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Need to connect and charge two Li-ion batteries in series. I'm not sure if it is possible to connect and use two Li-ion batteries and TP4056 like this. Are they going to be balanced? Do they need to?

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  • \$\begingroup\$ No, you can't do that. Probably IN- and BAT- are connected together. Now look at your 5 V in. It will connect to BAT- of the top board and put +5 V on the BAT+ of the lower board. \$\endgroup\$ – Transistor Sep 4 '18 at 15:03
  • \$\begingroup\$ That's why i connected top li ion between 10V and 5V, and the last one between 5V and 0. So top bat should be ok (?) \$\endgroup\$ – Рональд Sep 4 '18 at 15:26
  • \$\begingroup\$ No. It's just a very bad solution. You can see in Tony's answer that R2, for example, is between the BAT- and IN-. (It's probably a low value for current sensing.) If you connect BAT- to BAT+ of the lower board you will completely mess this up. \$\endgroup\$ – Transistor Sep 4 '18 at 15:34
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enter image description here

Thus your problem now looks like this.
enter image description here

"No Way, Jose".

Yes they need to be balanced to prevent accelerated aging from mismatch charge imbalance with high current and deep discharges.

  • It is analogous to runaway series incandescent lightbulbs ( where the faster weaker bulb achieves near full brightness and the other bulb stays cool.
  • Another example is like mismatched-Vf shunt LEDs sharing current. If one LED cannot support the current of 2 or more in parallel, one can burn out open cct as thermal runaway lowers the Vf of the hotter LED thus drawing more current.

    There is a better chance of extending end-of-life to achieve the max rated charge cycles if they are balanced or much lower if they are not!

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