I want to be able to start my truck with a li-ion pack I made. It is a 4S1P pack made from Headway 38120 cells. They have a max continuous discharge of over 100A. I can't recall the max pulse discharge rating, but the site I bought them from said they had been able to start a car from the same 4S1P configuration. When my car battery died, I connected the pack via jumper cables (14.4V fully charged), but it didn't quite have enough oomph to get it to start. Was this caused by a voltage sag resulting from a huge current demand? Or is it more likely that the impedance of the jumper cables combined with the cells' internal impedance was too high to deliver enough current? Can I fix this by going from 4S to 5S, or do I need to go from 4S1P to 4S2P?
Electronic – Why couldn’t the battery pack start the car
batterieshigh-current
Best Answer
Kirchoff’s Law would explain the failure if you had any measurments. Even a lead acid battery might not have enough with poor contacts or cables and good contacts might fail with a high ESR battery.
the best approach is to transfer the charge from the external to internal battery for a few minutes to eliminate the external ESR issues. The internal battery will have high ESR with low charge and the action is visa versa with external charging.
with sufficient charge Q=CV and low ESR the voltage sag = I* ESR should be less than 40% and it should start.