80. Terminal Voltage

Because any battery has an internal resistance ri its terminal voltage VT drops when current is drawn from it;

whereVo is the open circuit voltage. Measurements of VT for various i's are taken so ri can be determined. The circuit and operation are described over.

Students will be familiar with this effect from trying to start a automobile with the lights on. When the starter motor is actuated, the lights become noticably dimmer as the terminal voltage of the battery drops. A 12 V light bulb is provided in this circuit so the class can see it get dimmer as the large currents are drawn.

As of September 1979, the internal resistance of The J.C. Penney Battery was 0.02 ohms so it could deliver 600 A to a short circuit.

The circuit board for this demonstration allows the control and measurement of large currents (100 A) from a storage battery. The large currents are also used for magnetic fields of currents demonstrations.

High-Current Control Circuit

Basically the circuit routes the current from a 12 V storage battery through a series of 0.1W power resistors. When S2 is in the left position, the current passes through only one resistor, and will reach 120 A if there is no other resistance in the circuit. The single power resistor will heat up rapidly so this position must be used only momentarily. When S2 is in the right position, the current passes through the second power resistor. With S1 in the left position about 60 A will flow to a closed circuit; with S1 in the right position, the current flows through the third power resistor and 40 A flows.

The switch S3 is provided to short circuit the output for terminal voltage and internal resistance measurements.

The shunt w adjusts the Leybold 60 mV/300 mA meter to read 100A full scale at A, or 333 A full scale on its red scale when the series resistance r is switched in.

A voltmeter is hooked to V to read the terminal voltage of the battery.