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Example 16 identical battery cells rated as below:

  • 3.2V 100Ah
  • LiFePO4
  • Max Continuous Discharge 100A / 1C
  • Max Peak Discharge 200A / 2C for 100s
  • Max Continuous Charge 50A / 0.5C
  • Max Peak Charge 100A / 1C for 100s

Scenario 1 Cells arranged in 8S2P configuration (2 blocks in parallel, 8 of these in a row in series, 16 blocks total)

  • New voltage 25.6V
  • New capacity 200Ah

Scenario 2 Cells arranged in 2S8P configuration (8 blocks in parallel, 2 of these in a row in series, 16 blocks total)

  • New voltage 6.4V
  • New capacity 800Ah

Questions

  1. In each scenario, what are the values of continuous/peak charge/discharge currents and why?
  2. Does this change with differing battery chemistry (given otherwise same spec)?

Edit

  1. Purpose of this is to understand what loads can be attached to a given battery bank constructed from the cells (max discharge current) and what a safe charging current would/should be (max charge current).
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2 Answers 2

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First of all, I don't have real experience with LiFeO4, but I have some basic knowledge about Li-Ion battery. So just want to tell what I think, hope it can be somewhat useful.

  1. Because you connect batteries in series & parallel, it's sure that current can't be simple multipled by spec of a single cell. Ex: About discharge, I believe that if 1 cell can discharge at 1C, 2 cells in series also can discharge at 1C, 8 cell in a series is the same. But depend on real capacity of the lowest cell, that cell will impact to total voltage (dropped),especially when one cell is empty. About charge, surely you need to do balance charge, this will make the charge-time longer, so it also mean you can't reach max current charge (1C) on every cell, at everytime. When a cell's voltage lower than the others, it should be charged, other cell need to wait, and vice versa. For peak charge/discharge, it's truly the same. We know that when peak charge/discharge, cell's voltage will reach its max/min limit. And surely, the worst cell will reach this limit faster than the better cell. So you still can source (discharge) at max current, but the time of discharge (100s) maybe can't reach. The peak charge is as the above example, balance charge, so you can't apply max peak charge. About parallel cells, higher voltage series will be discharged first, so when first series will source 1C, second series will source only 0.8C (for example) to reach the same voltage. mean total is 1.8C max. The real value depends on the difference of cell, can't tell exactly.
  2. I don't know there is an ideal battery chemistry that can reach max powwer when you seri/parrallel them. Should be no, because always exist difference between cell.
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  • \$\begingroup\$ Hi Nam, thanks for the response! So do you think there may be a way to put a formula against this to show how both maximum discharge and max allowed charge current change based on Series and Parallel setups? \$\endgroup\$
    – DavidM
    Aug 14, 2020 at 10:28
  • \$\begingroup\$ If you care more about the discharge, I'm pretty sure that discharge is almost ideal, 2C for 8S2P for new batteries. Charge-time with balance can be a bit slower, but I don't think that it's a big problem in almost popular cases. But with aging of batteries, can't tell more. Anyway, LiFeO4 is strong of cycle - compare to other battery type, so hope it's not a problem, too. @DavidM \$\endgroup\$ Aug 14, 2020 at 11:09
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Once you consider that no individual cell can go beyond its current limit, the rest should come naturally.

Components in series all have the same current, so the current rating of cells in series is the smallest current rating of any cell in the string. If they're all the same, then the current rating of a series arrangement is simply the current rating of one cell. If they're not, well, better pay close attention to balance.

Components in parallel have the same voltage across them, and share current, so the current rating of N identical cells in parallel is N times the rating of one cell.

If the cells aren't identical then there are four possible answers for parallel connection, from most conservative to least:

  1. Don't do that, you'll probably blow something up eventually.
  2. If you never go over the current rating of one cell then you would theoretically be safe even if all the rest were replaced by open circuits.
  3. Using N times the lowest current rating is safe as long as all the cells are contributing equally — could develop problems due to uneven aging.
  4. Simply using the sum of all of the current ratings could be acceptable if the current ratings were proportional to capacity, and under the theory that each cell contributes current proportional to its capacity (a cell that did more than that would get discharged more than the others, which would lower its terminal voltage, which would make it receive current from the other cells, which would restore the equilibrium). This is a theory I would be willing to accept for lead-acid, but probably wouldn't chance with lithium.
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