Why do some batteries have different specific gravity readings across cells?

Causes of Variation in Specific Gravity

When measuring the specific gravity of a battery cell, the result can vary depending on the cell’s state of charge, electrolyte concentration, and temperature. For example, a 12-volt battery with 6 cells may have different specific gravity readings due to unequal electrolyte concentration or depth of discharge across cells. A 10% difference in specific gravity between cells is typically considered acceptable, but any greater variation can indicate a potential issue.

Measuring and Interpreting Specific Gravity

To measure specific gravity, a hydrometer or density meter is used to determine the specific gravity of the electrolyte in each cell. The specific gravity reading can be converted to a state of charge using a reference chart or table, but the accuracy of this conversion depends on the battery type and manufacturer. For lead-acid batteries, a specific gravity of 1.265 is typically considered fully charged, while a reading of 1.100 indicates a state of discharge.

Maintenance and Troubleshooting

To maintain consistent specific gravity readings across cells, it’s essential to follow proper charging and maintenance procedures. This includes monitoring cell temperatures, avoiding overcharging or deep discharging, and ensuring the battery is properly matched and sized for the application. If significant variation in specific gravity is detected, it may be necessary to perform a cell balancing procedure to equalize the state of charge and prevent premature aging or failure.