Hybrid Electric Vehicle Energy Storage Systems
An energy storage system is an essential component in hybrid electric vehicles (HEVs). Batteries used in HEVs should have high power (with high-peak and pulse-specific power), high specific energy at pulse power, high charge acceptance to maximize regenerative braking utilization, and long calendar and cycle life. See below to learn about HEV battery options, ultracapacitors (another energy storage device), and battery thermal management strategies.
- Lead-Acid Batteries
- Nickel-Cadmium Batteries
- Nickel-Metal Hydride Batteries
- Lithium Ion Batteries
- Lithium Polymer Batteries
- Ultracapacitors
- Battery Thermal Management
Lead-Acid Batteries
Lead-acid batteries can be designed to be high power and are inexpensive, safe, and reliable. A recycling infrastructure is in place for them. But low specific energy, poor cold-temperature performance, and short calendar and cycle life are still impediments to their use. Advanced high-power lead-acid batteries are being developed for HEV applications.
Nickel-Cadmium Batteries
Although nickel-cadmium batteries, used in many electronic consumer products, have higher specific energy and better life cycle than lead-acid batteries, they do not deliver sufficient power and are not being considered for HEV applications.
Nickel-Metal Hydride Batteries
Nickel-metal hydride batteries, used routinely in computer and medical equipment, offer reasonable specific energy and specific power capabilities. Their components are recyclable, but a recycling infrastructure is not yet in place. Nickel-metal hydride batteries have a much longer life cycle than lead-acid batteries and are safe and abuse tolerant. These batteries have been used successfully in production electric vehicles and are widely used in production HEVs. The main challenges with nickel-metal hydride batteries are their high cost, high self-discharge and heat generation at high temperatures, the need to control losses of hydrogen, and their low cell efficiency.
Lithium Ion Batteries
Lithium ion batteries are rapidly penetrating into laptop and cell-phone markets because of their high specific energy. They also have high specific power, high energy efficiency, good high-temperature performance, and low self-discharge. Components of lithium ion batteries could also be recycled. These characteristics make lithium ion batteries suitable for HEV applications. However, to make them commercially viable for HEVs, further development is needed, including improvement in calendar and cycle life, higher degree of cell and battery safety, abuse tolerance, and acceptable cost.
Lithium Polymer Batteries
Lithium polymer batteries with high specific energy, initially developed for electric vehicle applications, also can provide high specific power for HEV applications. Like lithium ion batteries, they could become commercially viable if the cost were lowered and lifecycle improved.
Ultracapacitors
Ultracapacitors are higher specific energy and power versions of electrolytic capacitors—devices that store energy as an electrostatic charge. They are electrochemical systems that store energy in a polarized liquid layer at the interface between an ionically conducting electrolyte and a conducting electrode. Energy storage capacity increases by increasing the surface area of the interface. Ultracapacitors are being developed as primary energy devices for power assist during acceleration and hill climbing, as well as recovery of braking energy. They are also potentially useful as secondary energy storage devices in HEVs, providing load-leveling power to electrochemical batteries. Additional electronics are required to maintain a constant voltage due to the low energy density.
Battery Thermal Management
The performance and life cycle costs of HEVs depend on the performance and longevity of their battery packs. Each battery chemistry must operate within a particular temperature range to achieve optimum performance. Thermal management is therefore critical for high-power battery packs used in electric vehicles and HEVs. For example, temperature variations from module to module in a battery pack can result in reduced power and capacity, reduced charge acceptance (during regenerative braking), and increased vehicle operating and maintenance expenses. To learn more, visit the National Renewable Energy Laboratory's Energy Storage Web site.