How is the nominal voltage of a battery determined?
A NiMH cell's usable voltage ranges between around 1.4-1.0V and the nominal voltage is quite in the middle of that at 1.2V.
Similarly, the nominal voltage of a LiPo is given most of the time between 3.7V and 3.9V, with the usable voltage being between 3.0V-3.5V and 4.2V-4.6V. So again, the nominal voltage looks to be in the middle of the usable voltage.
But for Alkaline cells the nominal voltage of 1.5V is at the very top end of the usable voltage of 1.6V to 1V.
So how is the nominal voltage determined, and is there a difference between primary and rechargable cells?
To clarify the question because there are a few answers that apparently misunderstood the question.
On some chemistries the nominal voltage is close to the maximum voltage of the cell (e.g Alkaline with 1.5V, where a fully-charged cell has 1.6V), while others have their nominal voltage at the average charge (e.g. NiMH at 1.2V or LiPo at ~3.8V).
Why are some nominal voltages close to fully-charged voltage, while others have are at 50% charge?
Is the difference that primary cells have their nominal voltage at 100% and rechargable cells have it at 50%?
I am NOT looking for answers to those questions:
- How can I find out the nominal voltage of a battery?
- Why have different chemistries different voltages?
- What are primary/rechargable cells?
Best Answer
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.