Power and Work

Power And Work







The electric power (P = power) is defined as the product of current and voltage (P = U x I) and is measured in watts (W) or alternatively in kilowatts (kW). The nickel-metal hydride battery in the Porsche Cayenne S Hybrid generates a maximum electric power of 38 kW with a DC voltage of 288 volts. On this basis, the calculated maximum continuous current is I = 131.9 A.

A higher current consumption is possible for a short time. A maximum pulse current of +120 A (charging) or -180 A (discharging) is specified. The battery manager is responsible for monitoring the battery. This control unit is part of the Battery Management System (BMS) and is located directly on the high-voltage battery in the rear of the vehicle.

Electric power (P) that is output or consumed over a certain period of time (t) is known as electric work (W = Work). The electric work W is therefore the product of power and time (W = P x t) and is recognized as an energy value because it represents the quantity of available energy.

With 12-volt batteries, the electrical capacity C in ampere hours (Ah) is usually specified and not the electric work W in watt hours (Wh) or kilowatt hours (kWh). The meaning is the same, however. Multiplying the battery voltage by the electrical capacity produces the electric work (W = C x t).

For comparison:
The 12-volt AGM battery installed in the Porsche Cayenne S Hybrid has an electrical capacity of 80 ampere hours (Ah) at 12 volts.

The resulting electric work or energy content value is:
960 watt hours (Wh), i.e. approx. 1 kWh at a nominal voltage of 12 V.
The high-voltage nickel-metal hydride battery installed in the rear of the vehicle has a total energy content of approx. 1.7 kWh at a nominal voltage of 288 V. The specified power is 38 kW and the capacity is approx. 5.5 Ah.

Efficiency

The efficiency refers to the ratio between the power output and the power consumed. One of the main arguments in favor of electric drives is the high degree of efficiency of all components.

While one can assume that the degree of efficiency of a combustion engine does not exceed 50 %, the efficiency of electric motors, electric generators and batteries can be as high as 90 %.