Electrical Power Systems Technology

Direct Current Power Systems 161

V

I = —————

Ri + RL

where:

I = the current in amperes,

V = the voltage or electromotive force (emf) of the cell in volts,

Ri = the internal resistance of the cell in ohms, and

RL = the resistance of the load in ohms.

Sample Problem:

Given: a 1.5 volt battery has an internal resistance of 0.8 ohms.

Find: load current when 10.0 ohms of load resistance is connected to

the battery.

Solution:

1.5 V

I = ——————

0.8 + 10.00Ω

= 0.139 A

This formula, and another circuit that illustrates it, are shown in

Figure 7-3. The formula shows that when a cell delivers current to a load,

some of the voltage developed must be used to overcome the internal re-

sistance of the cell. As load current increases, the voltage drop (I × R) with-

in the cell increases. This increased voltage drop causes the output voltage

(Vo) of the cell to decrease. Thus,

Figure 7-3. Illustrating the internal resistance of a cell