Battery voltage, current, and temperature are important operating parameters of the battery, but they do not reflect the internal state of the battery. However, internal resistance can reflect the performance indicators such as battery deterioration and capacity status.

The four main failure modes of the battery(water loss, negative plate vulcanization, positive plate corrosion and thermal runaway) are the main reasons for the increase in internal resistance of the battery.

As the capacity state of the battery decreases, the internal resistance of the battery increases. The larger the capacity of the battery, the smaller the internal resistance, and as the battery deteriorates, the internal resistance of the battery will also increase significantly. Therefore, the internal resistance of the battery has a close relationship with its capacity: the increase in internal resistance of the battery is an important indicator of battery performance degradation.

According to research, if the internal resistance of the battery exceeds the normal value of 25%, the capacity has been reduced to about 80% of its nominal capacity. If the internal resistance of the battery exceeds 50% of the normal value, the battery capacity is lower than the nominal capacity of 80%, need to be replaced in time.

The capacity status of the battery will decrease as the usage time increases. The battery will enter an unstable period after 2 years of use. In other words, Many battery cells will drop significantly after 2 years of use.

Through the internal resistance test of the single battery in the battery pack, it is possible to accurately grasp the performance state of each of the battery cells. At the same time, it is of great significance to ensure the stability of the battery power supply and extend the service life of the battery pack.