[Rick]

John,
First I apologise for not paying sufficient attention to the details, like failing to hit "shift" for a "+" and getting a "=" as you found, kinda like not checking my work.

One source for the formula giving voltage versus SG comes from an old IBE (a large manufacturer of batteries in this country) book and I've seen it quoted elsewhere as well. IN addition, I've verified the general accuracy.

I note a definite departure in the voltages versus SG for the first chart provided in your link in that it shows 12.478 Volts for a 1.271 SG @ 65 deg F when, for absolutely sure a 1.26 SG at 65 deg will show a much higher voltage (12.66 V). Of this I am sure. I question, therefore, all those voltages quoted.

The next chart down (in green and red) show a voltage for a 0% charged battery as being 10.5V. 10.5V is the end of test voltage for a 20 hour current discharge rate for making capacity tests. Obviously the voltage will rise after removal of the test current to a higher voltage (11.76 Volts) so this table is also suspect as not meeting standards.

According to Storage Batteries, by Vinal, aside from voltage variations caused by temperature of the electrolyte there are slight variations depending upon the grid makeup such as pure lead (like AGM and Gell-cell batteries and those grids of lead-antimony. There are also differences between the results made by various well-known electrochemists yet these variations occupy the second place past the decimal, i.e.; 2.1x on a volts-per-cell reading. Therefore, the third place past the decimal for nominally 12V batteries is of a highly dubious accuracy.

A comment was made about voltage during discharge rising after time. This is due to the discharge current being sufficiently high to cause electrolyte heating and plate heating which yields an increase in voltage. The electrolyte temperature increase also can result in a lower apparent cell resistance.

Please keep in mind that using open-circuit voltge to infer state of charge is not very accurate by itself. Using my method of determining capacity would be fortunate to be within 10% yet that is sufficient for energy planning purposes. I have long used (and advocate using) accurate battery monitors to make internal resistance checks and by noting voltage under noted load current along with actual Amp-hour discharge measurements one CAN infer just what is the general quality of one's battery. This is not supposed to be used to buy used batteries.