# Is it possible to only fill a Li-ion BMS partially?

I'm putting together an 18650 battery pack for an e-bike. The BMS is specified for 10s 36v 40a.

Is it possible to use the BMS with only, say, 2 or 3 of the 36v packs of cells I made? The use case would be for a lower capacity pack.

Edit: Removed part of question after clarification about proper BMS wiring.

• Even if possible , it's a bad idea to add mismatched cells in a string. Then you risk undervoltage on discharge and accelerated new cell wear – Tony Stewart Sunnyskyguy EE75 Nov 25 '19 at 22:17
• What if I made sure that each cell was identical, and the same voltage before I used them? Sorry, I assumed that was implied when building a pack with a bms. – FallenSpaces Nov 25 '19 at 22:20
• They have to be same Vbat, same Ah and same ESR within tight tolerances for the BMS to work since it can only handle maybe 10% overall mismatch in power during CV mode when current is declining not CC mode – Tony Stewart Sunnyskyguy EE75 Nov 25 '19 at 22:23
• So you're saying, even if it were possible, it wouldn't be very smart unless I'm planning to only use those cells together? And this is because once I use the cells like that, they might degrade from other cells I might have purchased alongside them? – FallenSpaces Nov 25 '19 at 22:33
• yes . Cells must always be very well matched then the BMS will extend the life to keep them matched longer. but not so if initially unmatched – Tony Stewart Sunnyskyguy EE75 Nov 25 '19 at 22:36

Is it possible to use the BMS with only, say, 2 or 3 of the 36v packs of cells I made? The use case would be for a lower capacity pack...

If so, would I have to wire it in a particular way? Such as connecting the charge/discharge positive to B2-B3 instead of B10?

You seem to be confused about capacity and number of cells. Connecting to B2-B3 would lower the voltage, not the capacity. It also probably wouldn't work with that BMS (Ali-express locks up my browser so I can't check its specs).

If you have several 36V packs then you can wire them in parallel for higher capacity, but each one should have its own BMS. If you make the packs yourself then make sure the cells are all identical (same part number, same voltage within 0.03V, same age - preferably new) or the BMS may not be able to balance them (assuming it even has a balancer).

• In other words do not deviate from the intended design for string size and choose only well balanced cells for capacity and voltage before using – Tony Stewart Sunnyskyguy EE75 Nov 27 '19 at 18:52

If your design is for several fully parallel connected strings of 10 cells, (ie. nP10S for some n,) then The bms should work fine for some lower n down to n=1 for a simple 10 cells in series. (but be sure to to exceed the maximum charge current etc)

Whether the BMS can manage fewer than 10 cells in series depends on the BMS

Combining mismatched cells in parallel is less of an issue than combining mis-matched cells in series; because of lithiums cell's large voltage swing during across charge state combined cells will self-level (but be sure to get their charge states close before combining them so that the leveling currents do not exceed the maximum charge/discharge current allowed)

It's fine to start with a 20 or 30 cell 36V battery during development and then to add more parallel cells afterwards. The cells in the first battery pack will experience more wear in the initial stage which will reduce the useful life of the battery pack by some amount. but if it saves you form a hernia, or better fits your cashflow to work this way I think it's still a good choice.

• Jasen how does the 36 V charger connect to 1 cell and not overdrive it? – Tony Stewart Sunnyskyguy EE75 Nov 26 '19 at 2:15
• Jasen with programmable supplies and cells it is possible but not dumb bare boards. aliexpress.com/i/32998117161.html or alibaba.com/product-detail/… How do you ensure heat dissipated is not excessive from voltage drop – Tony Stewart Sunnyskyguy EE75 Nov 26 '19 at 2:21
• it doesn't 2 or 3 of the 36v packs of cells should be fine. that's 2P10S or 3P10S (that plural "packs" makes all the difference) – Jasen Nov 26 '19 at 2:47
• it doesn't it says 2 or 3 of the 36v packs of cells not 2 or 3 cells of the 36V packs – Jasen Nov 26 '19 at 2:52
• What happens when 1 pack is fully charged at 3.8V while another pack is still in CC mode . Does this not degrade the charged pack lifespan? – Tony Stewart Sunnyskyguy EE75 Nov 26 '19 at 2:57