estimating battery discharge current approximately from ac load current

Question

I would like to know that if we have a data logger installed at a site and the logger measured the constant load current of 10 A from the Grid(Single phase@ 230 v) during a certain time say 10 am.

Now my question is if i replaced the grid with a battery back up s/m (for eg: say 48 v 150 AH Battery and a DC to AC converter System) what would be the battery discharge current from the battery for the time 10 am.?

Considering the absence of grid here what will be the approximate current drawn from the battery to match the out put current of 10A @ 230V for the DC-AC converter. And what all factors should we consider before approximating the battery discharge current

I would appreciate your valuable inputs

Answer 1

A good first approximation would be to:

1. Calculate the apparent power in VA. That's easy, just take the RMS voltage times the RMS current, in your case 10A * 230V.
2. Divide that power by the battery voltage to get the battery current. Note that this assumes a perfect converter, which doesn't exist; remember, this is a first approximation.
3. According to your question, you're done, but you may also be interested in maximum runtime. For that, divide the battery's capacity (150 amp-hours in your case) by the current that you got in step #2.

Okay, now that you have an unacheivable maximum performance figure, you can start accounting for inefficiencies, specifically:

• The converter isn't 100% efficient. Perhaps more like 80-90%. The other 10-20% represents extra current pulled from the battery and converted to heat.
• Battery capacity is not constant with load. It tends to decrease under heavy loads so that you may get 170-200 AH under a light load and closer to 100 AH under a heavy load. Exactly how much it changes depends on the specific battery.