Blog - Page 4 of 4 - Marcucci Consulting

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 on 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 (https://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 devices 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.

Hybrids and EV’s are becoming increasingly popular. Not just the passenger vehicles you see on the road, but also smaller vehicles such as NEVs (Neighborhood Electric Vehicles), LSVs (Low speed Vehicles), ESVs (Essential Services Vehicles), and the myriad of other small utility vehicles being imported or imported and finished in the US. Despite this increase, few options are available for these vehicles for overcurrent protection due to their unique challenges. Some of these challenges are:

Automotive reliability. These vehicles may not always fall under the strict requirements of 4-wheel passenger vehicles (NEVs, LSVs, and any 3-wheel vehicles do not), but they still are subjected to the same environment.
Medium voltage. While 72V, 144V, 288V may be considered medium voltage to those in the circuit protection industry, this is an extremely high voltage compared to the rest of the 12V (and less) systems normally found in an automotive environment. This may sound minor but it can be very hard to find fuses (and holders) that fit these voltage levels.
Unique activation current requirements and curves. Unique solutions for the EV world have unique requirements. Many Battery Electric Vehicles (BEV) and larger hybrids will run motor currents as of 300A, 400A, or higher. To avoid damage to high-cost items, and reduce the risk of fire, a very sharp time-current/I²t curve is needed. Also, the high cost of DC-DC converters to drive 12V accessories, and the lack of an internal combustion engine to power things like AC compressors or power steering, pushes many of these high power items to the traction battery. This means that overcurrent protection is needed that can handle large initial surges (such as with PTC heating elements), as well as repetitive surges.
Environmental requirements. Again, with the environment of the automobile, many different requirements come into play, primarily heat, moisture, and chemical or salt. This is typically not an issue for lower current devices operating at 12V, but for higher voltage branch or accessory circuits, or higher current devices (such as a 400A main fuse), there are fewer options available.

The “Big 3” and other established automakers already have strong relationships with existing suppliers. Even if they are working on a new product, prototype, or low-volume production vehicle, they can leverage that relationship to develop new devices. Many such devices exist that are often hidden in different supplier catalogs and can be very hard to find.

For the industrious engineer, devices can be found for higher voltage systems by searching for these devices simply by voltage. Devices made for forklifts, telecom (their battery systems), and light rail can be a good source of devices. Often, finding a viable fuse holder becomes an issue, as is the case with ATC/ATO types. High-current ANL/ANN fuses for the forklift industry are plentiful but holders are typically open to the environment. Mega, Maxi, J-Case, and Mini varieties have similar issues. Interestingly, glass fuses, which have seen a huge drop in popularity, provide a fair number of options for higher DC voltages than just 12V (usually rated at 32V) automotive fuses.

Far too many engineers simply ignore the voltage requirement which may risk delayed opening times, overheating, or fire. Even worse some engineers neglect to put circuit protection at all on their circuits, which can be disastrous. Some EV batteries can run as high as 400VDC and provide thousands of amps of current. Do you know what sort of welding you can do with that energy?