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A number of techniques are used to measure internal
battery resistance. One common method is the DC load test,
which applies a discharge current to the battery while measuring
the voltage drop. Voltage over current provides the internal
resistance (see Figure 9-5).
Figure 9-5: DC load test.
The DC load test measures the battery’s
internal resistance by reading the voltage drop. A large drop
indicates high resistance.
The AC method, also known as the conductivity
test, measures the electrochemical characteristics of a battery.
This technique applies an alternating current to the battery
terminals. Depending on manufacturer and battery type, the
frequency ranges from 10 to 1000Hz. The impedance level affects
the phase shift between voltage and current, which reveals
the condition of the battery. The AC method works best on
single cells. Figure 9-6 demonstrates a typical
phase shift between voltage and current when testing a battery.
Figure 9-6: AC
load test.
The AC method measures the phase
shift between voltage and current. The battery’s reactance
is used to calculate the impedance.
Some AC resistance meters evaluate only the load
factor and disregard the phase shift information. This technique
is similar to the DC method. The AC voltage that is superimposed
on the battery’s DC voltage acts as brief charge and discharge
pulses. The amplitude of the ripple is utilized to calculate
the internal battery resistance.
Cadex uses the discreet DC method to measure
internal battery resistance. Added to the Cadex 7000 Series
battery analyzers, a number of charge and discharge pulses
are applied, which are scaled to the mAh rating of the battery
tested. Based on the voltage deflections, the battery’s internal
resistance is calculated. Known as Ohmtest™, the mW
reading is obtained in five seconds. Figure 9-7
shows the technique used.
Figure 9-7: Cadex
Ohmtest™.
Cadex’s pulse method measures the
voltage deflections by applying charge and discharge pulses.
Higher deflections indicate higher internal resistance.
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