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I taken the necessary precautions in creating my own custom battery pack: I bought two 18650s from the same shipment and will attach a diode from each positive end of the 18650s before attaching the positive ends into one wire.

However, I am confused on how to charge the cells when the eventually deplete, all in one circuit.

  • I am trying to power an item that takes a constant 5V input in order to run.

Here is my design:

enter image description here

More Specifically,

  • I am going to connect the cells in parallel, with diodes on the positive terminals of the batteries/wires. Connect the positive wires with diodes and negative wires to form a parallel battery.
  • Connect the positive and negative "double wires" to the BMS B+ and B-
  • Connect the BMS P+ and P- output to the 5V Step Up Conveter Input
  • Connect the Output of the 5V Step up to a Switch so I can turn the whole system off or on.

Question:

How do I charge the parallel batteries in this circuit?

  • All of this circuitry is to make 5V output from a two 18650 cells in parallel, so how do I get power in? Would I make a Y Split at the P+ and P- of the BMS output, maybe?, not sure if that even makes sense?
  • Do I need diodes, wouldn't I not be able to charge the batteries if I put in diodes, blocking current from coming in?
  • Is the switch in the right place?
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  • \$\begingroup\$ This an example why it's better to buy a ready-made power bank, rather than design your own. A ready-made one has already solved all these issues, even ones you haven't yet thought about; it's reliable; it's guaranteed to work; it's housed in an appropriate enclosure; and, in the end, it's cheaper. \$\endgroup\$ Nov 13, 2022 at 16:55

2 Answers 2

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You're overthinking it. No diodes are required. 2-cells in parallel = 1 larger cell.

  1. Make sure that the DC-DC converter is current limited (for CCCV operation) and is trimmed for 4.2 V (not 5 V)
  2. Make sure the two cells are at the same voltage
  3. Connect the two 18650 cells directly and permanently in parallel.
  4. Connect a "single cell" BMS to these two cells in parallel
  5. If you picked a 1-port BMS, connect the charger and the load to the "P" terminals of the BMS. If you picked a 2-port BMS, connect the charger to the "C" terminals of the BMS and the load to the "P" terminals of the BMS.
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A BMS - Battery Management System is equalizing voltage an each cell, while the cells are connected in series. There is no such BMS for parallel connected cells. However you can use your BMS if you wish, it would give a balance charge and overcharge/discharge protection (possibly, I don't know the BMS model). Rather using a step up converter you can change to step down converter and you're fine.

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  • \$\begingroup\$ How would I go about wiring a 2S BMS then? I have an empty BM slot on the board and since It's made for 2S I'm not sure if it will work if leave BM unsoldered. How would I end up charging the board as well after I I hook up a boost converter? Do I charge it through the boost converter? \$\endgroup\$ Aug 25, 2018 at 16:46

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