Confusing state of health

[chris69]

I've mentioned in a couple of other posts that my Mk1 battery state of health reports as being lower than desired. Currently it is 88.4% (confirmed at last service and also my own OBD2 reader) after 28,000 miles and 4 years with relatively little rapid charging. So I decided to try to recalibrate the SoH measurement by draining the battery below 10% (I went to 6%) and recharging to 100% on a slow charger. The SoH reading hasn't changed, but the amount of energy put into the charger is somewhat inconsistent with the SoH reading. I appreciate there will be energy loss due to heat but this is what I found:

A 44.5 kWh battery has an (assumed) 7% top buffer and a 3% bottom buffer so there should be 40.05 kWh useable capacity.
With 100% SoH charging from 6% to 100% should use 37.647 kWh without additional energy loss due to heat etc.
With 88.4% SoH charging from 6% to 100% should use 33.28 kWh again without additional energy losses.
The energy consumed by the charger was actually 41.157 kWh which is more than the useable capacity of the battery and almost 8kWh more than expected with the reported SoH - a difference that surely cannot be explained by energy loss through heat and therefore suggests the SoH reported by the car is very wrong. In fact this logic suggests the SoH should be nearer 100% and not the 88.4% reported.

Could there be another explanation for these readings?

 

[MickeySw]

According to EV database a Mk1 has a usable capacity of 42.5kWh and charge efficiency is about 85% (if it’s similar to our MG5)

In order to fully charge a new battery (0-100%) 50kWh is required or about 47kWh from 6-100%. Less for a battery with a reduced SoH, in your example (88.4%) I’d say about 41.5kWh…

 

[chris69]

So I’ve done some more digging and need to correct my assumptions. The Mk1 requires 48.085kWh from empty to full. This equates to 45.199 from 6% to full. 41.157kWh is therefore equivalent to 91% SoH and would be closer to other cars of the same age and mileage and only just below where it was 6 months ago when I bought it. I think my car may be incorrect with the SoH reading.

 

[MickeySw]

I think my car may be incorrect with the SoH reading.

I’d say your SoH difference is well within system tolerance but if you’re bothered about it repeat your battery calibration on a warm summer’s day and all will be fine..

 

[chris69]

I’d say your SoH difference is well within system tolerance but if you’re bothered about it repeat your battery calibration on a warm summer’s day and all will be fine..

It was a warm summer day lol. I was more concerned about the rapid SoH drop over the past 6 months (hence the attempt to recalibrate) and hoped my thoughts may help someone else.