Battery as a backup power source when power system is outage, has been widely used in industrial production, transportation, communications and other industries. If the battery fails or the capacity is insufficient, it is possible to cause major accidents. Therefore, the on-line monitoring of the operation parameters of the battery must be carried out. One of the important signs of the battery state is its internal resistance. Whether the battery is going to fail, the capacity is insufficient or the charge and discharge is not proper, it can be reflected from its internal resistance changes. Therefore, the working state of the battery can be evaluated by measuring the internal resistance of the battery. The common methods for measuring the internal resistance of the battery are:
(1) density method
Density method is used to estimate the internal resistance of the battery by measuring the density of the battery electrolyte, which is often used to measure the internal resistance of the open lead-acid battery, which is not suitable for the internal resistance measurement of the sealed lead-acid battery. The application of this method is narrow.
(2) open circuit voltage method
The open circuit voltage method is used to estimate the internal resistance of the battery by measuring the terminal voltage of the battery. The accuracy is very poor, and even the wrong conclusion is obtained. Because even if a battery capacity has become very small, then the floating state of terminal voltage is still likely to be normal.
(3) direct current discharge method
The DC discharge method is to measure the instantaneous voltage drop on the battery by discharging the battery instantaneously, and calculate the internal resistance of the battery by Ohm's law. Although this method has been widely applied in practice, it also has some shortcomings. If we use this method to detect the internal resistance of battery, it must be carried out under static or offline state, and online measurement can not be achieved. Moreover, the large current discharge will cause great damage to the battery, which will affect the capacity and life of the battery.
(4) AC injection method
In the AC method, a constant current signal IS is injected into the battery, and the voltage response signal Vo at both ends of the battery is measured, and the phase difference between them is measured.
Study on the method of accurately measuring the internal resistance of the battery
By impedance formula