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Number of cycles for AGM battery with an inverter

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Member level 5
Mar 9, 2012
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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?

The time to failure depends on the degree of mismatch among all the cells for specific gravity and ESR.

Eventually one cell gets weak and destroys the battery. In practise it is better to put batteries in series to reduce the the probability for peak currents which can cause accelerated aging or shorted plates. As well it makes it easier to reduce conduction losses. But then reduces the time to failure a bit as there is no redundancy.

Total lifetime Ah capacity reduces with DoD on each cycle even for Deep discharge marine types. So get more capacity than you need.

Battery quality varies widely with supplier and high operating temperature increases sulphation rapidly which can be reduced with short pulse currents.

A central office for phones will use a large room full of 200 kg 2V cells in series and parallel to get 48V. This offers the best tradeoff.

For your calculations, it is easier to deal in units of power. Watts, watt-hours.

Your load is 840 W (120 x 7).

A battery is 12 x 200, equals 2400 W-hrs.

Two batteries in series is 4800 W-hrs. They will last 5.7 hrs (4800 / 840).

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