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ApplicationUPS and Battery UsageBattery Calculation And Selection

Battery calculation and selection

Question: UPS back-up time calculation

Method 1:

The formula to calculate UPS battery capacity: C = (P × T) ÷ (V × η × K)
C–Battery capacity of the required configuration; unit–Amp × hour (Ah)
P–Load power; unit–Watt (W)
T–Backup time; unit–Hour (h)
V–Battery pack rated voltage; unit–Volt (v)
η–Battery inverter efficiency
K–Battery discharge coefficient
  • When the battery backup time is <3h, K = 0.6
  • When the battery backup time is 3~5h, K = 0.8
  • When the battery backup time is 5~10h, K = 0.85
  • When the battery backup time is >10h, K =1

For example: If the load is 8000W and backup time is 1 hour, the required battery capacity C = (8000W * 1)/(12V * 16 * 0.92 * 0.6) = 75.5AH.
Therefore select a 100AH battery to guarantee the battery backup time is more than 1 hour    

Method 2:

The generally recommended method for UPS configuration is constant power algorithm, for which can achieve reasonable configuration in cost control and performance.
According to the total power value supplied by the battery, the formula to calculate the battery constant power discharge (W):  W = P * C0S φ/(Л * N * 6)
In the formula:
  • P–The nominal output power of UPS power (VA)
  • C0Sφ–The power factor of load (0.9)
  • Л–Inverter’s efficiency
  • N * 6–The number of series connected batteries required for calculation with 2v batteries in UPS

Configuration description (calculated according to the rated power for the actual loading power factor is unknown) For example: Load 8000W; backup time 1 hour; UPS efficiency 0.92; the batteries is 12V * 16 in series for each set.
Calculation steps: 

  1. Calculate the required battery power P total = 1 * 8000W/0.92 = 8695W
  2. Calculate the number of batteries in series for each set Sn = 16
  3. Calculate the single battery discharge power Pb = P total/Sn = 8695W/16 = 543.4W
  4. Calculate the single-unit-cell discharge power Pc = Pb/6 = 543.4W/6 =90.6W
  5. Look up the list of battery types and confirm the Pn number. The discharge power

  1. Notes: The cut-off voltage of the UPS battery is selected as 1.75V/cell and it may vary for different UPSs. For the details please consult the manufacturer.

Question: UPS battery type selection
Several suggestions for battery selection:
  1. Battery capacity, nominal 20HR, 10HR; please note how much V/C cutoff voltage is;
  2. Battery floating charge life: 6 years, 10 years and 15 years;
  3. Housing material, V0, HB;
  4. Refer to the internal resistance; lower the internal resistance is, fewer heats will be generated;
  5. Weight: The main material of lead-acid battery is lead. If the process is the same, the heavier battery is more reliable. It is recommended to choose the brand batteries;
  6. Battery consistency: It is recommended to choose the brand batteries because of their high quality consistency, to avoid affecting the entire battery pack because of the failure of a single battery.
  7. After-sales service: It is recommended to choose the brand batteries because of their good services.