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If I use a TP5100 module to charge two 3.7V lithium batteries, does the capacity need to be the same for each battery? If I connect 3 or more batteries in series and use another module (I don't know which module needs to be used) to charge those batteries, does the capacity of each battery need to be the same?

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    \$\begingroup\$ They must be identical otherwise runaway early failures occur. A BMS with 10% mismatch tolerance compensation will extend the life. \$\endgroup\$ Jun 5 '21 at 13:13
  • \$\begingroup\$ How different do you want them to be able to be? Like AAA vs D-cell? The small cell would always charge first, and discharge first, and the D-cell would act like an AAA. There are ways around it, but i haven't seen it in practice. If you are recycling cells, they should be tested and reasonably well-matched. That said, just attaching an extra cell in round robin fashion to all cells in the string should automatically balance their voltages and compensate for a larger than normal variation. But as always, it's the magnitudes that we fail to understand, so do your measurements, and your math. \$\endgroup\$ Jun 5 '21 at 14:39
  • \$\begingroup\$ If you are playing with LiIon batteries knowing that much about them, use safety goggles and have a fire extinguisher readily available. \$\endgroup\$
    – fraxinus
    Jun 5 '21 at 20:51
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Batteries connected in series or parallel should have similar discharge profiles.

When connected in series you need to stop using the battery pack when any one of the series batteries falls below around 3.0V. So the life of the pack is determined by the series battery which discharges the fastest.

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  • \$\begingroup\$ The more low the capacity is, the more fast the battery will be discharged. Right? \$\endgroup\$
    – Walid
    Jun 5 '21 at 18:02
  • \$\begingroup\$ If I connect one 1000mah and one 2600 mah 18650 battery with a 2S BMS board, how much ampere will it provide at the output of the BMS? Doesn’t the BMS will stop giving output when if a battery voltage falls below 3Volt? \$\endgroup\$
    – Walid
    Jun 5 '21 at 18:38

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