How to calculate Battery Capacity?

Battery capacity calculator

How to calculate battery capacity?

The battery capacity required for specific application can be calculated in the following manner. Let us take the example of

Inverter battery: An inverter battery is expected to work during power cuts, power outages. The duration of outage may extend from several minutes to many hours. But it is usual practice to design for Capacity discharge  for the extreme conditions. This will automatically apply for other situations as well.

Example 1

Current drawn is 33 amperes and duration required is 2 hrs 45 minutes .The duration can be varied to get a 400 W output-

  • Ah drawn= 33 Ax2.75 hr= 90.75 Ah. Since the discharge duration is near a 3 hr discharge rate, from a 10 hr rated battery, calculate the required Ah capacity.
  • IS specification gives the capacity obtained is 71.7 %.Therefore capacity of battery rated at 10 hr is = 90.75/  71.7  = 127 Ah

You should choose a battery having capacity near to 127 Ah, say 130 Ah  rated  at 10 hr. Such a battery will also meet say longer power outages like 5-6 hrs at a lesser load.

Engine starter applications

Example 2

When a higher current is required such as for Cranking railway locomotive engines, Gensets or passenger cars, we get

  • 2300 A X 5 sec/3600 = 3.2 Ah – 5 times =16 Ah
  • 300 A  x 3 sec/3600  =   0.25 Ah  ; 3.2 Ah

Comparison of starter batteries - an engine starter battery can sustain a current of over 4 - 5 times the Ah capacity of the battery

Characteristics Automotive Diesel Locomotive
Ah capacity of battery 70 Ah 450Ah
Cranking current 300 A 2300 A
Cranking duration 3 secs 1 to 2 times 5 secs 7 to 9 times
Cranking current per Ah capacity of battery 4.3 5.1

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