I have 3 identical lithium ion batteries, i want to connect two in series for about 8 volts, and then the last one in parallel for more capacity, and then I'm going to wire this to a 5v regulator and charge my phone (which i use as a wireless security camera). Since i don't want the lithium ion blowing up on me, i just wanted to make sure that i have wired it correctly:

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Does this seem right? Thanks!

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    \$\begingroup\$ This will definitely damage the single 4v Li-ion battery connected in parallel. You would be applying ~8v across the a battery which is rated for 4.2v. \$\endgroup\$ – R. Hirur Oct 21 '16 at 5:31
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    \$\begingroup\$ That's the best way to have a Samsung Galaxy Note 7. I mean having the battery blow up on you. \$\endgroup\$ – Passerby Oct 21 '16 at 5:37
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    \$\begingroup\$ If anything, put them all in parallel and use a dedicated Lith-Ion charger circuit that does both regulating 5V down to charge the batteries and step up to charge the phone. AKA a usb power bank. \$\endgroup\$ – Passerby Oct 21 '16 at 5:39
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    \$\begingroup\$ I think you are trying to essentially make your own USB power bank, right? I would put them all in series and use a 5V buck regulator. But you could put them in parallel and use a boost regulator. Either way will be fairly efficient. \$\endgroup\$ – mkeith Oct 21 '16 at 5:43
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    \$\begingroup\$ You can't connect one in parallel. R. Hirur is totally correct about that. Your choices are series and parallel. The calculation you are making is based on the assumption that your battery pack discharge current is equal to the cell phone charge current. If you use a buck regulator (or boost), this assumption will not be correct. The battery pack has 2.225*3.7*3=25 Watt hours of energy. Multiply by 85% for 21 Wh of energy. Load power is 5V * 0.8A = 4W. So 21 Wh / 4W = 5.25 hours of run time. \$\endgroup\$ – mkeith Oct 21 '16 at 6:03

You can put them all in series (with suitable balance for charging) or all in parallel (no balance needed for charging). Then use a switch mode dc-dc converter to get the final voltage you want.

If you connect the batteries as you have shown, please do it on a large open area of concrete, well away from buildings, pets, other people, flammable materials, and make the final connection with a wire on a long pole. They may not all explode at the same time. I would expect the top one to go up in flames first, however the lower two might also be damaged before the top one goes completely open circuit.

If you connect batteries in parallel, they absolutely need to be the same number in series in each arm. There are a few more requirements like same chemistry, same capacity, same state of charge, and same age and history of cell.

  • \$\begingroup\$ Thanks, no i won't even try to connect them that way, That's why i asked here first. Also, I use my power supply for charging (keeping an eye on it the whole time). I have set the voltage to 4.2v exact, and then 1450 mA for the current limit. Is it possible to charge them in series using this method? if so what current/voltage do i need to set it? \$\endgroup\$ – Xane Oct 21 '16 at 5:53
  • \$\begingroup\$ @Xane what criteria do you use to terminate the charge when using your 4.2v/1450mA method? \$\endgroup\$ – Neil_UK Oct 21 '16 at 5:59
  • \$\begingroup\$ I followed this tutorial: youtube.com/watch?v=0yhbof6_s64 So when the current drops down to almost zero in current, i terminate the charge. \$\endgroup\$ – Xane Oct 21 '16 at 6:08
  • \$\begingroup\$ @Xane just checking. 'Almost zero' might be too late, most guides suggest 3% to 5% of C as the end-point current. This method will work for series connections as well, as long as the cells are actively balanced for voltage, and the balancing current is taken into account correctly when assessing end of charge. \$\endgroup\$ – Neil_UK Oct 21 '16 at 7:09
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    \$\begingroup\$ @Xane If you are going to series charge the sells, I could ask 'you do understand about cell balancing, don't you?' but then I wouldn't know what you think you know. So I'll ask this. Explain to me what active cell balancing is, how you do it, why it's done, and what can happen if you don't do it. \$\endgroup\$ – Neil_UK Oct 21 '16 at 7:47

No, you cannot do that. The two cells in series would uncontrollably discharge into the single cell in parallel, charging it way above the rated charging current. Assuming that the cell survives the overcurrent, it will eventually get overcharged past the maximum safe voltage (typically 4.2 V), short out internally and blow up.

While cells can be connected in series to form higher voltage battery packs, the cell capacities must be equal. Even with well matched cells, the individual cells must also be protected from overvoltage when charging and undervoltage when discharging due to manufacturing differences. This is accomplished by a battery management system (BMS).

Your options are:

  • A 2-series 2-parallel (four cell) 7.4 V battery pack, with a complex BMS and a linear or switching regulator

  • A 3-series (three cell) 11.1 V pack with an even more complex BMS and a step-down DC-DC converter (switching regulator) for bringing the voltage down to 5V.

  • A 1-series 3-parallel 3.7 V pack with a simple BMS and a step-up DC-DC converter for boosting the voltage up to 5V. This is the configuration used by most commercially available "charge banks".

If I were you I'd buy a commercial charge bank and replace the original cell with your three cells in parallel.

  • \$\begingroup\$ Thanks for your answer, Then I'll think i go with A 3-series (three cell) alternative, since i need this today and can't really afford a charge bank atm, if i want to charge them i can just remove them and charge them one cell at a time, pretty annoying but safer. \$\endgroup\$ – Xane Oct 21 '16 at 6:13
  • \$\begingroup\$ Charge them in series. But monitor the individual cell voltages carefully. Don't let any cell get above 4.2. \$\endgroup\$ – mkeith Oct 21 '16 at 6:26

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