3550 Battery HiTESTER Series
27
Fig. 4: Change characteristics of the
internal resistance in a Ni-cd battery while
recharging
The output of the multiplier circuit is given by equation (3). In this equation, the first element is
the DC voltage component, and the second element is the AC voltage component. By inputting
these output voltages into the LPF, it is possible to obtain just the DC voltage component.
Assuming the secondary battery impedance
Z
is
Z = R + jX
(where
R
is the internal resistance
and
X
is the reactance), and considering that
R
=
Z
cos
θ, we find that the DC voltage
component is the real part of
Z
, that is, the voltage drop caused by the internal voltage
R
.
Therefore, by using the synchronous detection method, it is possible to measure internal
resistance without any effect from the reactance component.
4. Experimental Results of Internal Resistance Measurement
4.1 Change Characteristics of the Internal Resistance in Secondary Batteries
While Discharging
As a battery continually discharges itself in order to supply power to an external load, its capacity
decreases and its terminal voltage drops. In order to find out how the internal resistance
changes under these conditions, we used a prototype internal resistance tester to take
measurements. After charging a variety of brand new battery packs (Ni-cd, Ni-MH, and lithium
ion) intended for use in portable telephones fully, we measured their internal resistance and
terminal voltage as they were discharged at a constant current for one hour in an ambient
temperature of 23°C. We set the constant alternating current "
is
" generated by the equipment at
5mA (rms). Figs. 4, 5, and 6 show the measured results for each of the batteries. While
observing that the internal resistance increased as discharging continued in the case of both the
Ni-cd and lithium ion batteries, we saw that the opposite was true in the case of the Ni-MH
battery. When we tested other batteries of different capacities, we observed the same
phenomenon, making it likely that this characteristic is dependent on the type of the battery.
Making a judgement based only on these
results, it would seem that if a battery is
required to output a large current when it has
little capacity remaining, the internal voltage
drop in a Ni-cd or lithium ion battery increases
and the actual output voltage drops, while in
the case of a Ni-MH battery it is able to output
a large current without dropping in the output
voltage. The terminal voltage gradually
decreased for all of the batteries.
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