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I plan to use packs of 18650 Li-Ion batteries as power source for my hobby project. I would like to combine two 4-packs connected in parallel. Each 4-pack connects four batteries in a serie. So there is total 8 batteries. Assuming nominal voltage of 3.6V per battery each 4-pack will give 14.4V. Connecting two 4-packs in parallel will maintain 14.4V but double the capacity, at least that's what I expect.

I'm going to draw 1-2A from above setup. I will use identical 18650 batteries and will charge them individually with smart charger, outside of above setup. My questions are:

  1. Is it a valid circuit for Li-Ion batteries (are there any caveats)?
  2. Can I expect the combined batteries to have the capacity equal to the sum of individual capacities? My reasoning is: If one 18650 battery has 3000mAh at 3.6V then serie of four should have 3000mAh at 14.4V. Times two gives 6000mAh at 14.4V from combined 8 batteries.
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    \$\begingroup\$ I will use identical 18650 batteries and will charge them individually with smart charger, outside of above setup That is asking for trouble. If there is a slight voltage difference between battery packs, one will charge the other, large currents could flow causing damage/fire/explosion. So don't. You will need to balance the cells individually so they all have the same voltage, then connect then and leave them connected. \$\endgroup\$ – Bimpelrekkie Apr 3 '17 at 19:00
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    \$\begingroup\$ The basic questions you have about using LiIon cells makes me MOST STRONGLY advise you to not make your own battery pack but instead buy a ready-made pack of the required voltage and capacity. \$\endgroup\$ – Bimpelrekkie Apr 3 '17 at 19:03
  • \$\begingroup\$ @FakeMoustache If I balance the batteries and leave them connected permanently - What would be the best way to charge them? \$\endgroup\$ – PanJanek Apr 3 '17 at 19:03
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    \$\begingroup\$ With the proper LiIon charger, one where you can set the voltage/number of cells and maximum charging current. Yes those are expensive. Again, the fact that you have to ask this means you're not experienced enough for using these LiIon cells in this way. Treat them wrong and they can and will explode. You MUST know what you're doing. \$\endgroup\$ – Bimpelrekkie Apr 3 '17 at 19:06
  • \$\begingroup\$ @FakeMoustache Thanks for your response (put a proper response so I could accept it). As I suspected this couldn't be that easy :) I think I'll use ready-made battery pack. \$\endgroup\$ – PanJanek Apr 3 '17 at 19:13
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You will have some success with this for sure. It will 'work' but it is prone to error. Li-ion cells are dangerous.

That being said, to answer your question. Is it a valid circuit: Yes. Two pairs of 4s batteries connected in parallel at only the bulk nodes will work for sure. But you want to make sure that the bulk voltages of each of the packs is pretty close to the same when you hook them up. If one is 16.81v and the other is 16.73v well that is close enough (someone will complain that I said that but I can explain why if it comes to that, but that wasn't the question you asked so I won't get into it yet.)

Are there any caveats? Yes, What is monitoring the state of charge (SOC) of the individual cells? Does each 4s pack have its own battery management system (BMS)? You do not want any one of those batteries to go below 3.0v more or less. They will all reach 3.0v at a different time because of small differences in the cells. So that is a caveat.

This is also related to your next question: Will the bulk capacity be the same as the sum of the individual capacities. No, it won't. If you are indeed stopping the discharge of the unit when any one of the cells gets down to 3v then the capacity will be limited by the capacity of the weakest cell in the pack. Also it will not be at a consistent 14.4v. The voltage will change as the SOC changes. There are quite a few unknowns in your statement so it is difficult to give better advice.

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