[RedDevil]

That's alright, if the cells are very closely matched and if your car actually does output no more than 13.6 Volt. I'm skeptical about that, as my Insight will charge the battery at up to 13.95 Volt (verified) with load, so it would be even higher unloaded. IIRC most alternators produce about 14.2 to 14.4 Volt.

Then, if your cells are ever so slightly out of line you might get 3.3 + 3.3 + 3.3 + 4.5 Volt. The high cell may start to dissipate some current which may rebalance the pack, but up there you're already damaging it.
A BMS prevents that. Mine also cuts off if the temperature rises too much, at deep discharge, at overcurrent, you name it.

There's a downside on using a BMS too though. There's cost, it may limit the current below what you'd need for cranking, and the BMS itself may be faulty and cause imbalance.

Yet if you're willing to pay for a BMS, have the specs right and check whether it does in fact balance the cells right (like I did with all 3 of my packs) then it is the difference between hoping everything keeps working and knowing that it does.