Battery Power - Fall 2016 - 11

Feature Busbars & Thermal Management With the proper materials, a busbar can assist thermal management along with power distribution in an EV/HEV. A busbar's conductor material and the cross-sectional size of the busbar will determine its current-carrying capacity. Laminated busbars typically consist of copper or aluminum conductors, which may or may not be plated with an additional conductive metal, such as silver or gold. Busbars can be fabricated in a variety of shapes, including flat strips, solid rods and hollow tubes, with flat or hollow forms generally preferred for high-current applications. Although the AC drive motors used in EVs/HEVs generate very little heat compared to their internal combustion counterparts, the flow of current across any resistive junction in a vehicular power-distribution system can generate heat, including within the battery pack itself. Typically, good electrical conductors such as copper are also good thermal conductors. In a busbar, it is the blend of materials and differences in coefficients of thermal expansion (CTEs) for a busbar's composite materials that can pose challenges due to ohmic heating and at elevated environmental temperatures. Busbars designed for low electrical and thermal resistance can readily serve as part of an EV's/HEV's thermal path, from a heat source to a heat sink or coolant reservoir. Busbar materials can also limit process manufacturing temperatures for interconnection methods, such as when using lead-free-solder attachment methods, and expected reliability, particularly if busbar material temperatures are not rated above the temperature limits of a lead-free or reflow solder process. Along with combining the power cells within an EV/HEV battery pack, laminated busbars offer practical alternatives to multiple-conductor cables for distributing power to the many sensors, subsystems and other electronic components throughout an EV/HEV. Primary among these different electronic components is the AC drive motor and inverter system that provides drive power typically to the front axle in an EV/HEV (Figure 1). The inverter converts DC from the battery pack to the multiple-phase AC power needed by a three-phase induction or permanent-magnet motor. Power Conversion & Energy Requirements Power conversion in an EV/HEV usually takes place in several stages, often employing IGBT devices for power conversion. The voltage from the vehicle battery pack is first increased by a DC/DC boost converter to the minimum voltage required by the inverter. This can be a significant increase in DC voltage, from 200 V or less to 600 V or more. Once the voltage is boosted, it is converted to an AC voltage at the proper frequency by means of the inverter to drive the vehicle's electric motor. Busbars and different connectors provide practical methods for distributing energy initially from the high-power battery pack to these different power-generating components and on to other components within an EV/HEV electrical system. Busbars can also serve as a key part of charging process so essential to EVs and HEVs, as a portion of the low-inductance power transmission path that delivers energy from a home or charging station to the Li-ion cells of a vehicle power pack. Mechanically, busbars for EVs/HEVs must be durable, capable of withstanding high levels of vibration, and operate over wide ambient temperature extremes. Electrically, they must provide low-inductance conduction of electrical energy with high isolation from other circuits and potential ground points to avoid arcing. The energy requirements of an EV/HEV can vary widely, with the largest amounts of electrical energy required by the inverter and electric drive motor. An EV motor has a wide range of power levels, from lower-voltage operation Figure 1. Laminated busbars in EVs/HEVs are essentially for transferring electrical energy from the large high-power battery pack to the inverter for conversion to AC electricity for use by the electric engine. www.BatteryPowerOnline.com Fall 2016 * Battery Power 11 http://www.BatteryPowerOnline.com

Table of Contents for the Digital Edition of Battery Power - Fall 2016