Li-Ion Batteries

Li-ion Batteries

NOTE: Exhaustive discharge followed by a complete recharge for "regeneration" is only possible and appropriate with nickel cadmium batteries.

On a lithium-ion battery, the source voltage of the individual cells is generated through the migration of positively charged lithium atoms (=lithium ions). The material used for the positive electrode is lithium oxide (lithium-oxygen compound) and graphite is often used for the negative electrode.

During the charging process, positive lithium ions migrate from the positive electrode to the graphite layers on the negative electrode, which generates an excess charge that can be measured as a source voltage in the individual cell. The discharge process is identical, but runs in reverse.

Lithium is volatile and reacts with other materials much more readily than nickel. Although the energy density of a lithium-ion battery is significantly higher (approx. 150 Wh/kg), costly internal safety measures are required to protect the battery from exhaustive discharge, for example. In the initial development phase, isolated problems occurred in consumer electronics because insufficient protective measures were taken.

Lithium-ion batteries currently enjoy widespread use in consumer electronics. An advanced stage of development, superior safety measures and lower costs clearly make the nickel-metal hydride battery the better choice as a high-voltage energy store for hybrid technology.

Both types of battery described are extremely sensitive to overload and exhaustive discharge and the chemical processes that occur in the cells are irreversible. Conductive connections that can cause the battery to overheat also occur during exhaustive discharge of a lithium-ion battery (see above). The available overall capacity of the nickel-metal hydride battery decreases every time a charge/discharging process exceeds certain limits. For this reason, it is usual for the battery management system to limit the used bandwidth of the overall energy content. The battery ages in very small stages that are not perceptible for the driver. The high-voltage battery in the Porsche hybrid drive is designed to last for the vehicle lifetime.

Batteries reach maximum power within a defined temperature range. On the high-voltage battery used in the Porsche Cayenne S Hybrid, this range is between +50° F. (10° C.) and +99° F. (37° C.). The installation position of the battery provides protection against excessively low temperatures. A pulse charge/discharge can be actively applied to the high-voltage battery to help the battery reach the perfect operating temperature. An air-cooling system protects the battery from excessive temperatures.