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I have 4 18650 batteries and I want to make a battery pack of it. If all the batteries were new and had similar voltage/capacity I could easily charge them all together in parallel with my charger. However the batteries have about 10% difference in capacity so I need to charge them using a balance wire on each battery/cell.

My problem is this: I have 2 circuits that I came up on my mind (drawing below) in such a way that one circuit is for charging and the other for discharging. So how do I do the wiring in such a way that I can solder the wires to the batteries (spot weld) and have a DISCHARGE port (+ and -) and a CHARGE port (GND and cell 1, cell 2, cell 3 and cell 4)?

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    \$\begingroup\$ I don't think you want to be connecting two cells in parallel if they are not identical. A small difference in voltage could cause a large current to flow as one of the cells tries to charge the other one. \$\endgroup\$ Jan 18 at 23:10
  • \$\begingroup\$ @ElliotAlderson You are right, that's why I will choose 2 batteries that have the closest capacity to wire it in parallel. But I still dont know how to wire that battery to be able to charge and discharge at same time :( \$\endgroup\$ Jan 18 at 23:11
  • \$\begingroup\$ This is not my area of expertise but I think to be safe you should look for circuits that are specifically designed to manage the charging of series cells and the discharge of parallel cells. I don't think it is simply a matter of soldering some wires and a switch. \$\endgroup\$ Jan 18 at 23:26
  • \$\begingroup\$ With Lithium batteries this can result in a fire if not done correctly as @ElliotAlderson alludes to. With NiCd or NiMH worst case is you get a battery leak and a mess. (thinking in AA size terms). \$\endgroup\$
    – Aaron
    Jan 19 at 0:49
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Parallel charging works better then no balancer is needed. Just reduce the max CC for 3cell and the low cell will just charge up faster.

You may want to monitor the weak cell for low voltage or use as higher cutoff for low voltage as the imbalance will undercharge the weakest cell first if/when used in series.

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  • \$\begingroup\$ I accepeted your answer. One thing is not clear: should I parallel charge the batteries in series or should I parallel charge the batteries in parallel? \$\endgroup\$ Jan 20 at 13:15
  • \$\begingroup\$ Use In parallel so that voltage is controlled equally and none exceed CV. Ideally the string should be matched <1% for each series cell by selection and don’t include weak cells. Or combine in an array of 2PxS \$\endgroup\$ Jan 20 at 15:45
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how do I do the wiring in such a way that I can solder the wires to the batteries (spot weld) and have a DISCHARGE port (+ and -) and a CHARGE port (GND and cell 1, cell 2, cell 3 and cell 4)?

Cells that are wired in parallel act like a single cell, so you don't need a different configuration for charge and discharge. Just fully charge each cell individually, then wire them for discharge. Try to arrange them for the smallest capacity difference between each pair.

If the cell pairs don't have equal capacity then one pair will discharge faster than the other. A PCM (Protection Circuit Module) will prevent either cell pair from over-discharge, as well as protecting against overcharging and short circuit.

For charging you can either treat the pack as 2 cells in series and include a balancer, or charge each pair separately with single cell chargers. If you charge the pairs separately then you don't need a balancer, but at least one charger must have its power input isolated from ground (eg. by using a separate mains power supply or an isolated DC/DC converter).

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  • \$\begingroup\$ I cant chage the wiring every single time I will charge the batteries and change the wiring again when I am gonna use the batteries. The batteries are gonna be encolsed in a case, so I need some wiring that is fixed. \$\endgroup\$ Jan 19 at 13:14
  • \$\begingroup\$ In your question you suggest having two different circuits for charging and discharging. If you have now decided to stick with one circuit (2S2P) that's good. Just charge each cell separately (on a single cell charger) before wiring them together, to make sure they are balanced initially. \$\endgroup\$ Jan 19 at 17:35

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