Is this SoH OK?

[Data]

The BMS will use coulomb counting, rather than voltage for SOC calculation.

Because the upper knee is so steep the delta will be quite large (i.e. compared to NMC cells) when cell voltages are over 3.45V per cell, despite them being relatively well balanced.

Graph below is no-load voltage vs. SoC for typical LiFePO4 cells

That's perfectly correct Everest. Good info.

 

[Everest]

Anyone care to explain

My understanding, which is from LiFePO4-based home energy storage balancing (so may be different with EV's) is that balancing is based on cell voltages, but with LFP can only commence when the cells are above a certain threshold (typically about 3400mV) as below that voltage is not a good indicator of state of charge.

However, those voltages will only be reached when there is a charge current being applied, even if that current is small (e.g. my home cells will be charging with a current of only 4A at that point).

Once the charge current is removed, the cells will soon settle to a lower voltage (say around 3600-3800mV). Hence balancing - at least for home storage systems - is performed when cells exceed (say) 3400mV AND when a charge current is still present.

The challenge, therefore, is to be able to balance enough, in the short time there is before the highest voltage cell reach the designer's maximum voltage (likely to be between 3500mV and 3650mV (the latter being the absolute max recommended cell voltage for LFP))

So, in response to your specific points... my 2p worth would be...

Balancing (at least the way I understand it) commences when charging is completed (I.e. when the first cell reaches the cut-off voltage).

I suspect that it will be done during the upper SOC levels but during charging, rather than when complete.

Balancing therefore works with cell voltages (not SoC) which can be determined quite accurately, despite them being in the upper knee?

Absolutely based on voltage, but because, rather than despite them being in the upper knee.

Happy to be corrected by those on here who are more familiar with EV BMS's compared to home-storage BMS's though (e.g. @T1 Terry & @Coulomb)

 

[Data]

Balancing (at least the way I understand it) commences when charging is completed (I.e. when the first cell reaches the cut-off voltage).

Passive balancing:
The BMS then determines the lowest cell voltage and turns on the blalancing resistors of every cell above that level and thus ‘bleeds off’ excessive capacity (turning it into heat) until they are all equal.

Active balancing:
Excess capacity of the highest level cells is used to bring up the lower levels cells until an equilibrium is reached.

Balancing therefore works with cell voltages (not SoC) which can be determined quite accurately, despite them being in the upper knee?

None of these methods require additional energy from the EVSE therefore I’m puzzled why there seems to be a continuous (albeit reduced) power draw after charging is completed.

Both methods would also suggest that when balancing is completed, all cells would end up at a lower level than cut off voltage.

Anyone care to explain where I’m going wrong?

Cell voltages determine state of charge. I think you are going wrong because you are assuming that balancing brings all cells up to the same level. It doesn't always, even though that is the aim it isn't possible when some of the cells have started degrading. Cells degrade, charge & discharge all at differing levels & with differing timings. As such the BMS has to assume a median level of charge & will display that median charge. This is why as the battery degrades it can become more unbalanced, the range shown especially as the battery depletes nearer the lower end of soc can suddenly go from say 35 miles to 11 miles in the space of 3 or 4 minutes leaving the driver in panic, thinking they will not make it to the next charging station. Initially this whole process is happening in a very small way but can increase over long periods of time.

Let's not forget our batteries are very robust & last a long time whether they are LFP or NMC. They don't break easy! No one should be worrying to any degree if you are following the manufacturers instructions. The engineers who design the batteries know what they are doing & talking about.

 

[MickeySw]

Cell voltages determine state of charge. I think you are going wrong because you are assuming that balancing brings all cells up to the same level.

I’m pretty certain that (near) perfect top balancing is possible with degraded cells; SoH of my 15 year old EV is around 80%, yet top balancing is within a few milivolts??

It doesn't always, even though that is the aim it isn't possible when some of the cells have started degrading. Cells degrade, charge & discharge all at differing levels & with differing timings. As such the BMS has to assume a median level of charge & will display that median charge.

A HV battery is usually made up of a bunch of individual cells all connected in series.

The current going through each cell is therefore the same. Degraded cells will have a lower capacity than the rest, however their voltage/SoC ratio hasn’t changed.

Their voltage will therefore drop faster and the imbalance will become larger and ‘fall off the cliff’ rapidly at the lower end of the curve.

When changing, the opposite happens, degraded cell’s voltages increases faster and once they reach cut off voltage, they will have more or less caught up with the good ones, any difference can be corrected with balancing.

During a BMS calibration, coulomb counting (integrating current over time) is used to measure the energy added until the 1st (usually the weakest) cell reaches the cut off voltage.

Let's not forget our batteries are very robust & last a long time whether they are LFP or NMC.

Agreed, but I still like to know how this is achieved exactly..

 

[Data]

I’m pretty certain that (near) perfect top balancing is possible with degraded cells; SoH of my 15 year old EV is around 80%, yet top balancing is within a few milivolts??

A HV battery is usually made up of a bunch of individual cells all connected in series.

The current going through each cell is therefore the same. Degraded cells will have a lower capacity than the rest, however their voltage/SoC ratio hasn’t changed.

Their voltage will therefore drop faster and the imbalance will become larger and ‘fall off the cliff’ rapidly at the lower end of the curve.

When changing, the opposite happens, degraded cell’s voltages increases faster and once they reach cut off voltage, they will have more or less caught up with the good ones, any difference can be corrected with balancing.

During a BMS calibration, coulomb counting (integrating current over time) is used to measure the energy added until the 1st (usually the weakest) cell reaches the cut off voltage.


Agreed, but I still like to know how this is achieved exactly..

Not quite sure that you have quite a full grasp of EV batteries Mickey. No disrespect intended. However, in response to your question relating to how good long battery life is achieved, I refer you to my earlier post. It's simple.