The specific gravity of battery electrolye increases after charging, decreases after discharging, therefore, we could just battery state of charge(SOC) according to electrolyte specific gravity.

Keep the sight line be level with electrolyte level when reading with hydrometer, otherwise, the reading is not accurate. When measuring the density of electrolyte, electrolyte temperature should also be measured and conversed to the density corresponding to 25℃, since along with every 1℃ increase of the temperature, density of electrolyte will decrease by 0.00074g/cm3. Therefore, conversion should be made to plus or minus the increment of decrement if temperature is above or below 25℃. Simple reference table is shown below.