BMS began showing up in 2011 as a system to monitor the condition of the battery. The BMS detects when the battery is failing or getting weak. And that's still it's job today.
The BMS establishes a base line for the battery when the vehicle is off (BMS needs 8hrs of vehicle undisturbed off time to establish the base line of the battery).
Once the BMS establishes the baseline of the battery, it can then detect when the battery is getting weak overtime. This is especially important for the A.S.S. feature because the starting of the vehicle is the BIGGEST draw on the battery. Couple that draw with accessories running while the engine is off and that's an even bigger load on the battery when the starter engages.
Turn your truck on and off multiple times in a short period of time, the battery doesn't get enough time to fully recharge.
Cycle multiple heavy loads (Starting the engine) over and over and it wears on the battery.
Therefore, we have the BMS which monitors the health of the battery and will stop the A.S.S from engaging if the battery isn't fully charged or is getting weak.
The BMS does not determine voltage output of the alternator. The PCM does. PCM tells the voltage regulator (which is built into the alternator) what voltage to output and maintain.
If the PCM tells the voltage regulator to output 14volts that's what the alternator does. Turn off all of your accessories and you get 14volts. Turn on ALL the accessories including all of your added lights and you'll still get 14volts, but the current output of the alternator with a full load is increased to supply power for the additional load to keep the system at 14volts. It's Ohm's law.
