Specifications

the battery's chemical energy
is used up. The internal
resistance drops as the
temperature increases.
If V
L
is the voltage across the
load, we have
V
L
=V
o
iR
i
(2)
Thus,
R
i
=
∣
V
L
−V
o
∣
i
(3)
This gives us a way to
evaluate the internal
resistance using the DC load,
either manually or by a
program.
For smaller batteries, a
programming approach is an
advantage. Measurements
can be made quickly, causing
less drain on the battery.
Using an oscilloscope, a
millisecond-wide current pulse
can be used to make a useful
internal resistance
measurement. The approach
for measuring a “D” size
battery's internal resistance is
observed in the following.
Note that this is only one of
the practical ways for
measuring internal resistance.
There are other methods not
mentioned here.
Approach
Measure the battery's voltage
at two different current loads.
The remote sense capability of
B&K Precision's 8500 DC load
will be used to avoid errors
due to wire resistances. The
test method follows the
recommendation of [1] by
applying a stabilization current
of 5 mA, measuring the
battery's voltage V
0
, applying
a current of 505 mA, and
measuring the voltage V. The
internal resistance is
measured as:
R
i
=
V
o
−V
0.5
(4)
Setup
Connect the battery to the DC
load. Use a battery holder or
solder wires to the battery if
necessary. If soldering wires,
do not overheat the battery
due to risk of rupture.
Connect two wires as close to
the battery terminals as
possible and run these to the
remote sense terminals on the
DC load (be sure to observe
the proper polarity). Turn on
load with the configurations
under “DC Load Setup” below.
Make sure the battery voltage
has stabilized and write this
down as the starting voltage
V
0
. Then trigger the transient
current levels and immediately
watch the displayed voltage.
Within two seconds, make
note of the lowest voltage
displayed and label this as V.
Use equation (4) to calculate
the internal resistance. A
setup diagram is provided in
Figure 9 for the purpose of
illustration.
DC Load Setup
1. Turn on DC Load's
remote sense feature.
2. Set trigger to
immediate.
3. Set load to CC mode at
0.005 A.
4. Setup transient mode
with current levels from
0 A for 0.5 ms to 0.505
A for 2000 ms in pulse
mode.
Results
Using the test setup in Figure
9, the stabilization voltage
(V
0
) for the measured battery
is 1.496 V. With 0.505 A load,
9 of 17
python battery.py 5 cc 0.5 0.5 aa_test 78deg F
Figure 7. Command Used to Run Program
Figure 8. Circuit Model of Battery and Internal Resistance