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BMW is reserving quite a bit out of the official 9.2kWh battery capacity advertised.
When I am pretty close to 0% (as reported by the OBC or BMW Connected App), the most I've seen used for a full recharge is about 7.3 kWh from the wall. If we apply roughly 10% efficiency loss, it really took in about 6.5 kWh. It would mean close to 30% in reserve capacity, which is pretty conservative as compared to Tesla and other EVs.
This is also confirmed by the fact that when the OBC shows 3mi/kWh, I can go roughly 18 miles with a single digit % battery left.
Why does BMW do this? My theory is to prolong the battery life (and more importantly, to reduce costly warranty repairs due to premature battery failures). Lithium-ion batteries don't like to be drained all the way, and also don't like to be stay fully charged without inducing premature wear. So, BMW is probably leaving a good amount of reserve and not allowing the battery to be fully charged (e.g. when OBC reads 0%, it may actually be around 20%, and when OBC reads 100%, it may actually be around 90%). When the battery actually loses charging capacity over time, BMW can also mask it by charging it fuller, giving the impression that the capacity hasn't degraded.
So, why does BMW reserve more capacity than Tesla? I think the answer is that because the 530e has such a small battery that it needs to be charged a lot more frequently (2x per day isn't probably uncommon) than a Tesla with 75-100kWh capacity, it needs to be more conservative to prevent early battery failures.
This is all a speculation on my part, so take it with a large gain of salt.
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