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when you buy a 12 V lead acid battery, as in the example, the actual measured voltage will be greater than 12 V.
as in the example, about 12.6 or 12.7 V.
because it is difficult to build EXACTLY 12 V, manufacturers design the batteries to be slightly greater than 12 V, hence the 12.6 or 12.7V battery.
this "overvoltage" is the result of a design margin - it guarantees that all of the batteries will be at least 12 V
the goal here is not to get accused of selling an under-voltage battery.
it is not likley that someone will complain about getting a 12.6V battery instead of 12 V battery
but it is likley that someone will complain about getting a 11.5V battery instead of a 12 V battery
But there are some parameters that also have influence on the voltage:
* charging state (@ zero current: 1.8 V to 2.27 V)
* load/charging current: (down to 1.5V (still healthy) ... up to almost 3V)
* lead purity
* acid purity
This means: If you go to a shop with a voltmeter and buy the battery with almost exactly 12.0V then you still will see that it varies it´s voltage from 10.5V up to 14.4V. (for example)
Each battery chemistry has it's own intrinsic nominal voltage (which varies from the charge to the discharge state).
Thus the 0.6V "margin" of a nominal 12V lead-acid battery is just the result of the nominal cell voltage being about 2.1V, and a 12V battery is composed of 6 cells in series.
Similarly a NiMh cell has a nominal voltage of 1.2V, and a lithium-ion cell has a nominal voltage of 3.2V to 3.8V depending upon its specific chemistry.
The voltage is most stable at the midpoint (that value corresponds to E0)
The difference in voltage between the 90% and 10% (degree of charge) will be approx 2*60/n mV
(But this is a theoretical value and you can expect some variations; but this is the general theme).
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As pointed out in post #3, the nominal voltage of a lead acid battery is about 2.05. Once charged to 99% capacity, it reaches about 2.05+0.120=2.17 (real value is closer to 2.2V). Once discharged to 10% capacity, the voltage drops to 2.05-0.06=1.98 (real value is about 1.8V).
All battery voltages change with their state of charge (degree of discharge) and that is useful to measure the state of the battery charge). The actual relation must be determined by experiment.