Battery capacity calculator

Battery capacity calculator

Battery capacity calculator

Lead acid batteries

The battery capacity calculator helps to calculate the required Ah capacity for a specific application.

Let us take the example of inverter battery

Battery capacity calculator for inverter battery

While automotive starter batteries provide peak capacity over a short period of time. The automotive or SLI battery is not suitable for supplying long, even power demands. In such a case they quickly go flat, leading to early failures.

Inverter batteries, on the other hand, are designed to provide the required power consistently for longer durations of time. They are usually used as a power back up device called inverters. The inverter battery capacity calculator helps you to arrive at the correct selection of a battery to ensure that you get a continuous supply of power for the required load and for the required time duration. Not using a battery capacity calculator can lead to wrong selection of battery

Battery capacity calculator for inverter battery


An inverter battery is expected to work during power cuts, power outages. The duration of the 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.

Inverter battery capacity calculator

Battery capacity calculator
Inverter battery capacity calculator

Battery capacity calculator for Starter battery

Engine starter applications

Starter batteries are called SLI batteries, which is an acronym for Starter Lighting & Ignition. The starter battery’s function is to start the internal combustion engine by supplying the heavy current required to turn the starter motor of an engine. The starter battery draws a heavy current from the battery in a burst of a brief discharge usually 20% after which the engine starts and the battery gets charged by the alternator. Depending on the size of the engine & the motor that is required to crank the engine, the battery capacity calculator for this load is arrived as

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

Battery capacity calculator - comparison of starter 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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