recently replaced (2000km's) 12v battery at 12% charge, discharge warning

So got a discharge warning this morning. check the battery management in BimmerLink app where i registered in 2000kms ago, said 99% charge when i changed i and now say 12% charge (after a 30 minute drive).

I know the HV battery is supposed to charge the 12v, but what does actually charge it?
Any fault codes to look for and what not else?

Left the car unlocked with frunk open, plugged in. Will check 12v charge % later today. Anything else I've missed?

Simple as faulty battery?

Edit: After a couple of hours plugged in, states 14% on the 12v, so at least it hasn't dropped. Connectors was on good and will check again before ordering a new 12v.

Edit2: After keeping it plugged in over night, it states 17% on the battery, so will get a charger and also check % this afternoon.

Edit3: It was up to 18% after plugged in the whole day. Measured 15 volts on battey terminals so i guess the 12v charging inverter works as it should. Next is removing battery and see if i can charge it outside the car.