[JNoSol]

X5m has 2 batteries, 90mah + 50mah. X50i has a single 90mah while X35i has a single 105mah (which is strange, but I bet the X50i has lithium vs standard - the maintenance scheduled of the X50i is less frequent on the battery compared to X35i). X35d has 2 batteries as well.

I'm very confident that it has to do with e-CAT (electronically heated catylic converters). During cold start, the first battery is used to power the starter (smaller 50mah battery) while the larger battery heats up the e-CAT and all else electronics. This reduced the car's stress from heating up the e-CAT after the voltage is drained from the single battery design, which requires the car to run at the higher RPM to re-charge the battery from a cold start.

The F80 doesn't have an e-CAT. During cold start-up, the turbo wastegates are wide open and the engine RPM goes up + rich fuel to heat up the CATs. So the emission is worst on the F80 during a cold startup compared to an F85. This is also why the F85 sounds so much better at cold starts compared to my last F80.

I wouldn't imagine that if one fails, the other will need to be replaced as well. They're independent batteries tied to the charging grid.