3550 Battery HiTESTER Series
25
R
1
to R
4
: Lead and contact resistance
C: Coupling Capacitor
Fig. 1: Measuring Resistance with the AC
Four-terminal Method
3. Measuring the Internal Resistance of Secondary Batteries
3.1 Principles of Measurement
While the internal resistance of secondary batteries varies according to the battery type and its
capacity, the figure generally ranges from several mΩ to several hundred Ω. Generally, in
order to accurately measure a low resistance, the four-terminal method is used, in which a
constant current is applied to the resistor that is being measured, and the resistance is
calculated from the voltage drop that is detected. For secondary batteries, internal resistance
is measured by applying a constant alternating current, in order to avoid any effects from the DC
voltage generated by the secondary battery. This method is called the "AC four-terminal
method," and is distinguished from the DC four-terminal method in which direct current is
applied. Fig.1 illustrates the principles of the AC four-terminal method. A constant alternating
current "
is
" is supplied to the resistor that is being measured, and the voltage drop "
vis
"
generated across the resistor is detected.
In this case, "
is
" is always constant,
regardless of the resistance being measured,
the wiring resistance, and the contact
resistance between the wires and the
resistor being measured. In addition,
because the input impedance of the
voltmeter is sufficiently large, practically no
current flows to the voltmeter so it is possible
to detect only the voltage drop due to the
resistor being measured. Therefore, it is
possible, with this method, to measure the
resistance without any effect from wiring
resistance or contact resistance.
3.2 Equipment Configuration
Fig. 2 illustrates the configuration of the internal resistance measuring equipment. The AC
four-terminal method shown in Fig. 1 is being applied to a secondary battery. When a 1kHz
constant alternating current "
is
" is applied to the secondary battery, the voltage drop "
vis
" due to
the internal resistance of the battery is detected. In order to obtain the voltage drop that is due
only to the battery's internal resistance, a synchronous detector circuit is used. This circuit
operates to filter out the effects of reactance within the secondary battery and in the test system
without any effects from external noise. In addition, the DC terminal voltage "VT "of the
secondary battery is also measured. Then the CPU switches between the internal resistance
output "VR "and the terminal voltage output "VB "as the input to the A/D converter so that both
the internal resistance of the battery and the terminal voltage can be displayed simultaneously.
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