How Discharge Rate and End Voltage Affect Battery Capacity
When selecting and maintaining valve-regulated lead-acid (VRLA) batteries, capacity, discharge rate, and end voltage are always interrelated. The rated capacity marked on a battery is not a constant value; it only holds true under specific discharge conditions and a specified end voltage. Once the discharge current, discharge duration, or end voltage changes, the capacity that the same battery can deliver will also differ. For this reason, understanding the relationship among these three factors is essential to correctly interpret model data and reasonably plan backup time.
Discharge rate is generally expressed in two ways: discharge time and discharge rate multiplier. For the same battery, under long-duration discharge conditions (such as the 10-hour rate or 20-hour rate), the active material can be utilized more fully, so a relatively higher capacity can be released. Conversely, under short-duration high-rate discharge (such as the 5-minute rate or 15-minute rate), polarization and internal resistance have a more pronounced effect, and the deliverable capacity is significantly lower than the value measured at long-duration rates. This is also why, in product lines, capacity-type batteries are marked in ampere-hours while power-type batteries are marked in watts: power-type models are designed for high power density discharge, and their data are generally provided based on the 5- to 15-minute rate, making them suitable for applications requiring short-duration high current, such as uninterruptible power supplies.
End voltage serves as the cutoff threshold for discharge testing and directly affects the capacity reading. The lower the end voltage is set, the longer the discharge can continue and the greater the calculated capacity, but over-discharge accelerates plate aging and shortens life. Therefore, different applications use different end voltage standards: long-duration backup and deep-cycle applications often allow a lower end voltage to obtain more usable capacity, while high-rate short-duration discharge usually sets a higher end voltage to protect the battery and maintain a stable voltage plateau. The capacity tables in model data generally list both discharge time and the corresponding end voltage, and they must be matched as a pair when read.
Charging voltage and temperature also affect the above relationships. Official data use 25°C as the reference for marking float and equalize voltage ranges; for example, for a 12V battery, float is 13.5-13.8 Vdc and equalize is 14.4-15.0 Vdc. When temperature deviates from the nominal value, adjustment according to temperature compensation is required; otherwise, undercharging or overcharging will change the actual usable capacity. The discharge temperature range typically falls between -15°C and 50°C, and some series can be extended to -25°C to 50°C. Low temperatures reduce capacity, while high temperatures shorten life, so allowances must be made for these factors in capacity calculations.
In actual configuration, it is recommended to first clarify the load power and required backup time, then use that result to select the discharge rate basis and end voltage, and finally choose the appropriate specification by comparing it with the model capacity table. Applications such as data center uninterruptible power supplies, telecommunications 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.
