Advances in the miniaturization of electronic devices and construction of communications infrastructures have led to the rapid spread of compact, high-performance cordless equipment, such as portable telephones and notebook computers. In facilities where equipment downtime from power failures is unacceptable, uninterruptible power supplies (UPSs) are increasingly being installed.
Secondary batteries that allow repeated use through recharging are used in all of these devices. But as a secondary battery undergoes repeated charge/discharge cycles, its capacity decreases due to the effects of chemical matter deterioration inside the battery itself. As a result, the continuous discharge time of the battery is reduced, which can hinder equipment operation.
To accurately evaluate the deterioration of a secondary battery, it is necessary to perform a charge/discharge test and measure the capacity of the battery. Unfortunately, the test takes time to perform. That is why there is a need for a test method that can evaluate the performance of a secondary battery more quickly and easily.
Research now indicates a correlation between a decrease in a secondary battery's capacity and an increase in its internal resistance superscript . A secondary battery internal resistance tester that uses low-resistance measurement techniques has been developed to specifically address these issues. This article addresses this alternative test method and presents the results of its experimental testing.
Measuring Internal Resistance of Secondary Batteries Principles of measurement. While the internal resistance of secondary batteries varies according to the battery type and its capacity, the value generally ranges from several mohms to several hundred ohms. By using what is known as the four-terminal test method, a constant current is applied to the resistor being measured, with the resistance calculated from the detected voltage drop.
For secondary batteries, internal resistance is measured by applying a constant alternating current to avoid any effects from the DC voltage generated by the secondary battery. This method is called the "AC four-terminal test method" and is distinguished from the "DC four-terminal test method" in which direct current is applied.
Fig. 1, on page 20, illustrates the setup of the AC four-terminal test method. A constant alternating current "i subscript s" is supplied to the resistor being measured, and the voltage drop "v subscript is" across the resistor is measured. In this case, "i subscript s" is always constant, regardless of the resistance being measured (i.e., 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, thus making it possible to detect only the voltage drop due to the resistor being measured.
Equipment configuration. Fig. 2 illustrates the configuration of the internal resistance measuring equipment. The AC four-terminal test method shown in Fig. 1 is applied to a secondary battery. When a 1kHz constant alternating current "i subscript s" is applied to the secondary battery, the voltage drop "v subscript is" due to the internal resistance of the battery is detected. A synchronous detector circuit is used to obtain the voltage drop due only to the battery's internal resistance. This circuit operates to filter out the effects of reactance within the secondary battery and the test system without any effects from external noise.
In addition, the DC terminal voltage "V subscript T" of the secondary battery is also measured. The CPU then switches between the internal resistance output "V subscript R" and the terminal voltage output "V subscript B" as the input to the A/D converter. This makes it possible to display the internal resistance of the battery and the terminal voltage simultaneously.
Synchronous detection circuit. The synchronous detection method used with this measuring equipment determines the signal level of the in-phase components of a given signal and a reference signal. Fig. 3 shows a simple example of the circuit configuration of a synchronous detection circuit. This circuit consists of a multiplier and an LPF that removes the DC component of the output from the multiplier.
Assuming "v subscript 1" is the AC reference signal voltage generated by the measuring equipment and "v subscript 2" is the synchronous detection signal voltage, these values can be expressed by the following equations:
v subscript 1 = Asinvt..............(1) v subscript 2 = Bsin (vt + u)......(2)
Where A and B are the amplitudes of "v subscript 1" and "v subscript 2," respectively, and the phase in equation (2) is the difference from "v subscript 1" due to reactance.
If synchronous detection, as shown in Fig. 3, is performed using "v subscript 1" and "v subscript 2," the result can be expressed by the following equation:
v subscript 1 x v subscript 2 = ABsinvt sin(vt + u)= 1/2 ABcosu - 1/2 ABcos (2vt+ u)...(3)
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.
Using these output voltages as the input to the LPF, it is possible to obtain only 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 R = _Z_cosu, it was found 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.
Experimental Results Change characteristics while discharging. As a battery continually discharges itself to supply power to an external load, its capacity decreases and its terminal voltage drops. To find out how the internal resistance changes under these conditions, a prototype internal resistance tester was used to take measurements. After fully charging a variety of brand new battery packs (Ni-Cd, Ni-MH, and lithium ion) intended for use in portable telephones, their internal resistance and terminal voltage was measured as they were discharged at a constant current for 1 hr in an ambient temperature of 23C. The constant alternating current "i superscript s" generated by the equipment was then set at 5mA (RMS). Figs. 4 and 5, on page 22, and Fig. 6, above, show the measured results for each of the batteries.
Although the internal resistance increased as discharging continued in the case of both the Ni-Cd batteries and the lithium ion batteries, the opposite was true in the case of the Ni-MH battery. The same phenomenon occurred when other batteries of different capacities were tested. Therefore, if a battery is required to output a large current when it has little capacity remaining, then a Ni-MH battery is the best choice. This battery is able to output a large current without dropping its output voltage, whereas the terminal voltage gradually decreased for all other battery types.
Temperature characteristics. A battery's performance also depends on the temperature of the environment you use it in. It is reported that at low temperatures the amount of output current decreases, and at high temperatures the operational life of the battery is reduced superscript . We decided to fully charge a variety of batteries (Ni-Cd, Ni-MH, and lithium ion) and then measure their internal resistance and terminal voltage while changing the ambient temperature from -10C to +40C. Figs. 7, 8, and 9, on page 25, show the measured results for each battery type tested.
While the change in terminal voltage was small for all of the batteries, observation revealed that temperature increased the internal resistance of the Ni-Cd battery while it decreased the internal resistance of the Ni-MH batteries. On the other hand, the change in the internal resistance of the lithium ion battery was small. As you can see, a lithium ion battery can output a stable amount of current even at low temperatures.
Conclusion You can use an internal resistance tester quickly and easily to evaluate the deterioration of secondary batteries. By making a precise evaluation of the deterioration of a secondary battery on the basis of the results of a charge/discharge test, it should be possible to create a guide for determining the state of a battery by simultaneously measuring the internal resistance and terminal voltage.
It is now necessary for the industry to conduct long-term charge/discharge cycle testing on secondary batteries to confirm, in detail, the relationship between the change in internal resistance and decrease in battery capacity due to battery deterioration. We should also try to confirm whether internal resistance is related to other parameters of secondary batteryperformance.
References 1. M. Yamanaka, et al: "Operational Life Indicator for Sealed Lead Storage Batteries," Yuasa Jiho, No. 72, pp. 29 - 36 (April 1992).
2. M. Kanda: "Backup Battery Performance and Applications," Proceedings of '96 Battery Technology Symposium (April 1996).
3. T. Fukushima: Suitability of Batteries for Data-Related Equipment," Proceedings of '96 Battery Technology Symposium (April 1996).