An electric vehicle (EV) is as safe and reliable as any vehicle that uses gasoline or diesel fuel. But, as with any vehicle, an ounce of prevention is worth a pound of cure. By being aware of the potential hazards, you can prevent damage to the vehicle as well as injury to yourself and others.
No single component is capable of meeting the safety needs of your EV. The full defense comes from safety in numbers—the more components you have to protect your propulsion system, the less vulnerable your vehicle will be in the event of an emergency. Here’s a rundown of the main gear to make your EV ride as risk-free as possible.
The motor should be sized appropriately for your application—don’t try to use a 10 hp motor when 20 hp is required. An undersized motor can overheat and eventually burn up.
Even with a properly sized motor, you’ll still need to be conscious of insulation system ratings—which range from 90°C for low-end commercial motors to more than 300°C for high-end military ones. The higher the insulation’s thermal rating, the better the motor can withstand heat and overloading. If the motor’s insulation system is compromised, armature and field windings can begin to short out—eventually causing the motor to overheat and possibly ignite. The best commercially available EV motors have insulation systems rated for 180°C (UL Class H rating).
Most converted EVs use multiple flooded-cell, lead-acid (FLA) golf cart batteries because they’re available, affordable, and durable. But FLAs have very low internal resistance, and some can supply 2,000 amps or more into a short circuit. This magnitude of current can easily cause a fire or explosion if there isn’t a quick and reliable way to interrupt it. That’s why multiple emergency disconnect devices—such as fuses, circuit breakers, and contactors—are necessary. If one emergency disconnect device should happen to fail, others are there as backups.
Safety Fuses. When too much current flows, a fuse provides an automatic interruption of power by breaking the circuit. In an EV, use at least one safety fuse at the main battery pack to protect system power components from damage due to excessive current. To perform as needed, a fuse must be rated at the voltage, current, and time characteristics appropriate to its application. For example, if a propulsion-pack voltage is 120 V and the motor controller current limit is 500 A, the safety fuse should also be rated to handle these values. “Blowout time” can be slow or fast, depending on your preference: The fuse can blow instantly, or it can be designed with a delay curve. This allows it to tolerate a slight overcurrent for a short time, preventing “nuisance blows” due to a momentary current spike that can be safely ignored.
Several types of safety fuses can be used in an EV. An enclosed safety fuse, such as the Ferraz Shawmut A30QS500-4, is best because the fuse element is enclosed in a fire-retardant powder. This powder and its fiberglass or ceramic case can alleviate a fire or explosion in the event of a very high-current overload. With its open construction, a traditional fuse link is usable in most applications. It should, however, be enclosed in a piece of high-temperature insulated tubing, such as phenolic, when mounted above a battery. If an unenclosed fuse link should happen to blow, it will spew molten balls of metal that could burn right through a battery case—ruining the battery, or worse yet, causing a fire or explosion. For maximum safety, every implementation or control line that ties into the propulsion system should be protected with a small, high-voltage fuse rated at 1 A or less. Each fuse will protect its small-gauge wire from catching on fire if a short ever occurs within the instrumentation or control circuit. To provide maximum protection, the fuse should be located as close as practical to the propulsion system’s tie-in point.
Circuit Breakers. A circuit breaker provides a fail-safe manual and/or automatic interruption of power. It also can be used to shut off battery power during servicing or repairs. As with a fuse, a circuit breaker must be rated for the voltage, current, and time characteristics appropriate to its application.
For maximum safety, a circuit breaker should be mounted within easy reach of the EV driver. This can be accomplished by mounting the unit in the dashboard or console of the vehicle, but requires running the large-diameter battery cables into the vehicle’s interior. An alternative is to mount the circuit breaker with the power electronics. Then, it can be activated by a push-pull control cable and knob located on the dashboard or console. This shortens the length of high-current cable used, and keeps high voltage and high current out of the vehicle’s interior.
Contactors. In an EV propulsion system, high-voltage inductive loads and extremely high current levels are common. A contactor is an electromechanical device that switches high voltage and current by means of a low-level control voltage. It is similar to a relay, except that it is much more robust and can switch highly inductive loads that might otherwise burn up or weld the contacts of a relay when the circuit is opened.
This “main contactor” is installed in the positive or negative line between the propulsion battery pack output and the motor controller input so that battery power can be applied or interrupted with the vehicle’s key switch. Even though a main contactor’s primary function is to carry current, the type used should be capable of breaking current to an inductive load (i.e., motor) in case of a shorted controller.
The main contactor should be rated for the high voltage and current characteristics appropriate to its application. If a propulsion pack voltage is 120 V and the motor controller current limit is 1,000 A, the Albright Engineering #SW-200B is commonly used. A single #SW-200B contactor is rated only to 120 VDC. When a vehicle’s battery system is above 120 V, two #SW-200B contactors may be installed—one in the battery positive line and the other in the battery negative line. Two such main contactors, when activated simultaneously, possess a voltage rating of more than 240 VDC because of their collective interrupting capacity.
Albright contactors include magnetic “blowouts”—small magnets mounted in the switching head of the contactors that have the capability of forcing an electric arc outside of the device, effectively lengthening the path over which the arc must travel. The longer distance an arc travels, the better the opportunity to extinguish it.
Inertia Safety Switch. This device will disconnect the main battery pack from the propulsion drive system in case of an accident. The inertia safety switch is a normally closed, low-voltage/low-current device that switches to open circuit on impact. In an EV, this switch can be wired in series with the 12 V power that activates the main contactor. This switch is rated to perform only low-power switching; the main contactor does the heavy-duty work.
A “ground” usually refers to something that is electrically tied to earth ground, but it can also mean being electrically connected to a metallic structure. The high-voltage system in an EV includes the propulsion batteries, contactor, overcurrent protection, disconnects, controller, drive motor, onboard charger, and any lower-current devices that tie into this system. No part of the high-voltage system should be connected to the vehicle frame or metallic body structure. Isolating this system minimizes the possibility of being shocked when touching both a connection point (i.e., a battery terminal) and the body metal. This also minimizes the chance of having a short circuit to the metal body structure or frame if wire insulation becomes frayed or cut and comes in contact with the metal. Some components, including controllers and chargers, are designed not to activate for safety reasons if a high-voltage ground to the chassis is detected.
An EV’s wiring should be sized to safely handle the current. Too much current in an undersized wire can cause it to get hot or even catch fire. EVs typically have low-, medium-, and high-current circuits. Low-current circuits, such as instrumentation and control lines, can be wired using #16 to #20, insulated, stranded copper wire. Medium-current circuits, such as to the vacuum system, DC–DC converter, and battery charger, can be wired using #10 to #14 wire.
Most EV conversions use #2/0 welding cable for the high-current propulsion drive system and between the propulsion batteries. With its many strands of very fine wire, flexible welding cable is preferred over stiffer electrical cable types. #2/0 welding cable is suitable for most EV applications of several hundred amps. If the cable lengths are short (as is best) between the motor and controller, #2/0 cable can support up to 1,200 A intermittently. When deciding the proper size of wire or cable to use for an application, consult with the wire manufacturer’s ampacity tables. Keep in mind that these tables are for continuous current flow—not intermittent.
Heavy-duty automotive-type posts with crimped clamp-style—not pressure-contact—terminals are the best connections to use with propulsion batteries. The combination of an automotive post and a heavy-duty clamp terminal offers low contact resistance, low maintenance, and high reliability. Once a clamp terminal is crimped onto a cable end, the copper conductors in the cable can be sealed from potential corrosion by using heat-shrink tubing that contains a mastic glue sealer. Another advantage to using clamp-style terminals is that red and black rubber covers are available to further prevent corrosion and also to insulate them from electrical shock hazard. Ring lugs with L-terminals are a good second choice.
In a typical EV conversion, most of the propulsion batteries will be located in the rear of the vehicle, with only a few batteries in the front. This almost always results in running two interconnecting cables beneath the vehicle. To minimize the risk of these cables being scraped or cut by road debris, place them within jacketing, such as flexible or rigid PVC conduit or rubber heater hose, for protection. Likewise, wires of any size that are routed through sheet-metal holes or close to sharp metal edges should be jacketed with plastic wire wrap or grommets to prevent cutting or abrasion.
Ken Koch is the former owner of KTA Services Inc., an electric vehicle conversion kit and component supply business that has sold more than 1,200 conversion kits since 1984. His latest business venture—EV Consulting Inc. (www.evconsultinginc.com)—provides consulting, system design, and computer modeling for DC systems.