Using a Best Battery Selector for Highly Reliable Engine Starting Applications

A properly engineered Best Battery Selector ("BBS") consists of two large diodes, specialized heat sinks, voltage surge protection and high current terminals assembled and mounted in a metal housing. Each diode allows starter motor cranking current to flow in the forward direction, but blocks current from flowing in the reverse direction. The BBS got its name because the "best battery" is the one that provides the majority of the power to the starter. Since a BBS is a passive device, it does not perform any switching or control functions, so the best battery is "selected" in a passive manner based on the principle that current flows from the source of the highest voltage. In a system that has well-matched batteries and cabling, under normal operating conditions two good batteries will each provide about 50% of the total power delivered to the starter. In the case of a partial battery failure, the battery with the highest voltage will supply the majority of the starter current. In the case of a total battery failure, the remaining battery becomes the "best battery" and provides 100% of the power delivered to the starter.