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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?

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There are really two questions here.

  1. To calculate the run time you first need to estimate the amount of current being drawn by the inverter. If you assume an efficiency of 85% (your millage may vary) then you can calculate the current as
    ((120V / 24V) * 7A) / 0.85= 41A. This number is a bit on the high side since the voltage will initially start around 26V and drop down as you discharge the battery. The run time for 50% discharge is then calculate as a) (200 AH/41A) / 50% = 9.7 hrs. In case b) you have double the capacity so the runtime is increased by two.
  2. You don't get any more charge discharge cycles by combing batteries in series. However, if you combine them in parallel then you would only be discharging the banks by half as much for a given a given amount of time. So even though your charge/discharge cycles wouldn't change, you would get twice as much work out of the batteries.
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  • \$\begingroup\$ Can you please explain how did you come up with 9.7 hrs for (200ah/41amps)*50% ? \$\endgroup\$ – bhuvan Jul 29 '15 at 14:36
  • \$\begingroup\$ Oops. It is (200aH/41A)/50% ... sorry. If you discharged the battery completely it would roughly be 200AH/41A = 4.9 Hrs. I say roughly because the capacity of a battery is relative to how quickly you discharge the battery ( Peukert's law -> (en.wikipedia.org/wiki/Peukert%27s_law). \$\endgroup\$ – Argus Brown Jul 29 '15 at 16:17

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