The nominal voltage of lead acid is 2 volts per cell, however when measuring the open circuit voltage, the OCV of a charged and rested battery should be 2.1V/cell. Keeping lead acid much below 2.1V/cell will cause the buildup of sulfation.

A lead-acid battery's theoretical voltage is 2 V for each cell. For a single cell, the voltage can range from 1.8 V at full discharge, to 2.10 V in an open circuit at full charge.

Charging

The correct setting of the charge voltage limit is critical and ranges from 2.30V to 2.45V per cell or 13.8V to 14.7V for a 12V battery. 


Power Levels

PercentageCell VoltageVoltage
1002.1212.73
902.1012.62
802.0812.50
702.0612.37
602.0412.24
502.0212.10
401.9911.96
301.9711.81
201.9411.66
101.9111.51
01.8010.8



Batter Capacity Meter Readings

These readings were taken from my batter capacity meter. 


PercentageVoltage
13.7100
12.988
12.885
12.679
12.573
12.470
12.264



Extrapolating from above

VoltagePercentage
13.3+100
13.090
12.780
12.470
12.160
11.850
11.540
11.330
11.020
10.710
10.40



References

ReferenceURL
Wikipediahttps://en.wikipedia.org/wiki/Lead%E2%80%93acid_battery
Charging Lead Acid Batterieshttps://batteryuniversity.com/learn/article/charging_the_lead_acid_battery
Plot of Battery Capacity Meter Readingshttps://plot.ly/~jmehan/3/
The Best RV Deep Cycle Batteryhttps://www.batteryequivalents.com/the-best-rv-deep-cycle-battery.html
Renogy Lithium-Iron Phosphate Battery 12 Volt 50Ah for RV, Solar, Marine, and Off-grid Applicationshttps://www.amazon.ca/Renogy-Lithium-Iron-Phosphate-Off-grid-Applications/dp/B07QB25H47/ref=sr_1_1?keywords=Renogy+Lithium-Iron+Phosphate+Battery+12+Volt+50Ah+for+RV%2C+Solar%2C+Marine%2C+and+Off-grid+Applications&qid=1572447880&s=automotive&sr=1-1-catcorr