Battery Power - January/February 2012 - (Page 8)

Feature Usable Energy: Key to Determining the True Cost of Advanced Lithium Ion Battery Systems for Electric Vehicles Glenn Denomme Vice President of Engineering A123 Systems, Inc. The success of the passenger electric vehicle industry relies heavily on reducing cost. Wide-spread consumer adoption of battery-electric vehicles (BEVs) and plug-in hybrid electric vehicles (PHEVs) is only a possibility when these cars offer both a competitive initial cost and lower total cost of ownership over the life of the vehicle. To sustain long-term cost-competitiveness, automakers and suppliers must focus on reducing manufacturing costs, starting with the lithium ion battery systems that power these vehicles. The industry-wide standard for measuring battery costs is costper-kilowatt-hour (cost/kWh). However, while this will remain an important metric, there are other factors that contribute to the cost of lithium-ion systems that automakers must take into account when evaluating potential suppliers. One of the most critical, yet often overlooked, attributes that should be considered is how much usable energy a battery yields, which can have a significant impact on battery costs for BEV and PHEV applications. material (a phenomenon known as the Jahn-Teller distortion) or with the electrolyte (oxidation at high voltage.) Most battery chemistries have lower calendar life at high SOCs. By lowering the maximum SOC, it is also possible to extend the life of the battery by creating a more stable environment that is far less prone to damaging side-reactions. Abuse tolerance and concerns about safety may also limit the maximum state of charge of a battery. Most battery chemistries are more energetic at high SOCs, thus making them more prone to catastrophic failure, such as thermal runaway. One way to mitigate this potential issue is to not charge the battery as fully, which reduces the likelihood of the battery being involved in an overcharge situation that could result in a catastrophic failure. In the example provided in Figure 1, the vehicle requires discharge power of 25 kW and regeneration power of 20 kW over the life of the vehicle, regardless of SOC, or how full the battery is. The ideal lithium ion battery system would be able to meet these power requirements at any given SOC, from 0 percent (empty) to 100 percent (full), and would have flat discharge and regeneration curves that follow the green dashed line. The blue shaded area represents the usable energy of this ideal battery, while the orange shaded area represents the battery’s power that remains unused because the electric motor is not designed to handle it. Having a battery with higher power than the motor can handle is not of value, other than to provide a cushion for eventual power loss over the life of the battery. Often suppliers refer to a system’s “nameplate” or total energy when providing cost/kWh, but this can be deceiving since only a portion of the battery’s capacity can be used when a vehicle is in operation. Instead, automakers must consider their cost for the usable energy, which refers to the amount of a battery’s energy that can actually be utilized over the state-ofcharge (SOC) range. For passenger vehicles, batteries must be able to produce a certain amount of discharge (acceleration) and regenerative (braking) power over their lifetime, which translates into a pre-determined amount of usable energy necessary to meet these requirements. However, batteries have lower discharge power at low SOC and lower regenerative power at high SOC. To compensate, automakers must oversize the battery packs, deploying systems a nameplate energy that significantly exceeds the usable energy needed. This ensures that the battery can produce the necessary power and meet the energy and electric range requirements. In addition to discharge and regenerative power capabilities, the automaker and battery supplier must choose a window of operation that will allow the battery to meet other requirements, including life and safety. Some battery chemistries are unable to be cycled deeply at high depths of discharge (DOD). By lowering the depth of cycling or DOD, it is possible to extend the battery’s cycle life. However, while narrowing the window of operation allows for more cycles, it comes at the expense of using less of the battery. For example, manganese oxides experience degradation when fully charged or fully discharged, which impacts the life of the battery. This is due to chemical reactions in the cathode Usable Energy 101 Figure 1. The ideal battery system would have flat discharge and regeneration curves that follow the dashed line, meeting the vehicle’s power requirements over the full SOC range. The blue shaded area represents the usable energy of this ideal battery system while the orange shaded area represents the battery’s power that remains unused. To ensure consistent vehicle performance and driver experience over time, automakers generally set battery power requirements that enable the discharge of the same power (or accelerate at the same rate) regardless of SOC. As previously noted, battery systems have lower power at low SOC, so typically, battery systems cannot sustain a high enough power output to meet this minimum discharge requirements at a lower SOC. In order to ensure www.BatteryPowerOnline.com 8 Battery Power • January/February 2012 http://www.BatteryPowerOnline.com

Table of Contents for the Digital Edition of Battery Power - January/February 2012

Battery Power - January/February 2012
Contents
East Penn’s Batteries Play Key Role in Solar Energy Storage Project
The Rechargeable Battery Association Endorses UN Efforts to Improve Lithium Ion Battery Transport Safety Regulations
Usable Energy: Key to Determining the True Cost Of Advanced Lithium Ion Battery Systems for Electric Vehicles
Battery Chargers – Moving Towards Smart Chargers
Battery Evolution: Ensuring Batteries Meet the Safety Concerns of a Changing World
Batteries
Charging & Testing
ICs and Semiconductors
Components
Power Supplies
Industry News
Marketplace
Calendar of Events
Can Ultracapacitors Revolutionize the Electric Vehicle Market?

Battery Power - January/February 2012