System Control Electronics 105 Often used in conjunction with the contactor is a “precharge contactor” or “precharge resistor.” This is a secondary switch that is connected in parallel with the main contactor. The purpose of this switch is to prevent a large inrush of current when the main contactor closes. It does this by closing the secondary contactor first and allowing a small amount of current to begin flowing into the system so that when the main contactor closes to complete the circuit it will do so without damaging any other circuitry or causing the main contactors to become welded together in a closed position, which could be very dangerous ( Figure 3 ).
Figure 3 TE Connectivity precharge contactor. In Figure 4 below, three different schematics for the main and precharge contactors are shown. In the top figure, both the main and precharge contactors are open so no current is flowing through the system. This is basically in the “off” position. In the next schematic the bottom, precharge contactor is closed and thereby allowing a small amount of current to begin flowing into the system, while the main contactor remains open for a predeter- mined period of time.
Finally, in the bottom schematic, the main contactor has also closed and now the system is allowing full current to flow and the system is in the “on” position allowing current and power to be added or removed from the energy storage system. In most systems, there is only one contactor and precharge contactor. However, in some of the very large systems that are made up of several interconnected “strings” or packs in parallel, each string may have its own set of contactors thereby allowing for each one to be run independently.