Relationship Between Battery Capacity, Discharge Rate and End Voltage
In the selection and use of valve-regulated lead-acid (VRLA) batteries, capacity, discharge rate and end voltage are always interrelated. The nominal rated capacity of a battery is not a fixed value, but a result measured under specific discharge conditions and a specific end voltage. If the discharge current, discharge time or end voltage changes, the capacity that the same battery can deliver will also change. Therefore, understanding the relationship among the three is a prerequisite for correctly reading model data and reasonably configuring backup time.
Discharge rate is usually expressed as discharge time or discharge rate multiplier. For the same battery under long-duration discharge (such as the 10-hour rate or 20-hour rate), the active material is utilized more fully and the deliverable capacity is relatively higher; while under short-duration high-rate discharge (such as the 5-minute rate or 15-minute rate), polarization and internal resistance effects intensify, and the deliverable capacity is noticeably lower than the long-duration rate value. This is also why in product systems capacity types are marked in ampere-hours and power types are marked in watts: power-type models target high power density discharge requirements, and their data are usually given at the 5- to 15-minute rate, for short-duration high-current scenarios such as uninterruptible power supplies.
End voltage is the cutoff condition of the discharge test and directly determines the capacity reading. The lower the end voltage is set, the longer the discharge duration and the greater the calculated capacity, but overdischarge will accelerate plate deterioration and shorten service life. Therefore, different applications adopt different end voltage conventions: long-duration standby and deep-cycle applications usually allow a lower end voltage to obtain more usable capacity; high-rate short-duration discharge often sets a higher end voltage to protect the battery and ensure a stable voltage plateau. The capacity tables in model data generally mark both the discharge time and the corresponding end voltage, and they must be used in pairs when reading.
Charge voltage and temperature also affect the above relationships. Official data mark the float and equalizing charge voltage ranges under the condition of 25°C, for example, for a 12V battery, float charge 13.5-13.8 Vdc and equalizing charge 14.4-15.0 Vdc; when the temperature deviates from the nominal value, adjustment according to temperature compensation is required, otherwise undercharge or overcharge will change the actual usable capacity. The discharge temperature range is usually -15°C to 50°C, and some series can reach -25°C to 50°C. Capacity decreases at low temperature and life shortens at high temperature, so margin must be allowed in capacity calculations.
In actual configuration, it is recommended to first determine the load power and required backup time, then select the discharge rate convention and end voltage accordingly, and finally choose the specification against the model capacity table. Applications such as data center uninterruptible power supplies, telecommunications float standby, and renewable energy deep cycle 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.
