The battery should not, therefore, be discharged below this voltage. In between the fully discharged and charged states, a lead acid battery will experience a gradual reduction in the voltage. Voltage level is commonly used to indicate a battery's state of charge.
In general, lead acid battery comprises a flat terminal voltage in the range of 40% to 80% of the state of charge (SOC) . As shown in Figure 1, the voltage variation in this range is less than 0.44 V. ...
Temperature affects lead acid battery voltage levels. The voltage level of a lead acid battery increases as the temperature decreases and vice versa. Therefore, you need to consider the temperature when measuring the voltage level of a lead acid battery. At what voltage level is a lead acid battery considered fully charged?
A lead acid battery is considered fully charged when its voltage level reaches 12.7V for a 12V battery. However, this voltage level may vary depending on the battery’s manufacturer, type, and temperature. What are the voltage indicators for different charge levels in a lead acid battery?
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. While on float charge, lead acid measures about 2.25V/cell, higher during normal charge.
The battery may also fail as an open circuit (that is, there may be a gradual increase in the internal series resistance), and any batteries connected in series with this battery will also be affected. Freezing the battery, depending on the type of lead acid battery used, may also cause irreversible failure of the battery.