The BSU range all provide constant voltage to prevent loss of data and possible damage to control units during software updates, and module reprogramming as well as diagnostic processes when in a Key On, Engine Off state.
During diagnostic or programming operations, the vehicle's systems are operating solely from the battery. Maintaining battery charge during this process is critical. The status of all BSU units is indicated simply by three LED's - POWER, POLARITY, DEMAND. When all three are illuminated, BSU is supporting the battery.
