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Commercial vehicles usually adopt two 12-V automotive batteries in series to produce the needed 24-V supply rail. Figure 1-1 shows the typical architecture of commercial vehicle power system. Alternator charges the two 12-V batteries placed in series. The stacked 24-V voltage supports the 24-V electronics (or 24-V loads hereafter), while the bottom battery, namely Batt_B, supplies 12-V electronics in the vehicle. Such architecture allows the commercial vehicles to reuse the low cost 12-V electronics (or 12-V loads hereafter) developed for passenger cars, therefore reducing the overall electronic cost of the commercial vehicle.
In this architecture, the battery equalizer is necessary. Without the battery equalizer, Batt_B can be consumed faster than the top battery, namely Batt_T, which would become over-charged and Batt_B under-charged. The over-charge would degrade the lifetime of Batt_T, and the undercharge of Batt_B would lose sufficient supply voltage for the 12-V loads. Consequently, the overall system lifespan as well as performance will be adversely affected.
By introducing the battery equalizer, both Batt_T and Batt_B can stay well-balanced, resulting in maximal battery life time as well as improved system performance. In this report, a buck converter design based on the LM5170-Q1 controller is presented to implement the battery equalizer.
Battery equalizer has two modes to support stable power for both 12-V and 24-V loads.
Constant voltage mode
Figure 1-2 shows constant voltage operation of battery equalizer. When 12-V loads are low-powered, namely Iload ≪ Imax, the 24-V load becomes the main load of the two batteries. The battery equalizer keeps Batt_B voltage to be right at 50% of the total voltage of the two series batteries, such that the two battery capacities get well-balanced.
Constant current mode
Figure 1-3 shows constant current operation of battery equalizer. When 12 V loads consume high power, Batt_B is consumed faster than Batt_T. The battery equalizer will produce 12-V load current to prevent Batt_B from being discharged, therefore the two batteries remains the voltage balance. Obviously, the battery equalizer should provide Iout = Imax.