It’s a Gas
Q & A — January 2004
By Capt. Ken Kreisler
It’s a Gas
charging, troubleshooting a genset, fixing a leaking portlight, and more.
What does wet-cell
battery gassing mean? C.M., via e-mail
As the battery discharges, lead in the positive plates combines with sulfate in the sulfuric acid electrolyte to form lead sulfate. Oxygen, also present in the active material that makes up the positive plates, combines with the hydrogen in the electrolyte to form water. A similar reaction happens on the negative plates (also known as sponge lead plates), and as the amount of lead sulfate on the plates increases, so does the water in the electrolyte. The combination of decreased active material with weakened electrolyte results in a discharged battery that can no longer produce sufficient enough current to power the load.
During charging, as illustrated in the accompanying diagram, the input voltage drives the reaction in reverse. Lead sulfate in both plates splits into its original components of lead and sulfate, and water is split into hydrogen and oxygen. As the sulfate leaves the plates, it combines with the hydrogen to become sulfuric acid. At the same time, the oxygen at the positive plates will combine with the lead there and form lead dioxide. When most of the lead sulfate is converted to lead at the negative plate, hydrogen bubbles form and rise through the electrolyte. This gassing indicates the battery is nearly fully charged. The voltage at which this occurs is called the gassing voltage.
I have an older,
small auxiliary genset that will not hold a load. Before I call in a mechanic,
is there anything I can check? C. H., via e-mail
This article originally appeared in the December 2003 issue of Power & Motoryacht magazine.