Tuesday, December 1, 2015

Battery Calculation for UPS

It is important to select battery size according to UPS capacity otherwise appropriate backup of UPS can’t be obtained.

To calculate battery size according to UPS capacity is as below:-

UPS Backup [in hours]X UPS Capacity in VA = Battery Ah
            Volts* Power factor

From Above formula we see that:-
UPS Backup(in Hrs) means backup requirement of application

UPS Capacity is in VA
Volts is the voltage of battery is usually 12V
Power factor is the power factor of connected load. If you don’t know power factor then take Power factor as 0.8.
Let’s take an example of 500 VA UPS, Battery voltage is 12 V and Power factor as 0.8 and UPS backup required is of 2 hrs then battery size in Ah is as below:-
2X 500 = Battery Ah

Battery Ah= 1000/9.6
It comes approximately as 100Ah so battery requirement is 100 Ah for 500 VA capacity for backup of 2 hrs.

If there are UPS of higher ratings then there will be requirement of more batteries to be either connected in series or parallel.

Then following formula is used in this case:-

UPS Backup [hrs] =N(series) X VoltsX Battery Ah X N(parallel)/(UPS capacity in VA)

Where, Series and parallel of batteries combination is calculated as per user requirement.

If there are two or more batteries are required then following formula is used:-

Backup [in hours] = No. of batteries X Battery Ah X Volts / (Load/p.f.)