I'm believe that the "Low Battery" warning is voltage based, and not solely driven by the BMS system which is more to control charging (or more specifically over charging). That was certainly the case on P2 models.
The message should go away if you take the car for a decent drive (30 mins +) as the battery will be re-charged from the alternator. If it doesn't - you should get the battery tested at somewhere where they have intelligent testers available (Halford, Kwik Fit, Dealer).
If you search this forum, it's not an uncommon problem...
There is a hack to reset the BMS system without a diagnostic tool which is shown here :
https://www.youtube.com/watch?v=iu9z5m78Szw
The video also shows why the BMS reset is only completed when the car is left locked for circa 8 hours - during this time the quiescent drain is reset and the SOC (state of charge) calculated from the voltage drop.
If anyone's going to post saying this isn't correct - please share you references - happy to eat humble pie in deference of credible documentation....................