Introduction to the measurement of battery internal resistance in IEC 61960

In battery testing, we often receive customer testing requirements and related questions regarding battery internal resistance. Currently, the IEC61960-2003 standard requires testing battery internal resistance. The recommended testing methods in this standard are DC testing method and AC testing method. Below we will make a simple analysis of these two testing methods.

    DC test method (i.e. DC internal resistance): the DC method is to connect a discharge load at both ends of the battery pack, and calculate the internal resistance value based on the voltage changes U1 and U2 at different currents I1 and I2. From E-I1*r=U1 and E-I2*r=U2, we get: r=(U1-U2)/(I2-I1). Since the internal resistance of the battery is very small, the voltage change amplitude at a certain current is relatively small, which makes accurate measurement difficult. Due to the change in voltage during the discharge process, it is necessary to select a stable area to calculate the voltage change amplitude.

    AC test method (i.e. AC internal resistance), the AC internal resistance of the battery increases with the increase of the battery charge state. When an AC voltage is applied to both ends of the battery, u=Umaxsinωt, the generated AC current i=Imaxsin(ωt+φ) is measured, that is, the impedance is a complex impedance related to the frequency, its phase angle is φ, and its modulus r=|Z|=Umax/Imax.

    Theoretically, the internal resistance of the battery can be measured by inputting an AC current signal into the battery and measuring the voltage change caused by the signal.

    In actual use, due to the limited amplitude of the input signal, the internal resistance of the battery is in the micro-ohm or milliohm level, so the voltage change amplitude is also in the microvolt level, and the signal is easily interfered. Especially in online measurement, the impact is greater. The use of internal resistance measurement technology based on digital filters and synchronous detection methods can overcome external interference and obtain relatively stable internal resistance data.

    For the same type of battery, DC impedance and AC impedance are generally proportional or their difference is basically the same. DC impedance is based on the physical formula R=V/I. The test equipment forces a constant DC current to pass through the battery in a short period of time (usually 2-3 seconds), measures the voltage across the battery at this time, and calculates the current battery internal resistance according to the formula.

    However, this method has obvious shortcomings: (1) It can only measure large-capacity batteries or storage batteries. Small-capacity batteries cannot carry a constant large current within 2-3 seconds; (2) When a large current passes through the battery, the electrodes inside the battery will polarize and produce polarized internal resistance. Therefore, the measurement time must be very short, otherwise the measured internal resistance value will have a large error; (3) The large current passing through the battery will cause certain damage to the electrodes inside the battery.

    AC impedance: Because the battery is actually equivalent to an active resistor, we apply a fixed frequency and a fixed current to the battery (the standard requires the use of a 1KHZ frequency and a 20mA low current), and then sample its voltage. After a series of processes such as rectification and filtering, the internal resistance of the battery is calculated through the op amp circuit. The battery measurement time of the AC voltage drop internal resistance measurement method is extremely short, generally around 100 milliseconds, and the measurement is almost completed as soon as the measurement switch is pressed. The accuracy of this measurement method is also good, and the measurement accuracy error is generally between 1% and 2%.

    Advantages and disadvantages of this method: (1) The AC voltage drop internal resistance measurement method can be used to measure almost all batteries, including small-capacity batteries. This method is generally used to measure the internal resistance of laptop battery cells. (2) The measurement accuracy of the AC voltage drop measurement method is likely to be affected by ripple current, and there is also the possibility of harmonic current interference. This is a test of the anti-interference ability of the measuring instrument circuit. (3) Using this method to measure will not cause much damage to the battery itself. (4) The measurement accuracy of the AC voltage drop measurement method is not as good as the DC discharge internal resistance measurement method. In some applications where internal resistance is monitored online, only the DC discharge measurement method can be used and the AC voltage drop measurement method cannot be used.