Relationship Between CSB Battery Capacity, Discharge Rate, and End Voltage
For VRLA batteries, capacity, discharge rate, and end voltage are interrelated during selection and use. Rated capacity is not constant; it must be measured under specified discharge conditions and end voltage. If discharge current, discharge time, or end voltage changes, the same battery will deliver a different capacity. Clear understanding of their relationship is therefore essential to read model data correctly and size backup time.
Discharge rate is commonly expressed as discharge time or as a discharge rate. For the same battery, long-duration discharge (e.g., 10-hour or 20-hour rate) uses the active material more fully, so available capacity is higher. With short-duration, high-rate discharge (e.g., 5-minute or 15-minute rate), internal resistance and polarization have a stronger effect, so actual available capacity is much lower than at long rates. This is why capacity-type products are rated in Ah and power-type products in watts: power-type models target high power density, and their parameters are generally given at 5- to 15-minute rates for applications such as UPS that need short-time high current.
As the cutoff condition in a discharge test, end voltage directly affects the capacity reading. A lower end voltage allows longer discharge and a higher calculated capacity; however, overdischarge accelerates plate degradation and shortens life. For this reason, different applications use different end-voltage limits: long backup and deep-cycle applications usually allow a lower end voltage to obtain more usable capacity, while high-rate short-duration discharge often uses a higher end voltage to protect the battery and maintain a stable voltage plateau. Capacity tables in model data usually list discharge time together with the corresponding end voltage, so they must be read as pairs.
These relationships are also affected by charge voltage and temperature. When official data specify float and equalize voltage ranges, they are based on 25°C; for example, a 12V battery may have float charge 13.5-13.8 Vdc and equalize charge 14.4-15.0 Vdc. If temperature deviates from nominal, temperature compensation should be applied; otherwise undercharge or overcharge will change actual usable capacity. The discharge temperature range is generally -15°C to 50°C, and some series can reach -25°C to 50°C. Low temperature reduces capacity and high temperature shortens life, so margin should be reserved in capacity calculations.
In practical configuration, first define load power and required backup time, then determine the discharge-rate basis and end voltage, and finally select a model against its capacity table. Data center UPS, telecom float backup, and renewable-energy deep-cycle applications have different requirements for discharge rate and end voltage. Backup-time estimates are reliable only when capacity, discharge rate, and end voltage are checked together.
