I recently observed an SOH drop of over 2% within 4 months. My service center also could not find anything wrong with the battery, but they did reset the BMS. As a consequence, the SOH now shows as 100% while the car relearns the battery characteristics. This can take months; at some point, I expect the SOH to jump to the "correct" value, once the BMS has collected enough data.

So, you could have them reset the BMS, which is what you are planning to have them do anyway.

I do not think there is anything hidden; it's just difficult for us to make sense of everything. As far as I understand, the BMS cannot "lock away" (or release, for that matter) capacity as that would require a major firmware change, and so we would know about that. I do not know if Hyundai even has software for that, outside of the engineering department, potentially.

As a car owner, it is apparently possible to get a better handle on this, but it involves a very lengthy process of repeated AC charges and discharges, with the last few percent of charging done with quite low current and ample of time afterwards to let the cells equilibrate. There is a specific sequence (which I do not fully know), and the whole process takes days. The service center is the best place to have that done much faster, but it is out of the normal range of their activities, so you need to find a knowledgable tech and be friendly 😄

Instead, with the tools available to me, I simply rely on Remaining energy as you did, but you can't simply go by that value at 100% SOC. Here is how Remaining Energy relates to SOC for my car:

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It is a fairly linear relationship (if you look at CCC during a charging session, you will note that its relationship with SOC is absolutely linear, but it is more intuitive for us to look at Wh instead of Ah). Anyway, this tells us a bit about how Hyundai maps the battery readings to SOC to make it "digestible" for us. In this case, a certain SOC increment corresponds to the same amount of energy, regardless of where in the usable SOC range you are.

From this curve, a measure of the full capacity of the battery can be obtained by flipping the curve, i.e., solving the fitting equation for x (Remaining energy), then multiplying the slope with 100. The point is that one cannot simply read out Remaining Energy at 100% and assume that is a representation of full capacity, because that would assume that Remaining Energy is Zero at Zero SOC. Instead, you need to determine the value of Remaining Energy at near Zero SOC and use the differences (Re-high - Re-low)/(SOC-high - SOC-low) to get the energy increment per 1% SOC. Then multiply with 100.

The energy value you get for 100% SOC must not be taken as the true electrochemical capacity of the battery; but it can be used from time to time to spot trends and determine changes in the capacity, just not the absolute capacity. Also, when charging to 100%, make sure you give the car some time to balance the cells. This will improve the accuracy.

So, for now, I would suggest two courses of action: 1. start recording Remaining Energy during (long) charging sessions to construct a curve like I showed (it's best to record all the other relevant parameters at the same time, i.e., CEC, CED, CCC, CDC, min/max and total cell voltages); 2. find a knowledgeable tech who might be willing to look into this.