Is this SoH OK?

[MickeySw]

However, any reported SOH will need to be taken with a pinch of salt because, for EV applications, manufacturers build in a hidden "upper" buffer of unusable capacity. Hence any significant degradation in SOH can be mitigated by the EV's BMS software 'releasing' some of that buffer in to general usage so as to reduce the apparent degradation of SOH to the average user. So, without knowledge of the amount of upper buffer that has been deployed, the SOH is somewhat meaningless.

EV manufacturers deliberately underreport the max capacity of a HV battery by a few %, therefore SoH will remain 100% until that ‘buffer’ is ‘used up’.

I agree, a SoH of 100% is somewhat ‘meaningless’, however once you drop below 100%, SoH will become accurate.

Imho the only way for the BMS to release ‘unusable hidden upper buffer’ for ‘general use’ is to increase the cell voltage, has anyone seen evidence of that happening?

 

[Everest]

OK, I didn't mean increase the SOC to which the battery is charged, I meant a release of top buffer to keep the apparent SOH higher. From what I have read about on the Internet (so it must be true!!) some manufacturers, e.g. Tesla and VW for example, have algorithms to make some of that under-reported capacity available. I suspect that would be done over time, rather than all at the beginning - but who knows

 

[MickeySw]

OK, I didn't mean increase the SOC to which the battery is charged,

The battery is always charged to 100% SoC, however the actual capacity is a bit higher than the spec but all calculations, (range, SoH) are done using the nominal (lower) value.

Once the degradation has ‘used up’ the ‘overcapacity’ SoH will start dropping.
There is some ‘anecdotal evidence’ as handful of preFL SR MG5s still show 100% SoH after years of (ab) use, it’s thought they were actually equipped with ‘reject’ LR battery packs??

I meant a release of top buffer to keep the apparent SOH higher.

The top (and bottom) buffers are created by limiting the voltage range the BMS will use (e.g. 3V-4.2V) when the cells themselves are capable of more (e.g. 2.8 - 4.3V).

Increasing the top ‘cut off voltage’ will add more capacity but it’s also thought to increase degradation, not sure if there is any net gain?

 

[Data]

OK, I didn't mean increase the SOC to which the battery is charged, I meant a release of top buffer to keep the apparent SOH higher. From what I have read about on the Internet (so it must be true!!) some manufacturers, e.g. Tesla and VW for example, have algorithms to make some of that under-reported capacity available. I suspect that would be done over time, rather than all at the beginning - but who knows

All manufacturers make the upper & lower buffers available to the BMS. This normally happens only during the equalisation/balance charge. That is where all battery cells are balanced with, in effect, excess energy above 100%. This buffer must also be accessed to prevent premature degradation of the buffer. That's why doing these 'proper equalisations' is so important. Running for long periods with unevenly balanced cells reduces battery capacity (as you mention) & reduces state of health of the battery as the BMS will recognise the reduced capacity which after a short while becomes permanent.

 

[MickeySw]

This buffer must also be accessed to prevent premature degradation of the buffer.

Can you elaborate on the above?

That's why doing these 'proper equalisations' is so important. Running for long periods with unevenly balanced cells reduces battery capacity (as you mention) & reduces state of health of the battery as the BMS will recognise the reduced capacity which after a short while becomes permanent.

No doubt ‘proper equalisation’ will optimise battery performance, but the whole idea of a BMS calibration is to correct an inaccurate battery capacity, I.e. there is no permanent loss.

Apologies for using the MG5 as an example but I don’t have the details for a ZS EV

The LR model uses 96 cells with 174Ah, (163Ah usable). Each 10mV of cell voltage is therefore equivalent to about 1.4Ah (assuming linear distribution for simplicity).

Let’s say one cell is 30mV higher than the rest which means a loss of capacity of 95 x 4.2Ah or about 2.5% overall.

I don’t know at what imbalance the BMS is requesting driver action.

I do know however, that our MG5 currently has an imbalance of 13mV after 30kmiles (charged to 100% the odd time but without ever doing a BMS calibration), I recon this leaves me short less than 1%?

Therefore don’t loose seep if you haven’t done a ‘proper equalisation’, BMS knows best…

 

[Data]

Mickey, as per the MG manual (page 9), "battery equalisation assists in extending the battery service life". It maintains range also. If you don't do this often enough or at all your battery will degrade more quickly. It's proven, it's well known. The degradation becomes permanent as time goes by. State of Health drops. Of course there are other factors that can affect how quickly a battery degrades. We all know what they are. You keep mentioning cell voltages. You don't need to concern yourself with those if you equalise regularly. Sure you can check them with an obd2 reader but it's not required in normal use...unless you are really curious!