How Discharge Rate and End Voltage Affect Battery Capacity
When selecting and using Valve Regulated Lead Acid (VRLA) batteries, the three variables of capacity, discharge rate, and end voltage are always interdependent. The nominal rated capacity is not a fixed value; it is measured under specific discharge conditions and a specific end voltage. Once the discharge current, discharge time, or end voltage is adjusted, the amount of energy the same battery can deliver will also change. Therefore, understanding the relationship among these three is a prerequisite for accurately reviewing model data and properly planning backup duration.
Discharge rate is generally expressed as discharge time or discharge rate multiplier. For the same battery, if a long-duration discharge is used (such as the 10-hour rate or 20-hour rate), the active material is utilized more efficiently, and more capacity can be delivered; conversely, under short-duration high-rate discharge conditions (such as the 5-minute rate or 15-minute rate), polarization and internal resistance play a more prominent role, and the deliverable capacity will be significantly less than at long-duration rates. For this reason, in product lines, capacity types are rated in ampere-hours, while power types are rated in watts: power-type models target high power density discharge scenarios, and their data are often provided at the 5- to 15-minute rate, making them suitable for applications such as uninterruptible power supplies that require short-duration high current.
End voltage, as the cutoff point of the discharge test, directly affects the capacity reading. The lower the end voltage is set, the longer the discharge continues, and the larger the calculated capacity value becomes; however, over-discharge accelerates plate aging and shortens service life. In view of this, different applications adopt different end voltage standards: long-duration backup and deep-cycle scenarios usually allow a lower end voltage in order to obtain more usable capacity; high-rate short-duration discharge tends to set a higher end voltage to protect the battery and maintain a stable voltage plateau. The capacity tables in model data usually list both discharge time and the corresponding end voltage, and they must be read as a pair.
Charging voltage and temperature also affect the above relationships. Official data use 25°C as the baseline for marking float and equalize voltage ranges, for example, a 12V battery with float charge of 13.5-13.8 Vdc and equalize charge of 14.4-15.0 Vdc; when the temperature deviates from the nominal value, adjustment must be made according to temperature compensation, otherwise the actual usable capacity will change due to undercharging or overcharging. The discharge temperature range is usually -15°C to 50°C, and some series can reach -25°C to 50°C; low temperatures reduce capacity, while high temperatures accelerate life degradation, and both points require margin in capacity calculations.
At the practical configuration level, it is recommended to first clarify the load power and required backup time, then select the discharge rate basis and end voltage accordingly, and finally compare the model capacity table to determine the specification. Applications such as data center uninterruptible power supplies, telecom float backup, and renewable energy deep cycling have different requirements for discharge rate and end voltage. Only by checking capacity, discharge rate, and end voltage as a whole can a reliable backup time estimate be obtained.
