# Lithium Battery Pack - Do I need BMS within parallel connections?

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 (Battery Management System) 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.

• 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/… – Tagadac Mar 1 '17 at 11:25
• I believe this is what the Tesla pack does - they certainly don't have individual balancers per cell. – pjc50 Mar 1 '17 at 11:46
• @Tagadac how would one protect the four cells connected in parallel? – Shahid Thaika Mar 1 '17 at 12:29
• @ShahidThaika I don't know if I understand your question ? – Tagadac Mar 1 '17 at 12:36
• @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. – Shahid Thaika Mar 1 '17 at 12:54

For the most part, putting cells in parallel just makes them behave like a bigger single cell. So, if you take four cells and hook all of them together in parallel, it appears to a circuit to just be a single cell with four times the capacity.

BMS's are built to manage cells in series. Along with current and voltage protections, it monitors each "cell" in the pack to make sure its voltage is within limits, and if any one cell dies prematurely it will cut off the whole pack to prevent the other cells from reverse-charging the dead cell.

Your configuration is "3s4p" - three groups of four parallel cells wired in series. Thus, you need a BMS that can manage three cells in series - a "3S" BMS. Generally speaking, it's irrelevant how many cells you put in parallel in each cell group, as long as all the groups have the same number of cells at similar capacities (i.e. you do not want to put one parallel group of 3 cells in series with a parallel group of 4 cells), since the BMS will see your parallel groups as single larger cells and will manage accordingly.

• > you need a BMS that can manage three cells in series - a "3S" BMS. So do I need 4 such BMS for this configuration "3s4p"? (as 4 in 4p)? Each BMS for each block of cells connected in series? – The Godfather Jun 29 '20 at 13:44

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.

I'd err towards caution - everyone knows deep down that parallel batteries are not guaranteed to share charging and load currents evenly so, I'd use parallel arrangements of series batteries each protected by it's own BMS. So if you have a 3s battery then that has its own BMS. If you have another 3s battery then that should have its own BMS: -

With 4 parallel sets of 3s you'd have 4 BMSs and only make parallel connections at the ends of each series chain.

Of course this is an expensive solution but it has to be considered as viable if the cost and risk warrant it. If the cost and risk don't warrant it then just parallel 4 batteries and hope for the best with a single BMS.

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.

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.