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?
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.
100A is awfully light for starting a car engine. Certainly a truck engine is more. Yes, at these triple digit amperes, you need to think about the internal resistance of everything.
The person started a 1st generation Hyundai Santa Fe, a smallish SUV with a 3500 "Sigma" engine. It's a Korean fast-revving engine. That experience may not translate to a serious truck engine.
Also note in the video that the seller had the battery hooked up directly to the OEM battery cables (themselves with surprisingly thin wires, again reflecting the lower current of that starter). It's also possible this whole thing was a put-up and that the seller simply used the normal car battery.
Jumper cables, especially cheap ones, are surprisingly thin. They are not made to carry 100% of the cranking current, they are designed to short-term recharge a struggling battery so it can do most of the heavy lifting during start. So if you added a length of jumper cables to it, no wonder.
the site I bought them from said they had been able to start a car from the same 4S1P configuration
Yeah, well, that stuff is sold on eBay and Amazon Marketplace, the "caveat emptor" flea market of the internet. You need to be smarter about expectations from such unvetted China-sourced crud. You notice Digi-Key and Mouser don't sell those things? There's a reason for that...