Why do batteries need to undergo internal resistance testing?

As the internationally recognized most effective battery and the most convenient performance parameter measurement, internal resistance can reflect the performance indicators of the battery, such as degradation degree and capacity status, which are reflected by operational parameters such as voltage, current, and temperature that cannot be used.
The four main failure modes of batteries: (dehydration, solidification of the negative electrode plate, corrosion of the positive electrode plate, and thermal runaway directly affect the decrease of battery capacity and increase of internal resistance) are the main reasons for the increase of internal resistance in batteries.
As the battery capacity decreases, the internal resistance of the battery will increase. The larger the battery capacity, the smaller the internal resistance it reflects. Meanwhile, as the degree of battery degradation increases, the internal resistance of the battery also significantly increases. Therefore, the internal resistance of a battery is closely related to its capacity: the resistance of a battery is an important indicator of battery performance degradation.
According to the research results of the International Telecommunication Power Conference, if the internal resistance of the battery exceeds 25% of the normal value, the capacity of the battery will be reduced to 80% of its nominal capacity. If the internal resistance of the battery exceeds 50% of the normal value, the capacity of the battery will decrease to below 80% of its nominal capacity and needs to be replaced in a timely manner.
In most cases, batteries are used in series, and the performance status of a single battery directly affects its performance status. Meanwhile, outdated batteries in the battery pack will accelerate the degradation of other batteries connected in series. Therefore, single battery monitoring is a necessary condition for protecting battery capacity status and service life.
By testing the internal resistance of individual batteries in a battery pack, the performance status of each individual battery in the battery pack can be accurately understood. Ensuring the stability of battery power supply and extending battery life is of great significance.