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I have some Toshiba SCiB cells that I wish to string in 4s configuration. These are Lithium Titanate (LTO) cells. They have a 20Ah capacity and operate safely between 1.5 V and 2.7 V. The cells were obtained after some light use. I was told they still have good capacity and should still have a solid amount of cycles left.

I have 14 cells and I want to know how to determine which four cells will perform the best when strung together. I could just grab four cells and place them in series ( with appropriate cell managing of course). Would it be worth it to measure the internal resistance of each cell and place in series the four cells with the most similar internal resistance?

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It will be almost impossible to match internal resistance since this is defined by the state of charge, temperature etc.
The best way to match the cells is to fully charge to the same endpoint voltage and then discharge each cell with the same load current (a simple load resistor) to a fixed endpoint voltage measured with a decent multimeter. This will give you the approximate capacity of each individual cell.

Select 4 cells with the closest capacity (ie time to your discharge endpoint can be used).

The internal resistance of all four cells you select is effectively in series for discharging, so the impact on the terminal voltage of the 4S under load is impacted by all the internal resistance.

If you match the capacities of the batteries closely, you may not need a BMS. However a BMS is always better.

This may help your understanding of the subject.

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  • \$\begingroup\$ If the internal resistance of any of the cells is significantly worse, this method is not good enough. You have to match on both capacity and internal resistance to make a good match. But even if you match well, the cells may age differently, so keeping them balanced is also very important for long life. \$\endgroup\$ – MicroservicesOnDDD Mar 21 at 19:32
  • \$\begingroup\$ I argue this way because they sell two different basic kinds of 18650's -- those tuned for greater capacity, and those tuned for delivery of high power (as in vaping). A high capacity 18650 has a higher internal resistance (the jellyroll has thinner Lithium foil), and should not be connected in series with a high power delivery 18650 for a low-ohm vape machine -- you would risk "venting with flame" because it's the difference between the ability to deliver only 8 Amps, and being able to deliver 20 Amps. The higher power cell will overheat the other one because of internal resistance. \$\endgroup\$ – MicroservicesOnDDD Mar 24 at 17:28
  • \$\begingroup\$ @MicroservicesOnDDD If batteries are to be connected in series they should be from the same batch or certainly from the same specification sheet. Your second comment is just nonsense, the lower internal resistance battery does NOT overheat the higher resistance battery. If you have two batteries of differing internal resistance ....the load defines the current flowing ....the internal resistances define the terminal votage for the batteries under that load current. \$\endgroup\$ – Jack Creasey Mar 26 at 0:05
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Matching cells by hand would be better than doing nothing, but if one or more cells has significantly lower voltage/resistance it can cause heating issues when placed in series. If the cells have been charged to a state that isn't known it would be best to use a Battery Management System (BMS) or balancer to charge the batteries. This helps balance the cell voltage and prevent problems with unmatched cells.

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  • \$\begingroup\$ Am I correct in saying that the charge capacity of the four series-connected cells will be limited by the lowest capacity cell? \$\endgroup\$ – jRRRR Apr 8 '19 at 23:14
  • \$\begingroup\$ No, it will have the most resistance and could heat to a dangerous level \$\endgroup\$ – Voltage Spike Apr 9 '19 at 5:54

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