When Is Battery Discharge Considered Over-Discharge?

Battery discharge is a complex physical and chemical process involving multiple stages. Different types of batteries have varying standards and requirements for discharge. The discharge regimen for a battery includes not only the discharge rate, discharge pattern, and termination voltage, but also temperature and other key factors. For common battery types like lead-acid batteries, the discharge process is typically divided into several stages, each corresponding to different voltage levels and discharge rates. According to the “Guangyu” VRLAB discharge regimen, the discharge process of a battery can be divided into three main stages.

Stage 1: Rapid Drop from Float Voltage to Open Circuit Voltage

At the beginning of the discharge process, the battery’s terminal voltage quickly drops from the float voltage. The float voltage is usually around 2.23V, while the open-circuit voltage drops to about 2.13V. This stage is characterized by a rapid voltage decrease because the battery‘s voltage has not yet entered the normal open-circuit voltage level but is transitioning from the float voltage. As a result, the voltage decreases rapidly, but this does not represent actual discharge, as the battery has not yet entered a regular discharge state.

Stage 2: Steady Decline After Open Circuit Voltage

Once the battery voltage starts to decline steadily from the open-circuit voltage, it enters the second stage of the discharge process. Under normal circumstances, the voltage typically transitions smoothly between 1.80V and 2.06V, which is considered a stable discharge period. During this stage, the battery voltage decreases gradually, and the discharge rate remains relatively stable. This phase is essential for battery performance, as excessive discharge may affect battery lifespan. Therefore, it is critical to maintain voltage stability during this stage.

Stage 3: Over-Discharge After Reaching the Termination Voltage

When the battery voltage approaches 1.80V, it enters the final discharge stage. If discharge continues, the battery voltage will decrease further, and the discharge rate will accelerate, entering the over-discharge phase. Over-discharge means the battery voltage has dropped to a dangerously low level, and further discharge may cause irreversible damage to the battery.

Over-discharge is particularly dangerous for lead-acid batteries and similar types. If the voltage falls below 1.80V, continuing discharge can lead to instability in the internal chemical reactions, which may cause sulfation. Sulfation occurs when lead electrodes react with sulfuric acid to form lead sulfate, significantly reducing the battery’s effective capacity and making it difficult to restore the original capacity. Therefore, once the voltage reaches 1.80V, discharge must be stopped immediately, and charging should commence to prevent further damage.

Impact and Prevention of Over-Discharge

Over-discharge not only leads to chemical instability within the battery but also accelerates battery aging, shortening its lifespan. Prolonged over-discharge reduces charging efficiency and may render the battery incapable of charging. Therefore, it is crucial to stop discharge promptly and recharge the battery.

To prevent over-discharge, some Battery Management Systems (BMS) incorporate low-voltage protection mechanisms. When the battery voltage nears the pre-set over-discharge level, the system automatically disconnects the battery from the load to prevent further discharge. Additionally, users should monitor battery voltage regularly during use to avoid deep discharge.

Conclusion

When is battery discharge considered over-discharge? The answer depends on the battery type and discharge standards. For lead-acid batteries, over-discharge begins when the voltage drops below 1.80V. Over-discharge can cause significant damage, so it is essential to monitor battery voltage closely during the discharge process and stop discharge promptly for recharging. By managing discharge properly, the battery’s lifespan can be extended, and its operational efficiency can be maintained.