In order to meet and exceed the performance offered by internal combustion engine (ICE) vehicles, electric vehicles rely on sophisticated high-voltage systems that deliver the staggering amount of power needed to propel the vehicle wherever the owner needs to go. Of course, storing and utilizing this power is not without risk. To be clear, HV battery systems represent inherently safe technology to the end user, but to a service or repair technician, the risks are greater.

In this article, we’ll take a look at some of the safety systems commonly used in high-voltage electric vehicle systems.

Safety Systems in High Voltage EV Systems

First Things First: Insulation Guards

Insulation guards are one of if not the most crucial safety system components in high voltage battery control units, and here’s why. If one of the other high voltage electrical components has an electrical leak, the result will be an insulation error. The purpose of the insulation guard is typically two-fold. First, insulation guards are used to detect the insulation error and communicate said error to the user via the display panel (by way of a warning). Second, since electrical leaks can pose very serious harm, the vehicle will essentially become inoperable upon the detection of the insulation error until such a time that the vehicle can be serviced. It is worth noting that insulation guards can only function properly provided there are sound connections between all of the high voltage components. If some of the components are not bonded correctly, there is a chance that the insulation guards cannot test the system effectively.  

HV Interlock Loop

What exactly is a high voltage interlock loop? How does it keep owners and service personnel safe? An interlock loop consists of a low current, 12-volt safety loop circuit. These safety circuits are tied into all high voltage components and system connectors. When a fault in the current is detected, the main battery connectors will open thereby removing power from the high voltage system. In the event the connectors are already open when the current fault is detected, the battery connectors cannot be closed. In either case, all high voltage components are effectively cut off from the power supply, which in turn creates a safe environment in which the vehicle can be serviced.

Active/Passive Discharge – What is it? 

Active and passive discharge has to do with the capacitors that typically store the same voltage as the associated HV battery. Capacitors are used to stabilize and store the AC voltage that is converted to DC voltage using a rectifier circuit within the HV system. When a vehicle requires servicing, the technician will need to dissipate the electricity stored in the capacitors which can be done by active and passive discharging. 

Active discharge uses a low ohm switched resistor that is wired in parallel with the capacitor. When the ignition is turned off, the switched resistor will close, and the capacitor will quickly be drained of all remaining voltage. The reason this discharge method is referred to as “active” discharge is that the circuit must be manually (i.e. deliberately, or actively) closed in order for the electricity to dissipate.   

Passive discharge, as one might infer, uses a high ohm resistor that is wired in parallel with the capacitor. Unlike active discharge, the resistor used in passive discharge does not have a switch, meaning that it is always a closed circuit. Since the circuit is always closed and there is no switch, the associated capacitor is always being drained, albeit much more slowly than that observed in active discharge. In order for a technician to drain the power from the capacitor, power must first be shut off to the inverter. 

HV Battery Contactors

If you’re familiar with a relay, consider a high voltage battery contactor as its heavier duty cousin. With any high voltage battery, there is always a risk of creating an electrical arc anytime time the ignition is turned on or off. Electrical arcs are by no means universally an HV battery problem. Any electrical switch can result in an electrical arc whenever a circuit is closed.  The manner in which these harmful arcs are mitigated in HV batteries is ingenious in its simplicity. 

The first of the three contactors within the HV battery will close as soon as the ignition is turned on. An arc cannot be created with the closing of the first contactor because the circuit is still open at this point. The second contactor will complete the circuit, but it is coupled with a high ohm resistor that reduces the voltage to such a low level that an electrical arc will not occur as the second contactor closes. The third contactor can also close without risking an electrical arc because the circuit has already been closed, and the power reduced. With the circuit fully closed, and a path of least resistance established, the second contactor will open (as the path with the resistor is not needed) and full power can be delivered as needed. 

Managing Thermal Energy

Managing heat energy within an HV system is vital to its longevity, but there are more dire consequences associated with poor thermal energy management. Consider for a moment the optimal conditions for an HV battery include temperatures in and around 77 degrees Fahrenheit (or 25 degrees Celsius). Because EVs are used in all types of climates that are a far cry from optimal, it’s important to know how thermal variations can negatively impact an HV battery. For example, in low temperatures, the chemical reactions within the battery can slow, which in turn can mean reduced output and performance. High temperatures can also impact an HV system, though perhaps more drastically. At incredibly high temperatures, a phenomenon known as “thermal runaway” can occur which can result in a battery bursting into flames. All this to say that both a battery heater and one or more battery coolant systems must be used to ensure thermal energy remains within range. 

Mitigating Risks in HV Battery Systems

Even the safest, cutting-edge technologies can pose a risk to health and life under specific conditions. With that in mind, anyone who is considering making a career out of installing or repairing high-voltage battery systems in EVs should have a good understanding of the safety systems that are used to mitigate those risks. 

As technologies evolve, the safety systems that are used will in turn increase in sophistication. This in turn means the technicians that work on the systems will require equally sophisticated training. If you enjoy working on vehicles or see yourself employed as an electric vehicle technician, contact a Program Consultant by phone, toll-free at 1-888-553-5333, or email at [email protected]