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as my title says, I would like to design a rechargeable circuit made out of 18650s in parallel. I understand that placing batteries in parallel isn't ideal, so my question is:

If I utilize same batteries, with the same age, and allow them to discharge while connected to each other with a resistor to achieve the same voltage, before using them daily (discharge and recharge) is this safe? I see laptop batteries using 18650s in parallel, so I was wondering how did they achieve this safely.

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  • \$\begingroup\$ Connecting them initially via a resistor allows the cells to "balance". Once balanced, they are as safe as one large cell. How safe that is, depends on how you use and charge it. \$\endgroup\$
    – user16324
    Apr 21, 2020 at 14:43
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    \$\begingroup\$ I strongly suggest using a battery management system that has voltage and current protections while dealing with multiple cell parallel configured battery packages. \$\endgroup\$
    – emre iris
    Apr 21, 2020 at 15:03
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    \$\begingroup\$ While a few cells in parallel is OK, subject to balancing before connecting, a lot of cells in parallel ideally needs each cell protecting with a series fuse. If one cell fails short circuit, the others could drive enough current through it to turn it into a disaster. \$\endgroup\$
    – Neil_UK
    Apr 21, 2020 at 15:07
  • \$\begingroup\$ 100% you have to "balance" these. If you don't, you might make a really cool fireworks display out of your project ;) \$\endgroup\$
    – Kyle B
    Apr 21, 2020 at 15:22
  • \$\begingroup\$ Thanks for the advice everyone, I understand that balancing is necessary now. In that case, would fuses be necessary if I'm utilizing a BMS? I'm following this instructable, and the lack of fuses between batteries in his circuit worry me greatly, since battery-battery flow seems possible with his circuit. \$\endgroup\$
    – Zhi Yong Lee
    Apr 21, 2020 at 15:41

2 Answers 2

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Li-Ion rules:

  • You mean Series! Series, to increase the voltage
  • Parallel is absolutely ideal, since there is no balancing required, since they are parallel, they basically share the same voltage
  • Do NOT EVER permanently add a resistor to the batteries! This over time will discharche them and yield in a fire hazard
  • if you have no idea, how to make that circuit, do not make it please. I know, that is the typical answer, but the understanding of the principles is mandatory for working with high power batteries, which can set fire to anything if done improperly.
  • To connect them in series you will need a balancer capable of:
    1. Under voltage detection and cut off
    2. Over voltage detection and cut off
    3. Balancing the load (either pump the power from cell X to cell Y or at least limit the voltage of each cell by discharging to precisely 4.2000V max (It never should go over that for even a second. At 4.3V the Cell gets basically destroyed in an unpredictable manner)! Rather use 4.05V as the "fully" charged voltage. You barely loose any capacity, but realy improofe safety my an order of... no idea, but it is much much safer and gives room for errors - that is the main reason. Also storing a battery at 100% charge will shorten its life and results in a slight fire hazard again.

My tip for you: If you want a power bank, just parallel them up and boost the voltage afterwards. This is 100 times more simple to implement and increases safety a ton. The only problem with parallel cells: If one is damaged and self discharges, it will discharge every other cell. So make sure, this never can happen. Each discharged cell is a hazard, especially when getting recharged.

Have fun. And never exceed 1C charging (better 0.5C) and the rated C discharging current. Than you should have fun. I also reuse old 18650 cells. But store them in a steel container for safety (plastic layer, to prevent shorts etc...)

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  • \$\begingroup\$ Thank you for your answer, but my question was more related to preventing the cells discharging into each other, and how this is done safely by laptop battery manufacturers since I don't see fuses in tear downs of their batteries.. and I mentioned the resistors for balancing cell voltages, not to permanently discharge them.. Thanks for the tips on containing the cells though, that is good advice. \$\endgroup\$
    – Zhi Yong Lee
    Apr 22, 2020 at 1:48
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After doing more research, I found out that Tesla is also stacking these 18650s, and they use fuse wires to ensure cells are protected when placed in parallel.
For someone else doing similar research this thread shows lots of useful info: https://endless-sphere.com/forums/viewtopic.php?t=75336

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