Lead-acid batteries are the most widely used rechargeable (or secondary) batteries in the world. Applications for lead-acid batteries span the range from small portable devices (such as flashlights and video recorders) to large batteries for submarines. The duty cycle can vary from a short duration pulse (milliseconds) to sustained discharge periods of over 100 hours in length.
The popularity and widespread use of lead-acid batteries are due to the following factors: mature technology (in use over 100 years), wide temperature range for cycling and non-cycling operations, low cost materials, recycling of lead. Both the battery product and the battery manufacturing process are proven, economical, and reliable.
All lead-acid batteries have the same general overall
reaction for charge and discharge (Figure 5-1):
On discharge, lead dioxide (PbO2) of the
positive electrode and sponge lead (Pb) of the negative electrode are both
converted to lead sulfate (PbSO4). On charge, the lead sulfate in
the positive electrode is converted to lead dioxide (PbO2) (with
oxygen evolution on overcharge), and the lead sulfate sponge lead in the
negative electrode is converted to sponge lead (with hydrogen evolution on
overcharge). The electrolyte, sulfuric acid (H2SO4
an active component in the reactions at both electrodes.
In flooded batteries, the oxygen generated at the positive electrode escapes from the cell. Concurrently, at the negative electrode, hydrogen is generated from water and also escapes from the cell. The overall result is water loss and cell gassing, therefore, flooded cells require periodic water replenishment.
In valve-regulated (sealed) batteries, oxygen combines
chemically with the freshly formed lead at the negative electrode in the
presence of H2SO4
to provide lead sulfate and water. This oxygen recombination suppresses the generation of hydrogen at the negative electrode. Overall, there is no water loss during charging. A very small quantity of water can be lost as a result of self-discharge reactions; however, such loss is so small that no provision need be made for water replenishment.