Three common SoC monitoring methods – voltage correlation, current integration, and Impedance Track are discussed. State of charge of lead acid battery is the ratio of the remaining capacity RC to the battery capacity FCC . The FCC (Q) is the usable capacity at the current discharge rate and temperature.
Lead acid comes with different plate compositions that must be considered when measuring SoC by voltage. Calcium, an additive that makes the battery maintenance-free, raises the voltage by 5–8 percent. In addition, heat raises the voltage while cold causes a decrease.
The hydrometer offers an alternative to measuring SoC of flooded lead acid batteries. Here is how it works: When the lead acid battery accepts charge, the sulfuric acid gets heavier, causing the specific gravity (SG) to increase.
State of charge of lead acid battery is the ratio of the remaining capacity RC to the battery capacity FCC . The FCC (Q) is the usable capacity at the current discharge rate and temperature. The FCC is derived from the maximum chemical capacity of the fully charged battery Q MAX and the battery impedance R DC (see Fig. 1) .
State of charge can be monitored by measurement of current, voltage and temperature. State of charge can be determined from the battery impedance R DC. Impedance Track method must be temperature and discharge current compensated.
Texas Instruments uses the Impedance Track method to determine SoC of lead acid batteries . While current off, the OCV is measured, which is used to determine the SoC and to update Q MAX. When discharging, both discharge current and voltage are measured.