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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.

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    \$\begingroup\$ You realize that there is no guarantee that the cells will remain precisely balanced after you assemble the packs, right? \$\endgroup\$ – Elliot Alderson Jan 16 at 16:30
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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.

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    \$\begingroup\$ Yes, and I would recommend a resistor between every pair of cells to prevent high currents as they equalize. \$\endgroup\$ – Elliot Alderson Jan 16 at 16:30
  • \$\begingroup\$ OK, I have since learned just how basic a question this is. And that the units being paralleled need to be close to the same voltage before doing so. \$\endgroup\$ – HansBKK Apr 27 at 16:18
  • \$\begingroup\$ The need for resistors to limit current flow would be reduced by fat wires between the units being paralleled, as long as the voltage difference does not cause such a high rate that current exceeds the mfg C rating. \$\endgroup\$ – HansBKK Apr 27 at 16:22

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