Switching Heaving Loads in Electric and Hybrid Electric Vehicles

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November 4, 2012

I have already blogged about the popularity of EVs and Hybrids and the challenges of overcurrent protection. What about switching power? As I’ve mentioned loads upward of 400V and 400A are not uncommon in the electric motors that power EVs and Hybrids. Numerous other loads exist at these voltages where the traction battery (the high voltage battery used to drive the electric motor) is also used to power accessory loads such as air conditioning, heating, the 12V DC-DC that charges the 12VDC accessory battery, and other items.

As with many other components, EVs and Hybrids provide a unique set of requirements for switching loads:

  • Automotive environment: Shock and vibration ratings, IP67 or better sealing , -40 to +85C (or wider) temps
  • Wide variance between control & contact voltages: 12V (or lower) control, 400V or higher switched
  • High isolation: As high as 1GΩ between coil and switched contacts
  • Low contact resistance: High current loads, high efficiency needs, and packaging/temp constraints mandate low contact resistance

For very small current loads (under 1 amp) there are many options by the major manufacturers in DIP reeds. This will work for smaller reference circuits and very light loads. Similarly, FETs and other solid state devices can be used (albeit with constrictions switching heavy loads in evs1on isolation and PC board trace routing). Solid state solutions are also usable at somewhat higher current loads, such as some accessory loads, such as an electric heater or air conditioner. These might  draw 20A, 40A, 60A, or more. Several mechanical relay options do exist from TE, Omron, Schnieder, and others. Formats may not lend themselves very well to automotive use where socketed solutions such as the Form A or Form C have dominated the industry. Solid state devices may prove the most flexible when handling these medium loads.

Unfortunately, there is one heavy load that should be addressed for safety concerns. Most EVs and Hybrids feature a main relay to disconnect the traction battery from the drive electronics when not in use. This functionality also extends to much smaller EVs such as golf cars and sub-25 mph NEVs (Neighborhood Electric Vehicles). Even still, these smaller vehicles may draw hundreds of amps continuously. This main relay may have to handle 400A, 600A, or more, depending on the vehicle. All such vehicles have devices to deactivate the main electronics from the pack with the key off, or to enable the connection to the charger, or both. I have even seen main relays like this used for smaller loads to provide a high-reliability, high-isolation break in the circuit.

There are several devices that can handle this function that are available, but none used as widely as the Kilovac family (http://relays.te.com/kilovac/). Kilovac is a brand that has been around since 1964. In 2002 Kilovac was purchased by TE Connectivity and experienced a whole new surge in popularity through their distribution network. Many switching heavy loads in evs2devices use prior to Kilovac’s recent rise in popularity were “open frame” type devices that were primarily used in telco central offices to connect the DC battery supply used in central office equipment. I have seen more than one of these (many more, in fact) that welded shut, sometimes with disastrous consequences. Terminal contamination, FOD, and any number of issues can cause these units to weld up. While they were designed to activate heavy loads, they were NOT intended for automotive duty or exposure to the elements, which is inevitably how they were implemented in vehicles (not in-cabin, but in an engine bay or trunk area).

The Kilovac units provide a hermetically sealed unit that has excellent resistance to vibration, high isolation, and is even available with a “coil economizer” to help keep the continuous current draw down on the coil. It is compact and really an ideal solution for the EV world for these heavy loads.

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