Don't know for sure, but suspect it will show up on the display screen as a battery or alternator fault. The X3 does not have a dedicated battery or alternator dash fault light. The BMW BER (Brake Energy regeneration) system is programmed to consider the state of the battery and only works when the battery is fairly well charged. On my car, the BER will start working when the battery reaches about 80% charge and will stop working when the battery drops to around 55-60%.
How do I know since I don't have the display meter? I drove around for a month with a plug-in digital voltmeter connected to the 12v. outlet.
http://www.amazon.com/Vector-VEC008-...2438843&sr=8-3
It took about a week after the car was delivered to accumulate enough charge for the BER to begin working. When the battery is partially charged, the voltage will run around 14.1 or so all the time (alternator charging normally - light charging). Once the BER is activated, the charging voltage will drop to around 12.9 to 13.2 volts during steady speed driving, but will shoot up to 14.6-14-8 volts (very heavy charging) under moderate braking.
Without the BER gauge or a voltmeter, the BER function is transparent to the driver. The only possible indication of a very low battery charge is a loss of the highest blower fan speeds when using the automatic mode of the climate control. (X3 manual pp. 104).
The X3 has a "monster" alternator capable of generating up to 3kW for short periods. If they did use a "motor/generator" like a real F1 (electronic) KERS, it could add about 4hp for a long period of time (not the 8 sec F1 limit). About as much as a low restriction exhaust system.