The health of an SMF battery can be determined by checking the OCV (Open circuit voltage) of the battery. A healthy battery will have an OCV above 12.8V. Any voltage below this means the battery is not sufficiently charged and needs a charge. The typical OCV readings should be as follows

  • Just after unpacking at site: Minimum 12.80 V
  • Just after connecting on float: Minimum 13.30 V
  • After 2 days of float charging: Minimum 12.95 V

The health of a flooded battery can be determined by checking the OCV and also by checking the specific gravity of the electrolyte. Specific gravity is a unit of measurement for determining the sulphuric acid content of the electrolyte. The recommended fully charged specific gravity of flooded batteries is 1.255 to 1.265 taken at 80°F. More than .025 spread in readings between fully charged cells indicates that the battery may need an equalization charge. If this condition persists, the cell is failing and the battery should be replaced. Since water has a value of 1.000, with electrolytes having a specific gravity of 1.260, it means that it is 1.260 times heavier than pure water even while pure concentrated sulphuric acid has a specific gravity of 1.835.

The following table illustrates typical specific gravity values for a cell in various stages of charge:

  • 100% Charged…….1.255 – 1.260 Sp. Gr.
  • 75% Charged…….1.220 – 1.225 Sp. Gr.
  • 50% Charged…….1.185 – 1.190 Sp. Gr.
  • 25% Charged…….1.150 – 1.155 Sp. Gr.
  • 0% Charged…….1.115 – 1.120 Sp. Gr.