Questions about how BMS balanced charging works

[StjepanV]

Hi good people of EEVblog forum!

I have a question how does actually BMS' balanced charging works. I'm interested in the passive balancing where the charge of a particular cell is drained through bleed resistor. I also have questions about active balancing but let's leave that for later...

Let's say we're charging a battery that has 3 cells in parallel. All of them have different capacities and charge at different speeds. Let's say we're talking about LiIon cells where we want to reach 4.2 when fully charged. Lets say cell 1 charges the fastest and reaches the set cut-off voltage of 4.2 before cell 2 and cell 3. Does BMS immediately starts bleeding cell 1 as it still is charging cells 2,3 to keep cell 1 at 4.2V and not to go over that voltage? Or is it done when all of the cells reach cut-off voltage and then it drains them when the charging is done?

[opampsmoker]

If they are in parallel, they are all clamped to the same  voltage.

[tom66]

You can't charge batteries of significantly different capacity in parallel.  Cell balancing is usually used to balance out sub-1% capacity variation and is usually performed near the end of the cycle.  For instance, on the Tesla Model S, the cell balance resistors draw only 0.1A from each 239Ah "brick", so it would take over 100 days to fully discharge any given cell.

[Siwastaja]

Did you mean to write "in series"? As it stands, the question doesn't make a lot of sense, because no BMS or "balanced charging" is involved. 3 cells in parallel act as one cell, the voltages are definitely the same unless there is significant wiring resistance between the cells. As a result, the cells are also at approximately the same state-of-charge, regardless of their capacity differences. At large currents, they can be at slightly different states, temporarily, but will even out quickly because of the hard parallel connection. With very large cell mismatches in either capacity or resistance, you'd better derate charging/discharging current well below the sum of the three cells.

[tom66]

If you put different capacity cells in parallel, how is the charge current distributed?  While they will likely be at a similar SOC, I think that due to any variations in the individual batteries ESR, which changes with SOC, will make assessment of the end charge difficult (C/10 termination for instance, will not be guaranteed to be distributed evenly between cells.)

Bottom line would be, only put same capacity and same type Li-Ion cells in parallel with each other.  If they are close enough then they will all have very similar charging behaviour. 

[Siwastaja]

If you put different capacity cells in parallel, how is the charge current distributed?  While they will likely be at a similar SOC, I think that due to any variations in the individual batteries ESR, which changes with SOC, will make assessment of the end charge difficult (C/10 termination for instance, will not be guaranteed to be distributed evenly between cells.)

Bottom line would be, only put same capacity and same type Li-Ion cells in parallel with each other.  If they are close enough then they will all have very similar charging behaviour.

They are forced to the same terminal voltage. As long as the chemistry is similar enough so that voltage vs. SoC curves are close, they follow the same SoC and share current accordingly. A larger cell is more active foil than a smaller cell. You can similarly discuss how a large cell share current internally. External parallel connection isn't that different from extending the foils internally.

But I'd definitely recommend some current derating, because the ESR works against this. At C/10 charge termination, I don't think there is any problem unless one of the cells is in such poor shape it's totally dying.

Personally, I have paralleled some tiny 1Ah cells with massive 100Ah cells no problem whatsoever; same chemistry (LFP). Why? Because I had a large 18kWh second-life pack where the capacities were all over the place, so to maximize the series pack usable capacity, I added the tiny extra cells to the smallest-capacity elements that defined the low-voltage cutoff of the whole pack.

Of course, in reality, it usually doesn't make much sense to parallel different cells. Second-life applications are always iffy, anyway. A proper design uses high-quality cells where the capacities and ESRs are quite well matched anyway, and which are designed to be paralleled.