercharge will vary by which lead-acid charger you are using. (I'm using a Schulze isl 6-330D charger, which charges LiFe cells BTW) This will actually increase cell life by undercharging them, so I think it's ok. To maximize life of a lead-acid cell, you're only supposed to use 50% of it's capacity anyways. The PC680 is rated for 17Ah at the 20 hour rate, and I was able to only get slightly more than 9Ah at a 6.8A discharge rate.