By SK Ng

The following steps are guidelines in determining the capacity required for your back up system. Some end results may differ, depending on how the actual requirement shall be determined as well as the characteristics of the batteries used.

Step 1 Check the current drawn by your equipments and the operating period required
ie. total current drawn under 24V supply is 30A and 2 hrs of full load operation is required
Capacity C1 = 30 x 2
= 60 Ah

Step 2 Residual voltage percentage to remain at battery to avoid total battery discharge
ie. 30 % of voltage shall remain to avoid damage to the batteries
Capacity C2 = 60 / 0.7
= 85.7 Ah
Also refer to the battery data sheet for recommended residual voltage

Step 3 Determine optimum discharge rate
ie. If it is recommended to draw 5 amp/hour from 10Ah battery
Capacity C3 = 85.7 / 0.5
= 171.4 Ah
Also refer to the battery data sheet for recommended discharge rate
Higher discharge rate shall not be able to provide the required capacity and the voltage shall drop sharply

Step 4 Duty cycle and Capacity required
ie. 20% of current shall be drawn in an hour, ie.duty cycle = 20%
Battery capacity required = 171.4 x 0.2
= 34.28 Ah

In voice or music broadcast, the amplifiers shall not be constantly loaded at the rated value. Lower audio output, music or speech may only consume a fragment of power unlike siren. Thus using a factor of 20% shall be reasonable.