LiFePO4 cells - self-balancing in series or parallel?

Question

I am building various banks / packs from individual prismatic LFP cells, nominal 3.2V each. New and pretty large, 60-400AH.

I have a good power supply, charge each cell independently, Bulk stage CC to a standard voltage setpoint, then hold CV Absorb until a standardized trailing amps, then stop, no Float.

This results in a batch of cells already pretty well balanced, but I want to ensure as precisely equal a resting voltage at that top SoC point as possible, by hooking a batch of them up (say 18 cells) and letting any potential differences equalize passively over time.

During that time, no loads, no charging, no devices attached at all other than the top-quality connecting wiring between cells, the block as a whole being isolated.

Should I do this by connecting them in series, or in parallel?

These connections are just temporary, before later assembling into production packs / banks.

Answer 1

Parallel.

If you put them in series, you'll either have an open circuit (nothing will happen) or a short-circuit across the 57 volt pack with a lot of energy behind it. They'll try to discharge in a hurry, making lots of heat, possibly melting one of the interconnects - before it starts a fire, if you're lucky.