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Let's assume I am going to build a Li-ion battery pack with 12 18650s, where I connect four cells together in parallel and then the three sets of four in series.

My understanding is that a BMS keeps an eye on the voltage and keeps it from going too high or too low. Thus, would I then use a BMS module that connects three batteries in a series, or would I need to have a BMS with 12 connections, including the cells that are connected in parallel.

TBH, I am not at that level yet, but just trying to understand the nuances of li-ion batteries before I get my hands dirty.

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  • \$\begingroup\$ Avoid putting lithium batteries in parallel without any protection against voltage disparity or self balancing currents. (see : electronics.stackexchange.com/questions/288288/…). But for your question here is maybe a lead of answer : electronics.stackexchange.com/questions/289450/… \$\endgroup\$ – Tagadac Mar 1 '17 at 11:25
  • \$\begingroup\$ I believe this is what the Tesla pack does - they certainly don't have individual balancers per cell. \$\endgroup\$ – pjc50 Mar 1 '17 at 11:46
  • \$\begingroup\$ @Tagadac how would one protect the four cells connected in parallel? \$\endgroup\$ – Shahid Thaika Mar 1 '17 at 12:29
  • \$\begingroup\$ @ShahidThaika I don't know if I understand your question ? \$\endgroup\$ – Tagadac Mar 1 '17 at 12:36
  • \$\begingroup\$ @Tagadac You said not to put lithium batteries in parallel without any protection. My question described a scenario where three sets of 'four 18650s connected in parallel' are connected in series. I know that a BMS can manage the connection within the three packs connected in series, but what about the four batteries connected in parallel within each set. \$\endgroup\$ – Shahid Thaika Mar 1 '17 at 12:54
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The cells you put in parallel are no longer considered 4 cells in parallel but are now considered one cell with more capacity and able to source more current safely (if your bus is up for it.) There is no need to put multiple leads to this grouping.

The leads monitor each series groupings only. So if you have a battery that is 10 cells in series and 7 in parallel, You monitor the 10 and not the 7.

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Someone please educate me if I'm wrong, but to give the correct solution he's looking for, he should make a 3s4p pack with only one bms connected to only the 1St set of 3 in series.

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My understanding is you have a number of options here with the most common being 1 BMS connected across the series cells as just described. The issue I see with this is that in a large pack 1 or more cells could fail (lose capacity/voltage)and you would not know with this arrangement. All that would happen is you’d lose capacity in that group of parallel cells. If your BMS does balancing then this will probably lead to excessive balancing

You’d need some kind of capacity tracking or balance tracking in the BMS logic to trigger alarm/action if this occurred.

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