Battery Power - Fall 2016 - 10

Feature Sorting Busbar Choices for Electric Vehicle Power Distribution Dominik Pawlik, Technical Marketing Manager Rogers Corp., Power Electronics Solutions (PES) Interest in electric vehicles (EVs) and hybrid electric vehicles (HEVs) is growing steadily as battery technologies improve and the driving range of such vehicles increases. Perhaps as important, EVs/HEVs offer a "green" alternative to traditional vehicles powered by internal-combustion gasoline engines. EVs and HEVs rely on robust electric motor drives, large-capacity battery pack, power inverters, and efficient distribution of power from charging source to battery and then throughout the vehicle. Busbars, which comprise a system of electrical conductors for collecting and distributing current, provide the means to efficiently distribute power to the vehicles' various subsystems. A number of different types of busbars are commercially available, so specifying the best match for an EV/HEV application is a matter of understanding the various requirements of an EV/HEV and how different types of power busbars can meet those requirements. Efficient use of a limited amount of energy is critical to any vehicle design and EVs and HEVs typically work with rechargeable lithium-ion battery backs as their source of energy. Fortunately, the cost of these battery packs has been dropping in recent years, to around a current cost of about $300 USD/kWh. The cost of EV/HEV battery power is expected to continue to drop during the next decade, making EVs and HEVs more affordable and priced more in line with vehicles based on internal-combustion engines. Acceptance of EVs/HEVs is growing to a point where more than 200,000 Nissan Leafs have been sold worldwide. In fact, in some countries, such as Japan, electric vehicle charging stations now outnumber gas stations. Many EVs are now referred to as "plug-in" EVs for their recharging capabilities and are equipped with "fast-charge" charging modes to provide quick energy for shorter driving distances. Power Distribution & Driving Range Power storage and driving range have been chief concerns with EVs/HEVs, with power typically provided by large banks of battery cells combined in sealed packs to achieve the required operating voltage and current to power a vehicle's electric motor. Of course, induction motors, whether dedicated to each wheel in some novel designs or with power transferred to the front wheel axle through a transmission system, are not the only subsystem in an EV/HEV requiring electric power. All of the basic vehicular functions found in a vehicle powered by an internal combustion engine, including heating, cooling, lights, and warning systems, require electric power in an EV/HEV, and each subsystem when in use can subtract from the total driving range possible with a charged vehicle battery pack. 10 Battery Power * Fall 2016 Fortunately, effective power distribution within an EV/ HEV can contribute to improved driving range. A number of battery technologies are employed in new EVs and HEVs, with lithium-ion (Li-ion) technology providing the most popular solution for the large battery packs that operate the electric drive motor. Traditional lead-acid batteries are still commonly used within EVs and HEVs as auxiliary power sources, for functions such as sensors, cooling fans and lights. Li-ion battery systems can provide 130 Wh/kg energy capacity and handle thousands of charging cycles with minimum degradation of storage capacity. For a typical EV or HEV with a driving range of 100 miles or more, this translates into a useful Li-ion battery lifetime of 6 years or more (or the typical warranty period for a new EV/HEV). Several manufacturers are developing variants of lithium-based power cells, including lithium-iron-phosphate batteries and lithium-titanate batteries, attempting to add range to EVs and HEVs, although cost must be within certain limits for competitive automotive markets. Additional vehicular battery technologies include nickel-metal-hydride (NiMH) cells, which are heavier and less efficient than Li-ion batteries, but considerably less in cost, and zinc-air batteries, but Li-ion batteries currently represent the dominant technology in EVs and HEVs. Cells in a high-power EV/HEV battery pack can be combined in series or parallel to achieve voltage ratings approaching 400 V. Individual cells of about 1.5 to 2.0 V are typically combined using busbars rather than insulated cables. A busbar is essentially an electric conductor and ground plane separated by an insulator. It can be fabricated as a single layer component or with multiple layers, including circuit paths for signals as well as for distributing power. As with other basic circuit components, a busbar can be characterized by its resistance, capacitance and inductance, ideally with its electrical contributions distributed as evenly as possible across its length to avoid performance inconsistencies. While the lowest possible resistance and inductance values are to be preferred in a busbar for EV and HEV power distribution, some busbars for that purpose have capacitance added in different ways to increase the chargecarrying capabilities of the power-distribution structure. Because even low resistance will cause heating effects from large current flow at high enough power levels, it is important to minimize the contact resistance at all connection points along a busbar, including solder joints. To minimize contact resistance, groups of battery cells are often laser welded to a busbar in the process of assembling the large, high-power battery packs for EVs/HEVs. Laser welded connections can also be made as part of an automated assembly process to minimize manufacturing costs in large-volume production. www.BatteryPowerOnline.com http://www.BatteryPowerOnline.com

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