Sunday, April 13, 2014

Battery Rating Calculation

Batteries are usually connected in Series as in Series Voltage get added up and current remains the same.


The formula employed rating calculation is as stipulated below:-

Capacity of Battery (AH) = (DC Current X Duration in Hrs)/( %age capacity utilization)
Where, DC Current = (UPS (KVA) x 1000 x Load Power Factor (=1))/( Inverter efficiency x End Voltage)
 Hence, VAH = AH x Nominal Voltage
 For e.g.: for 3 KVA UPS, 1 hour Backup having 18 no. batteries:

DC Current = (5 x 1000 x 1)/ (0.95 x 10.5 x 18) =27.85A
Battery “AH” =   (27.85 x 1.0)/ (0.62) = 44.91

Hence , 18 no.s of 12V/42AH batteries will suffice .
TOTAL VAH = 18 x 12 x 42 = 9072 VAH

The following assumptions have been made in the above calculations:-
1. DC Voltage – 216 for 5 KVA
2. End cell voltage / battery of 10.5 V
3. Load Power Factor = 1.0
4. Inverter efficiency = 95%

% Capacity utilization is: 

Duration % Capacity utilization
½ hr (30 min’s) à52%
1hrs à 62%
2hrs à74%
3hrs à83%
4hrs  à85%

  • The amp-hour is a unit of battery energy capacity, equal to the amount of continuous current multiplied by the discharge time, that a battery can supply before exhausting its internal store of chemical energy.
  • An amp-hour battery rating is only an approximation of the battery's charge capacity, and should be trusted only at the current level or time specified by the manufacturer. Such a rating cannot be extrapolated for very high currents or very long times with any accuracy.
  • Discharged batteries lose voltage and increase in resistance. The best check for a dead battery is a voltage test under load.