Nominal voltage of a battery
The nominal voltage of a battery is not determined. It is specified.
If there was such a thing as 'nominal voltage' that could be measured, then it could determined by measurement.
The clue is in the name 'nominal'. Amongst its meanings are 'existing in name only' and 'within expected and acceptable limits'. It's a convenient way of summarising the cell's performance, and is not meaningful for any practical evaluation of the cell, or design for equipment using it.
If a rechargable battery has a voltage at which it delivers most of its stored energy, then that tends to get picked for the 'nominal' voltage, so 2 v for lead and 1.2 v for nickel chemistries, otherwise 'somewhere between max and min' is all you can do. Below 1.2 v, there's very little deliverable energy left in a nickel cell.
The choice of 1.5 v for the nominal voltage for alkaline cells has driven the market for using these cells in a bad direction. There are far too many AA-using devices that say 'battery low' when down to only 1.2 v or even 1.3 v. When I was in engineering in a professional scientific instrument company, our internal rule was that battery-driven products had to work down to 0.75 v per cell. Perhaps that was excessive, but there's still quite a lot of energy to deliver from an alkaline cell between 1.2 v and 1 v, which would be a more reasonable lower limit.