Advances in hybrid electric vehicle battery technology pose new challenges to contactor designers.

By Earle Alldredge, Product Manager, Tyco Electronics

As hybrid electric vehicle (HEV) batteries have improved in storage capacity and efficiency; associated protective components such as high voltage DC contactors have been pressed to keep pace with the performance issues of newer systems.

The typical HEV battery system consists of the battery pack itself, as well as contactors - the key component in providing the connecting link from the battery pack to the vehicle drive system.

The contactors provide safe isolation of the battery pack when the vehicle is powered-down, or being maintained, as well as being required to carry the motor drive current. In fault situations, such as those that could occur during vehicle impact or when a down-stream component fails, it is the job of the contactors to interrupt high current at battery system voltage.

The challenge to the contactor designer is to provide a device that can quickly interrupt this current so that subsequent system and component failures do not occur.

The contactor itself must be able to perform this function, and be useable after the fault event, unlike other components that are typically intended to be replaced, such as fuses.

Some critical system parameters that must be considered are:

Inrush Currents on Closing: Vehicle drive systems almost always contain significant filter capacitors on the motor drive input.

These capacitors can have upwards of 20-30 millifarads of storage capacity, and due to the low series resistance of the capacitors and very low resistance of the new battery technology, many thousands of amps could flow from the battery pack to the uncharged  capacitors on initial contact closure.

The contactor must be able to withstand these inrush currents without welding, in the event that the system pre-charging function fails for any reason.

Pulsed Drive Currents: While the average currents flowing from the batteries to the system can be manageable, the contactor designer must consider the peak currents also (the "on" time of the switch-mode control), and ensure that contact resonances do not develop. These conditions can lead to contact trembling and overheating, and must be prevented.

Maximum Possible Fault Current: Maximum fault currents that can be seen with new battery systems can approach 8 to 10 kA at DC voltages of approximately 400 VDC.

The contactor must safely carry then interrupt these currents without welding of the contacts. It is also important that the cut-off time is short, so that associated wiring and down-circuit components are not damaged.

The contactors are typically required to perform the fault interruptions more than once, and still be ready for continued regular use.

Check out high voltage relays designed for high voltage DC in a small, hermetically sealed package for operation in electric vehicles and other harsh environments.