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9.2 How is the internal battery resistance measured?
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.
