I have an AC system which has a continuous load of 7 amps at 120 volts. To run this system, I need a set of AGM battery and an Inverter system (power factor- 0.8). The inverter is 24 volts and rated for 2 Kva. I have a few 12 volt, 200 AH batteries. Now, how do I calculate the run time of my battery at full load mentioned above for the following situations ?

a.) Two 24V, 200 AH batteries.

b.) Two 24V, 400 AH batteries.

Also, from the battery manufacturer, I know that the cycles available for the battery are 1200-1300 for 30% DOD and around 600 for 50% DOD for each 12volt, 200AH battery. Now, will I get more cycles if I connect two 12V, 200ah batteries in series/parallel (More than 1200-1300 or 600 cycles ?) for the entire battery system, or even more cycles if I connect four 12V, 200AH batteries in series/parallel or a combination of series and parallel for the entire battery system?

 

Your load is 7A*120V=840VA.

Ássuming an efficiency of the inverter of 80% this translates into 840VA/0.8=1050VA at the input of the inverter.

As the inverter runs from 24V, 1050VA lead to a current of I=1950VA/24V=43.75A, let's use 44A to ease calculations.

A battery with a capacity of 200AH can operate at 44A for T=200AH/44A=4.5h (rounded).

As it is a good idea not to fully discharge the battery and as you'll have to take into account loss of capacity with time, you're probably good for <=4h operation with a 200AH battery, <=8h for a 400AH battery and so on. Or less if you discharge only by 30% to 50% to prolong battery life.

The number of batteries in series or parallel is of no interest here (as long as the input voltage to the inverter is 24V, that is).

For example 1*24V@200AH is equivalent to 2*12V@200AH. Since the limiting factor to battery life is how deep you discharge the battery, both setups (1*24V vs. 2*12V) will be equally discharged by the same current.

The only way to improve lifetime is to use batteries with higher capacity (more AH) and recharge them early so they will not be drained by more than 30% to 50%.

 

For example 1*24V@200AH is equivalent to 2*12V@200AH. Since the limiting factor to battery life is how deep you discharge the battery, both setups (1*24V vs. 2*12V) will be equally discharged by the same current.

thanks hurold, makes sense.