Emergency batteries are vital for ensuring continuous power supply during outages, but over time, their capacity degrades, reducing their effectiveness. There are several underlying causes for this capacity degradation.

　　One of the primary causes is the number of charge - discharge cycles. Every time a battery is charged and discharged, chemical reactions occur within the battery. Over repeated cycles, these reactions can lead to the formation of internal resistance. For example, in lead - acid batteries, sulfate crystals may form on the electrodes during the discharging process. If not properly removed during charging, these crystals can build up, increasing the internal resistance and reducing the battery's capacity.

　　Temperature also has a significant impact on battery capacity. High temperatures can accelerate the chemical reactions within the battery, leading to faster degradation. In addition, exposure to extreme cold can limit the battery's ability to deliver power. For instance, lithium - ion batteries are known to experience a decrease in capacity at low temperatures due to the reduced mobility of lithium ions.

　　Another factor is over - charging and under - charging. Over - charging can cause excessive gassing and heat generation, which can damage the battery's internal components. Under - charging, on the other hand, can lead to sulfation in lead - acid batteries, further reducing their capacity.

　　Moreover, the quality of the electrolyte can degrade over time. Evaporation, contamination, or chemical changes in the electrolyte can affect the battery's performance. By understanding these causes of capacity degradation, proper maintenance and management strategies can be implemented to extend the lifespan of emergency batteries.