Hybrid Electric Vehicle batteries

Which Batteries are used in Hybrid Electric Vehicle?

1. Li-ion batteries:

           In the lithium-ion system, a lithium-carbon electrode acts as the negative electrode material. The lithium is not present in the form of metal, but either as an ion in the electrolyte or chemically bound to the positive or negative electrode. The carbon electrode is characterized by its high life-cycle stability. Small cells using lithium-ion systems have so far displayed energy density of 120 Wh/kg. The lithium-ion system used at the moment for portable batteries already satisfies two key conditions for an electric vehicle i.e. high specific energy and a long service life. Cobalt, the main element in the positive electrode, is a relatively expensive metal. The cobalt is replaced by manganese oxide. Cells using these new materials achieve comparable specific energy and specific power. The next generation of lithium-ion is expected to contain a solid polymer electrolyte. Estimation of material costs show that there is a possibility of producing batteries for electric vehicle at costs considerably more favourable than those of Ni-MH.

Specifications:

Specific energy: 100 to 120 Wh/kg

Energy density: 200 to 250 Wh/L

Maximum power: 200 W/kg

Charge time:4 hours

2. Sodium Nickel Chloride Batteries:

       Sodium-nickel-chloride batteries are under development but not yet widespread, because the high operating temperature currently still results in too high self-discharge losses. These batteries show energy density over 80 Wh/ kg and specific power over 110 W/kg at full charge. The battery has the potential to meet a life goal of 5 years.

3. Sodium Sulphur Batteries:

        Research is underway to improve battery technology to have a higher energy density for electric vehicles. A potential contender however is the sodium sulphur (NaS) battery, which has reached the production state in near past. The NaS battery offers high specific energy 100 Wh/kg with relatively low-cost battery materials. Specific power value is 130 W/kg. The sodium sulphur (NaS) battery uses a cathode of liquid sodium into which a current collector, a solid electrode of B-alumina is placed. The complete assembly is surrounded by a metal can, which is in contact with the anode, a sulphur electrode. A running temperature of 300°C is necessary with NaS system, which is the major problem. A heater in the capacity of a few hundred watts forms part of the charging circuit, which maintains the battery temperature when the vehicle is not running. The battery temperature is maintained when it is in use due to losses in the battery

          cell of this battery is very small, using only about 15 gm of sodium. This is a safety feature because if the cell is damaged the sulphur on the outside causes the potentially dangerous sodium to be converted into poly-sulphides, which are comparatively harmless. The additional advantage is that the cells can be located around the car. The capacity of each cell is about 10 Ah with an output voltage of about 2 V. These cells fail in an open circuit condition and hence this must be taken into account. A problem yet to be solved with this system is its casing material, which is prone to fail due to the very corrosive nature of the sodium. Presently an expensive chromised coating is used.

         This type of battery combined with an electric motor, seems to be a very good competitor to the internal combustion engine. The servicing and charging infrastructure needs to be developed but looks promising. It is estimated that the cost of running an electric vehicle may be little around 15% of the petrol version, which may absorb the extra cost of production.

4. Fuel Cell:

         The energy of oxidation of conventional fuels, which it usually manifested as heat, may be converted directly into electricity, in a fuel cell. The process of oxidation involves a transfer of electrons between the fuel and oxidant and in a fuel cell works on this principle where the energy is directly converted into electricity. All battery cells involve an oxide reduction at the positive pole and an oxidation at the negative pole during some part of their chemical process. For the separation of these reactions in a fuel cell an anode, cathode and electrolyte are required. The electrolyte is fed directly with the fuel. When hydrogen fuel is combined with oxygen it is found to be a most efficient design. Fuel cells are very reliable and silent in operation, but at present are very expensive to construct. Figure shows a simplified representation of a fuel cell. In one type of fuel cell hydrogen is passed over an electrode (the anode) of porous nickel, which is coated with a catalyst, and the hydrogen diffuses into the electrolyte. This causes electrons to be stripped off the hydrogen atoms. These electrons then pass through the external circuit. Negatively charged hydrogen anions (OH-) are formed at the electrode over which oxygen is passed, such that they also diffuse into the solution. These move through the electrolyte to the anode. The electrolyte used is a solution of potassium hydroxide (KOH). Water is formed as the by-product of a reaction involving the hydrogen ions, electrons and oxygen atoms. If the heat generated by the fuel cell is used, then an efficiency of over 80% is possible together with a very good energy density. The working temperatures of these cells varies but about 300-400°C. High pressure 2.4-40 MPa is also used. The pressures and storage of hydrogen are the main problems to overcome with fuel cells before they can be realistic alternatives to other forms of storage for the mass market. It is believed that hydrogen fuel cell cars will hardly become commercially viable economically competitive with other technologies because they have inefficiency of producing, transporting and storing hydrogen and the flammability of the gas.