Stage 1 Tasks
Measure the battery terminal voltage
(This is the voltage that the battery has
when it is not connected to a circuit and
Measuring the Internal Resistance of a Battery or Power Supply (Stage 1) is not doing any work)
The terminal voltage is always higher than the on
Required for P3 Electronic Measurement and Testing load voltage because of the internal resistance of
In this example the battery terminal voltage reads
9.3 volts. By the time I had finished the test the
battery terminal voltage had dropped to 8.9 volts.
Stage 1 Basically the older and more worn out a battery is,
the higher its internal resistance and the faster its
Connect the battery to the meter as terminal voltage will drop.
shown. Set the range to V – DC.
Read the terminal voltage of the battery
from the display.
Stage 1 Schematic
Stage 2 Tasks
Connect a variable resistance box and an
ammeter in series with the battery.
Measuring the Internal Resistance of a Battery or Power Supply (Stage 2 & 3) Set the ammeter to measure mA DC
Adjust the value of the resistance box until the
current reads 0.005 Amps which is the standby
Ammeter current of the LM386 amplifier
Read the value of the battery voltage
Calculate the value of the internal resistance
R = V/I = (Terminal voltage – on load voltage)
Current drawn by circuit
Stage 3 Tasks
Given the internal resistance of the battery
calculate the on load voltage when the full load
current of the amplifier is flowing through the
Remember on load voltage: Stage 2 Schematic
Voltage = Current drawn x internal resistance
Now adjust your resistance box so that the full
load current is flowing through this circuit.
Measure the on load voltage of the battery.
Do your calculations and measurements agree?
If not then why not?