All of the computers, lights, etc. are powered by the 12vdc system. The 12vdc battery gets recharged during operation once the high voltage circuit turns on. If the 12vdc battery dies, and you plug in an EVSE, the car won't immediately start to charge since the controls won't be active, but there's a backup trickle charger to the 12vdc battery that will try to bring it back up to a high enough value to then run the car. But, in the interim, you have a big brick unless you change the battery. There's a very special procedure to try to recharge the battery...you must disconnect the high voltage first, or you'll end up damaging things or yourself.
Note, the charging logic in BMWs adjusts based on the type, size, and age of the battery installed. When you change to a new one, even if it's the same in every way (except being new), it will be treated as if it were an old one based on the timer. So, you need to reprogram the logic to restart the timer, and treat the battery as a new one. Should you change the type or size, that would need to be updated as well, or you could damage the new battery.
Some time, the car is too smart...
2014 i3 BEV, 2021 X5 45e
(The i3 will be sold soon, <17K-miles, interested?)