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I have made a lipo battery pack with 2 identical 3.7V 1400mAh battery packs connected in series.

This pack is being used in a wearable project where I wear this pack on my wrist.

Now to charge it up, I have a TP4056 1A single cell lipo charger. Now my charger allows me to charge one cell at a time. I was thinking that if I wire individual cells to the charger in parallel, I could use the same charger for both the cells and could keep my battery pack compact and would have a simple plug-in solution for charging it up.

Will this method work?

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That will not work.
Not without extra care, attention and electronics.

If one cell when you start charging is 3.3V and the other is 3.4V, depending on the quality of the cell and thus its internal resistance, already you can have a situation where one cell forces several amps into the other the moment you connect them in parallel. Doing this regularly will severely reduce the life of your batteries. If the difference is larger the currents can be big enough to cause serious and immediate problems.

Apart from that, you should be aware that your device or pack should turn off the current drain once either one of the cells falls below a safe minimum voltage (3.2V for example, or 3.0V), so that they can't discharge to a point where damage occurs. This is why many 2cell packs have an internal BMS board that monitors both cells for over and under voltage and turns them off if either is at danger.

With home-made wearable tech I'd also strongly suggest you add current-limit protection inside the battery pack, so that a single mistake while wearing has a much lower risk of short-circuit based fire and/or injury.

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  • \$\begingroup\$ Any alternatives you suggest? \$\endgroup\$ – YaddyVirus Jul 13 '17 at 8:06
  • \$\begingroup\$ @YaddyVirus A properly protected pack (BMS inside), charged with a balance charging set-up, failing that, something at least safe. \$\endgroup\$ – Asmyldof Jul 13 '17 at 9:03
  • \$\begingroup\$ Hey I found a very good guide here scriptasylum.com/rc_speed/lipo.html This tells you exactly how to wire multiple lipo cells in any config you want and shows the balance wiring as well. Solved my problem \$\endgroup\$ – YaddyVirus Jul 14 '17 at 4:10
  • \$\begingroup\$ @YaddyVirus Be careful with some of those set-ups, very few important warnings are given and those that are are hidden behind a question-mark link. Putting batteries in parallel we discussed here, and applies the same as when building a pack, please remember that. \$\endgroup\$ – Asmyldof Jul 15 '17 at 19:14
  • \$\begingroup\$ Sure thing.. The pack seems to work okay for now \$\endgroup\$ – YaddyVirus Jul 17 '17 at 13:09
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If you made a 2s battery pack you can't actually connect the same batteries in parallel, otherwise you'll short circuit them. Assuming that you can disconnect the cells from their series connections we can proceed with the discussion.

If you want to connect in parallel and charge 2 unbalanced cell with a single TP4056, you can do that as long as the two batteries are connected by a (small) resistor to avoid the divide-by-zero issue: A=V/R. With zero ohm you will have short circuit current, which is usually dangerous, but you can proceed this way.

Battery charger (BC), battery (B1 and B2) , resistor (R).

BC+ --- B1+ --- R --- B2+

BC- ---- B1- ----------- B2-

The resistor R should likely be the smaller you have near you because with unbalanced cell even by 0.5V (which is a lot for two cells in series!) and 2ohm resistor you will only have 0.25A balance current, which is not dangerous for a lipo battery.

ps: you can't charge the cells at their maximum potential with the TP4056, and you should often swap B1 and B2, but you can have a "good enough" charge. With high ohm resistor you won't have a proper CC charge phase on B2, but that's all.

ps: i've done this "trick" several times with li-ion cells, but i can't guarantee that it will work flawlessly

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