As a core component of DC power systems, batteries serve a crucial role in storing electrical energy, responding to grid anomalies and special operating conditions, and maintaining normal system operation. They are the last line of defense for the normal operation of power systems. Currently, online battery monitoring is gaining increasing attention and is being used more widely in industries such as power and communications. However, the internal resistance AC discharge method—a key technology used in online battery monitoring and condition assessment—is still poorly understood and poorly understood. The misleading term “maintenance-free” has led users to neglect routine battery maintenance and management, resulting in premature capacity loss and damage. Accidents in substations and power plants caused by insufficient or failed batteries are common. Therefore, proper battery use and maintenance to extend their service life is crucial.

Key factors affecting battery internal resistance include:

Battery age: As battery age increases, factors such as electrolyte dehydration, corrosion of the plates and connectors, plate sulfation, plate deformation, and active material shedding cause battery capacity to decrease and internal resistance to increase.

Battery Charge: Battery internal resistance, and therefore charge capacity, can vary significantly due to differences in the depth of electrolyte injected into the battery, the thickness of reactants on the electrode surface, and the porosity of the electrode surface.

Temperature: Changes in ambient temperature, such as an increase, accelerate the diffusion of reactants, facilitate charge transfer, electrode kinetics, and mass transfer, and thus reduce the battery’s internal resistance. Conversely, a decrease in temperature increases the internal resistance.

Battery Model: Internal resistance varies between batteries of different manufacturers, types, and models due to differences in electrode, electrolyte, and separator material formulations, battery structure, and assembly processes.

Measurement Signal Frequency: Many current battery internal resistance measurements actually measure the battery’s impedance, which includes capacitive reactance. The magnitude of capacitive reactance is related to the measurement signal frequency, making the battery internal resistance measurement results less objective. To achieve objectivity, the influence of battery capacitance on the measurement results should be removed using analytical methods based on the phase relationship between the measurement signal current and voltage. This ensures that the measurement result is independent of the signal frequency, meaning that the internal resistance measurement result is unique at any measurement signal frequency.

Measurement time and current: When using a high measurement current, polarization builds and stabilizes during the instants between applying and removing the measurement signal. Different measurement currents and measurement times will result in different polarizations, making the battery internal resistance measurement result less objective. To achieve objectivity, internal resistance measurements should be performed with a minimum signal current. Based on experimental results, the measurement current should be less than or equal to 0.05C10 (where C10 is the battery capacity at a 10-hour discharge rate).

Impact of Overcharging

Long-term overcharging causes water to be consumed at the positive electrode due to oxygen evolution, resulting in increased H+ production. This leads to increased acidity near the positive electrode, accelerated grid corrosion, and grid thinning, accelerating battery corrosion and reducing battery capacity. Furthermore, the increased water loss can put the battery at risk of drying out, shortening its lifespan.

Impact of Overdischarge

Battery overdischarge primarily occurs after an AC power outage, when the battery is left powering a load for an extended period. When a battery is overdischarged to the point where its voltage is excessively low or even zero, a large amount of lead sulfate within the battery is adsorbed onto the cathode surface, causing sulfation. Lead sulfate is an insulator, and its formation negatively impacts the battery’s charge and discharge performance. Therefore, the more sulfate formed on the cathode, the greater the battery’s internal resistance, the poorer its charge and discharge performance, and the shorter its service life.