Check out "Energy Primer Solar, Water, Wind, and Biofuels" by Richard
Merrill and Thomas Gage- Dell Publishing--accompaning and discussion on
page 125. Graph shows both charge and discharge cycles with respect to
amp hours, volts per cell, specific gravity, and amp hours returned
over time. It appears that the specific gravity during discharge is
linear with respect to amp hours discharged, but not linear with
reguard to volts per cell. During charging, specific gravity of the
cells over the charge time is not linear in respect to amp hours
returned or volts per cell. It's a mindfull---good luck!