Electrical Power Systems Technology

162 Electrical Power Systems Technology

Vo = V – IRi

where:
Vo = the output voltage of the cell in volts,
V = the no-load cell voltage in volts,
I = the load current in amperes, and
Ri = the internal resistance of the cell in ohms.

Sample Problem:
Given: the no-load voltage of a battery is 9.05 volts. Its rated load
current is 200 mA, and its internal resistance is 0.1 ohms.
Find: the rated output voltage of the battery.
Solution:
Vo = V – IRi
= 9.05 V – (.2 A × 0.1Ω)
Vo = 9.03 Volts

Modifying this formula to solve directly for Ri, we obtain

V – Vo
Ri = ———
I

Sample Problem:
Given: the rated output of a battery is 30.0 volts, and its no-load volt-
age is 30.15 volts. Its full-load current is 350 mA.
Find: the internal resistance of the battery.
Solution:

V – Vo
Ri = ———
I

30.15 - 30.0 V
= ———————
0.35 A

Ri = 0.43 ohms

The same method is used to determine the internal resistance of all
voltage sources, including primary cells, secondary cells, and genera-
tors.