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In practice, this method is sufficiently accurate for determining the size of a battery, assuming that the current consumed remains constant. 

For example, to calculate the capacity of a flooded tubular plate battery of 125 VDC to supply an installation consuming 500A at a constant rate over 30 minutes – with standard margins and no additional factors applicable.  

The steps are as follows:

• Determine the maximum and minimum voltage margins of the installation. The usual industrial standard is +10% -15%, but due to local regulations, voltage Drops etc. this margin may vary.  For a 125 VDC battery, we would have a maximum of 137.5 VDC and a minimum of 106.25 VDC. 
• Determine the number of battery cells. If maximum voltage is 137.5 VDC and I need 2.4 VDC for a rapid charge, I can only charge 57 cells.  This is the maximum number of cells possible.  (Also see #6 - Use of Voltage Reducers in Rectifiers to determine the number of cells in batteries – practical examples.)
• Determine minimum cell voltage at end of discharge by dividing the final discharge voltage, 106.25 VDC by the number of cells, 57 – each cell will have a minimum voltage of 1.86 VDC.
• In the constant current discharge table provided by the manufacturer (see https://www.norwatt.es/files/products/pdf/batteries_lead_block_gnb_classic_opzs_en.pdf )

(N.B.  Not all manufacturers provide these tables.)  For the desired range of batteries, we have 1.86 VDC per cell.   Therefore, we would choose the table corresponding to 1.87 VDC final voltage.  In order to achieve 30 minutes of battery autonomy with a consumption of 500 A, we would choose battery 14 OPzS 1750 LA which would consist of 57 cells in series.

When purchasing a battery, it is essential to indicate how the cells will be arranged on racks or in cabinets so as to be supplied with the appropriate connectors.  When this is not taken into consideration, problems can arise in the assembly of the battery which can be especially difficult and costly to remedy when located in remote areas.