Batteries (or accumulators) are electrochemical energy storage devices used for a wide variety of purposes ranging from watches and cell phones to cars and industrial applications.
As can be seen from the Ragone diagram in Figure 1, batteries have very high energy densities compared to other storage devices, but suffer from low power density and resulting high charging times. The reason lies in the conversion from electric to electro-chemical energy and back. Some modern high performance batteries especially the Nickel-metal hydride type do however reach power densities that are promising for braking energy storage at least in automotive applications.
Figure 1: Ragone diagram
The most relevant requirements posed on high-power batteries for transport applications are high power and energy densities. A high charge acceptance to maximize regenerative braking utilization, and long calendar and cycle life, electric and thermal balance and recycleability are additional technological challenges.
In the following some of the more relevant battery types are briefly presented.
Lead acid batteries can be designed to be high power and are favoured by low price, high safety and reliability. A recycling infrastructure for lead acid batteries is in place. Drawbacks are low energy densities, poor performance at low temperatures, and short calendar and cycle lifes. Lead acid batteries are currently used in many electric vehicles. Advanced high-power lead acid batteries are presently being developed for application in hybrid-electric vehicles.
Nickel-cadmium batteries are used in many electronic consumer products. They have higher specific energy and a better cycle life than lead acid batteries, but do not deliver sufficient power and are therefore not promising for braking energy storage.
Nickel metal hydride batteries, used in computer and medical equipment, have good energy and power densities. Recyclability is satisfactory, but a recycling infrastructure does not exist yet. Nickel metal hydride batteries have a much longer life cycle than lead acid batteries and are safe. They are used successfully in electric cars and recently in hybrid-electric cars. Challenges of nickel metal hydride batteries are high prices, high self-discharge and heat generation at high temperatures, the need to control losses of hydrogen, and low cell efficiency.
The lithium ion batteries are characterised by high energy density and are therefore an attractive option for laptops and cell-phones. Further benefits are high specific power, high energy efficiency, good performance at high temperatures, and low self-discharge. Recycleability is also acceptable. These characteristics make lithium ion batteries suitable for braking energy storage. A commercial use in transportation is however still impeded by high costs and calendar life is quite high but could still be improved.
Lithium polymer batteries have the potential to provide the high specific power needed for braking energy storage. In addition, they are safe and have good cycle and calendar life. For a commercial use in transport cost has to drop and higher specific power batteries have to be developed.