Common port BMS in series with...

I'm trying to work through the logic of how a common port BMS functions under this particular scenario. I have a 14S lithium ion pack (18Ah) with a common port BMS. I have two paralleled 12Ah 6S lipo packs (24Ah total) connected in series with the 14S pack. I bulk charge the packs together with a 20S charger.

I'm using the higher capacity lipos in order to only use the middle part of their capacity (max charge at around ~4.1V per cell and min voltage higher than around ~3.5V) when in series with the 14S pack. The 14S pack will be charged to around 4.15V per cell and cutoff charge a little before the lipos reach 4.1V per cell*. *The desired voltage range of the lipo packs will be adjusted after gathering some data while in use, in order to place them in the middle of their discharge range, and be used that way going forward.

I believe that I can simply monitor the 6S lipo voltage to ensure they stay within a range where there is headroom at the top, and bottom, under normal day to day use, and per cell voltage periodically (I've used different Graphene lipos in the past and they never went out of balance for years, so I expect similar performance).

For bulk charging, I always cutoff charging manually, but my question for this thread is, given the above set up, I believe the common port BMS will cutoff all/bulk charging when the 14S pack is "fully charged" to 4.1V per cell (setting the charge cutoff the 4.1V in the BMS settings), if I let the packs charge until the cutoff point rather than manually cutting off. I'm only trying to determine what protections I have in place, even though I'm always present monitor the charging anyway. None of the packs will ever be fully discharged, but I believe given the capacity of the 6S vs 14S,, the 14S would cutoff discharge before the lipos ever reach a low enough voltage to be a concern.

I doubt it could do it safely. Full charge for a 20s pack is 84v, right? The breakdown voltage for the FET that blocks charging in the 14S BMS is likely only 60V. And FETs can fail in such a way that closes the circuit...

lnanek said:

I doubt it could do it safely. Full charge for a 20s pack is 84v, right? The breakdown voltage for the FET that blocks charging in the 14S BMS is likely only 60V. And FETs can fail in such a way that closes the circuit...

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Per the model number displayed in the app, the BMS works with 8S to 24S and up to 100A continuous. It seems like the FETs might be fine from a voltage perspective.

OK,, no problem with open circuit voltage across the common port BMS. The two 6S packs have no BMS. You're asking if the whole combo will charge evenly, or will the 14S cells reach 4.10 volt far sooner than the 6S cells.

Lets assume a charge current of 3A, and all three packs sitting at 3V/cell. With 6 hours of charging,, you've put 18AH into the combo. That will have charged the 14S, but leaves the two 6S at 66% charge when the 14S cuts off charge current, Sure, that won't work,

But suppose you start with all three packs fully charged, If you take out X AH, the voltage will distribute proportionally across the 14S and the 6S according to capacity, The cells in the 6S combo will be less discharged. When you recharge, they should all equalize near max voltage when you put X AH back in..

I did this experiment last year with a 36V6AH and a 12V10AH combo. Both were full charged. I ran them down from full charge to 48V. Recharged and was expecting them to go unbalanced, but they followed the above scenario and I was back at 54.6V.

I'm sure things can go wrong. I only did it once. Maybe the whole stack will drift in voltage if all the cells aren;t similar., Different charge curves might have the 6S getting to 4.2V/cell before the 14S shuts off. Put common port BMS on the 6S too.

With two single port packs in series, if either one triggers cutoff and goes open circuit, the voltage across the FET switch will be in the normal ballpark, so I wouldn't worry about blowing up the FETs. The larger capacity pack may never get fully charged though.

During discharge, it's another story. If one switches off, then you have the combined pack voltage across the switch and things can blow up.