Installing Solar Electric Since 1983

 

Proper Procedure for Checking Specific Gravity
in Flooded Storage Cells

I thought I had been instructed to check specific gravity in flooded cells after equalization but prior to rewatering. The problem is that prior to re-watering the electrolyte level in the cells can vary quite a bit. I created a theory that if the electrolyte level in a cell was low the SG would test inaccurately as artificial high. This theory is based on the assumption that the sulfuric acid volume does not change but the water volume does. If the water volume is reduced, the ratio of sulfuric acid to water (and the Specific Gravity) would increase.

I did a test on some batteries being maintained. I tested SG before and after rewatering. I found that cells with low electrolyte level tested generally with higher than expected SG and those SG readings decreased after water was added to bring electrolyte levels up to a uniform level. The more water that was added, the greater the decrease in after watering SG readings.

Here is a link to the spreadsheet in which I recorded my results.

Tools used:
1. To test SG: Misco Palm Abbe digital refractometer.
2. To fill water: Philadelphia Scientific Water Gun-X and Hydrofill cart. I recorded the sound of the water system filling the cells and assigned the amount of water added to different cells based on the duration of the automatic water filling as regulated by the X-Gun.

Conclusion: Here is the proper order of steps to check Specific Gravity:
1. Verify water level in cells is above the moss-guard. Add water only to bring water to top of moss-guard.
2. Run Equalizing charge.
3. Add distilled water to all cells to bring electrolyte to recommended level.
4. Apply bulk charge voltage to batteries for 30 minutes to mix electrolyte.
5. Test and record Specific Gravity.