VRLA Batteries: Vent Caps and Gassing

Has anyone come across a VRLA battery bank that was installed and the shipping vent caps were never removed. The battery bank has been in service for 3 years. It’s obvious that gassing has occurred. The fluid levels were low. Some of the jars were bulging. The flame arrestors were found in a box on the floor never installed. We were able to remove the shipping caps and fill the batteries with distilled water. We installed all of the flame arrestors when we were done filling the batteries. The contractor wanted us to test the batteries. We advised that the batteries needed to equalize charge and we should wait a few months before we test. The owner insisted we test the batteries. All the specific gravity results came out low and the owner wanted us to add sulfuric acid to the batteries.

I pushed back pretty hard on the situation and informed them that all of the water had evaporated, leaving a much higher specific gravity. We added water and there wasn’t enough time for the water to mix with the sulfuric acid, which lead to the lower specific gravity readings. We returned a month later to retest and the specific gravity of almost all batteries are back in the required range. But the cell voltages still appear high.

Does anyone know what effect “gassing” has on individual cell voltage? Literature on this matter is very hard to come by.


Gassing can cause an increase in cell voltage because the generation of hydrogen and oxygen gases creates additional chemical reactions within the battery, resulting in a higher cell voltage. Excessive gassing also leads to loss of electrolyte, increased pressure, and reduced battery life.

Some potential sources of gassing to watch out for:

  • Overcharging
  • High Charge Currents
  • High Temperatures
  • Impurities in Electrolyte
  • Battery Age

Sounds like that bank is in pretty bad shape for only being in service for 3 years. What is the operating environment like? What have the charging practices been since install? Fixing the electrolyte may just be a Band-Aid as the batteries could be internally degraded at this point.

So the generation of hydrogen and oxygen can cause an increase in cell voltage? I’m assuming the hydrogen will dissipate with flame arresters being installed and the voltage will eventually come back down within range.

The bank was in a newly installed state. I believe the lifespan of the bank has been degraded by an improper installation. But if we give it a few more months and retest, we may see a change in the cells that still had a higher voltage. We did not add any more electrolyte. As I suspected, you need to allow time for the water to mix with the acid before performing specific gravity. The results will be high if you take them before adding water, and low after you add water. Batteries do not typically loose acid when the liquid levels are low.

It will be interesting to see how they re-test in a few months and perhaps again a few more months after that. Definitely sounds like a bad install, not removing the caps along with high temps probably led to the increase in pressure as you witnessed with the bulging jars. Letting it sit like this for several years is concerning.