Electrical connections for high-power EV applications must be capable of handling sustained high voltages and high currents to provide the power needed to propel vehicles electrically.
The common unit used to measure engine performance is horsepower (HP), which is equivalent to 750 watts of electrical power. As such, a 100 HP EV motor would require 75 kW of electrical power to have the same performance. A high-performance EV sports car could easily require more than 300 kW of electrical power. Using a 750V power inverter, that would be 400A, even before taking into account the varying phase alignment of applied voltage with applied current. In reality, the power equation is a more complex rotating vector calculation.
Viewing these high-voltage and current requirements from the perspective of typical automotive electrical connector designs, a number of issues need to be resolved in order to create a reliable, high-performance EV electrical connector.
Figure 1: Imperium high-voltage high-current harness connector and header suitable for EV use. (Source: Molex)
EV Battery Thermal Management System
High current produces power losses that are proportional to the contact resistance between the mating surfaces of the connector. If the connector had only 1 mΩ of contact resistance, a 400 A current level would produce 160 watts (I2R) of power lost as additional connector heat.
Automotive connectors must survive in environments with broad temperature extremes that may range from -40° C to 105° C. In the connector industry, it is standard to limit the application-induced temperature rise on the terminals to no more than 30° C to maintain the desired mechanical tolerances as the terminal material expands due to internal heating.
A 160 W power loss can easily create such a temperature rise, if the heat cannot be dissipated effectively through the connector to the surrounding environment. The connector in Figure 1 is designed to have less than 50 µΩ of contact resistance, which is 1/20th the resistance of the 1 mΩ example above.
Contact Resistance
Contact resistance is also a function of the force compressing the two connecting components of a terminal together. High compression forces result in a high-insertion force, which is undesirable in a modern automotive manufacturing environment. In response, manufacturers use various lubricants to both protect the surfaces of the contacts and reduce insertion force. The shape, design, material, and plating of the terminals are also important design considerations in reducing insertion force.
High voltages can introduce a phenomenon known as dendritic growth, where certain metal ions are encouraged to move from one physical position to another due to the force of the electric potential between the connector terminals. Metal migration can eventually result in a short circuit between the connector terminals. Prevention requires a careful selection of metal compounds used for plating the terminals.
High voltages can also introduce arcing between the terminals and ground connections due to the breakdown of the insulating properties of plastics and air in the presence of moisture or other environmental contaminants. Preventing these effects requires a larger mechanical spacing of the contacts relative to the circuit ground as part of the design. This is commonly known as providing the necessary clearance and creepage.
EV Charging Connectors Dimensions
Mechanical dimensional requirements can result in EV connectors that are larger and heavier than desirable, which works contrary to the industry trend to smaller, lighter and more economical electronic controls and connectors. Managing the physical size and mass needed to conduct high currents is one of the more significant challenges in designing EV connectors.
The metal wires used in EV applications must maintain high electrical conduction and efficiency. Copper is the usual choice because of its low-cost, high-conductivity, ductility (which allows it to be more flexible during installation), and the fact that it conducts heat well. Unfortunately, copper is also relatively heavy.
Many factors, including alloy composition, annealing properties, strand number and size, and frequency of the signal being conducted, influence the amp capacity of a wire. Standard industry tables indicate that a 4/0 (0000) stranded-copper wire can carry 380 A of current in open air. The copper in that wire is approximately 0.6 inches in diameter. Large amounts of copper, combined with required protective insulation covering, can become very stiff and heavy.
Mechanical Stress Load, EMI/EMR
During vehicle operation, the movement of a significant copper wire mass results in high-vibration stress loads on the connector body holding the cable, the power electronics enclosure and the connector terminals. Automotive applications, where power components are exposed to road vibration and environmental conditions, such as dust, water, solvents and steam cleaners, additionally require strategies to maintain protective seals along the cable and connector body-to-power enclosure interface. The design goal is to keep environmental contaminants out of the power electronics enclosure, while preventing stresses caused during vehicle operation from damaging the connector body, power electronic enclosure, terminals or the wire.
Compounding these design challenges is the fact that most EV applications control the power applied to the motor by pulse-width-modulating (PWM) the applied current, which introduces high electromagnetic radiated and conducted emissions; the wire must be shielded (usually with a grounded braid) to prevent corruption of other vital electrical signals in the vehicle. The quality of the connection between the shielding braid and the ground of the power electronics enclosures is vital for overall system performance.
EV Charger Lifespan
Every stage of EV design—even the connections of terminals to the large copper cables—requires solving long-term reliability challenges. For decades, the method of choice for assembling terminals to large copper cables has been to crimp the terminal (lug) onto the cable. The crimping process inserts high stress on the terminal body, deforming its shape and gripping the cable tightly. EV systems pulse high power through the joint between the cable and connector terminal. The cycle of localized heating and cooling of the interface and joint causes minute expansion and contraction of the material. Frequently, as the material relaxes during cooling, it does not completely return to its original crimped dimensions.
Thousands of hours of operation can cause long-term degradation of the joint contact resistance between the cable and the terminal, which can eventually result in connection failure requiring cable/connector replacement.
The high-vibration automotive environment can also cause issues between the male and female connector terminals. With such large masses involved, the position, location and assembly of EV cables can inadvertently allow road vibration to be transmitted down the length of the cable and ultimately affect the interface integrity between the two halves of the connector terminals. Pure copper corrodes quickly, so depending on the choice of protective plating applied to the terminals a phenomenon known as vibration-induced fretting can introduce microscopic wear and surface corrosion of the male and female terminals. If the terminal plating thickness and assembly process is not precise, an increase in the contact resistance between the two terminal halves can result in a reduction of connector performance and overall reliability.
High-Power EV Connector Safety
Safety is always a major consideration in automotive design, especially when dealing with high voltages and currents. UL does not consider voltages below 60 VDC hazardous enough to require special safety designs to prevent unintended human contact with connector pins. As voltages rise above that threshold, the internal resistance (impedance) of the human body to the flow of electric current begins to decrease, potentially allowing current to rush through the body producing injury and even fatality. Because of the significant risks, high-voltage EV connectors must be designed with utmost consideration for operator and manufacturing safety.
Incorporating a High-Voltage Interlock (HVIL) circuit and following international safety agency design requirements for touch-safe connections are important strategies to enhance EV connector safety. An HVIL circuit is a separate closed circuit built into the connector design that is a mate-last/break-first type of connection. As an EV connector starts to disconnect, the HVIL circuit detects that movement and signals the power electronics to discharge high voltages present at the terminal below 60 V before the final disconnection of the terminal. This typically must happen within a half-second of the HVIL detecting the beginning of the connection break within the power electronics unit. Ideally, that results in no high voltages being present at the EV terminals when the connector is fully separated.
As a secondary safety strategy, EV mechanical designs require insulating material between the electrical contacts and potential human interfaces. The mechanical dimensions of the materials are small enough that a “standard” finger cannot make contact with the electrical voltage at the metal terminal, in the event dangerous voltages are still present. One of the challenges for EV connector designs is to make insulating protective covers around terminals that are small enough to protect a “standard” finger, yet able to accommodate large enough metal terminals to efficiently conduct high current when connected.