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My son is doing an Eagle project where he is creating a solar charged LED light set up using two 18650 cells in parallel. We are working to ensure they are safely protected. From the research we have done, a small BMS for the pair of cells and a fuse for each cell seem to be the way to go. We are thinking PTC's from reading, but will need a little help sizing them. The load is a 0.10 amp LED light string that he is powering.

What's the best way to size the PTC so we can give it try? Also, is there a better way to protect the cells? I can provide part numbers and circuit details if needed to help.

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For just two 18650 cells in parallel, fuses are not required.

  1. Measure the voltage of the two cells, to make sure they are the same
  2. Connect the two cell directly and permanently in parallel
  3. Use a BMS for a single Li-ion cell
  4. Use a charger for a single Li-ion cell
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    \$\begingroup\$ The #2 is an excellent safety advice - terrible things will happen if battery holders are used, and there is a bad contact like oxidation which allows one battery to be full while another is empty, and when due to a shock etc there is again good contact, the full and empty battery gets shorted together. Another note is that the single cell BMS must be able to handle twice peak current when two cells are bolted together as single larger cell. \$\endgroup\$
    – Justme
    Commented Sep 5, 2023 at 21:34
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    \$\begingroup\$ thank you for your help! do you think I need a fuse on each cell to protect each one from the other? Also, on the peak current - is it the peak output of the batteries or the peak draw from then LEDS he will be using? \$\endgroup\$
    – Shy Dinosaur Productions
    Commented Sep 7, 2023 at 23:33
  • \$\begingroup\$ "do you think I need a fuse on each cell to protect each one from the other? " I already answered that question: "For just two 18650 cells in parallel, fuses are not required." \$\endgroup\$
    – Davide Andrea
    Commented Sep 7, 2023 at 23:42
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    \$\begingroup\$ Davide - thank you again for your help! \$\endgroup\$
    – Shy Dinosaur Productions
    Commented Sep 9, 2023 at 4:28

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