A lead-acid storage cell is the most common type of storage cell used. It is basically a prepackaged reversible chemical reaction. Its primary components are a positive terminal connected to plates filled with PbO2 (lead dioxide) and a negative terminal connected to plates filled with spongy metal lead (Pb) with both sets of plates immersed in an electrolyte solution of 33% sulfuric acid (H2SO4)

When an electrical circuit is made between the positive and negative terminals, the sulfuric acid is allowed to react with the lead on the negative plates to form lead dioxide (PbO2) and water (H2O). Electrons have to be jettisoned to allow this reaction to drive forward.

At the same time the sulfuric acid also reacts with the lead dioxide (PbO2) on the positive plates to form lead sulfate (PbSO4). Electrons have to be added to allow this reaction to drive forward.

When the terminal voltage is increased with a rectifier or other power source and electrons are forced back onto the negative plates, and the reaction is reversed. The original lead dioxide forms on the positive plates, metal lead forms on the negative plates and sulfuric acid returns to solution.

A cell fails for only one reason.  Each cell has a finite number of lead, sulfur, and oxygen molecules available for reaction. When enough of these molecules are tied up in superfluous compounds or become electrically disconnected, the reaction can no longer take place.

Common problems

Excessive discharge and the production of PbSO4 (lead sulfate)

As noted above, when the reaction runs forward lead sulfate is created. Lead sulfate is a powdery material that tends to disconnect from the plates. When excessive discharge occurs, lead sulfate flakes away to become electrically disconnected. Lead, oxygen and sulfur molecules are in this way removed from the reaction, often to the point of diminished capacity.

Bulging and damaged cell jars

Lead sulfate and lead dioxide are physically larger than metal lead. When these compounds are produced as a result of the reaction driving forward, they cause the plates to "grow" and bulge or crack the container. Also, the terminal posts may be pushed out.

Oxygen and hydrogen gas is created as a result of charging and discharging the battery. A valve-regulated battery is designed to recombine these elements into liquid form rather than vent them. However, if gas production is too rapid, bulging or other damage may result.

Electrically disconnected plates

The lead dioxide and lead sulfate produced by discharging the cell are softer and more brittle than metal lead. It is not unusual for a plate thus converted to crack in two or even break away from its bus. Electrically disconnected, many molecules are in this fashion removed from the reaction.

Contamination of electrolyte

If the electrolyte becomes contaminated, other reactions then occur, tying up molecules needed for the reaction. This is why you should add only pure distilled water if the electrolyte gets low. Also, portions of plate material may become coated with oxides or oily compounds, preventing contact with the electrolyte.

Ripple voltage

A defective rectifier that creates an unacceptable ripple (more than 20 mV) in the sine wave of the charging current may cause excessive heat within a cell. If this results in excessive lead sulfate, diminished capacity may result.

Excessive charge/discharge cycling

As the reaction runs back and forth with each charge and discharge, some molecules do not reform into their original compounds or elements. With each cycle, therefore, fewer molecules are available to react. At some point enough loss will occur to reduce capacity.
 

Common environmental parameters

Temperature variations should remain within 5 degrees F.

Battery temperature should not be affected by direct sunlight, heaters or air conditioning systems.

Optimal ambient is 77 degrees F. Temperatures over 80 degrees F will adversely effect battery life. (30% reduction at 86 degrees F)

The importance of float and equalize voltages

Float voltage above recommended limits reduces service life, while float voltage below recommended limits will not maintain full charge. GNB recommends for its valve regulated lead acid batteries 2.23 VDC to 2.27 VDC per cell. ( 27.24 VDC max for a 24 volt system) Above 2.28 VDC ( 27.36 VDC ) will result in 50% reduction of service life and 2.33 VDC ( 27.96 VDC ) reduces service life by 75%!

Due to potential cell damage, equalize charging should be done rarely if at all, and the manufacturers’ recommendations as to voltage and duration must be strictly observed.

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