Thursday, March 8, 2012

The Lead-Acid Battery - Its Care and Feeding for Long Life

G. Plante' began his study of electric polarization in 1859. As a result of his experiments, he devised a battery consisting of two sheets of lead rolled into the form of a spiral. The lead sheets were separated by strips of rubber and this assembly was immersed in dilute sulfuric acid. He studied the charge and discharge of this simple cell and described how to materially increase the capacity by a process called formation which is the multistep creation of a layer of sponge lead on the surface of the negative plates and a layer of lead dioxide on the positive plates. These layers are the chemically active portions of the cell. This process is different from that used for developing the pasted plate cells, but "formation" is commonly used when referring to them as well. Formation of pasted plates means the reduction or oxidation of the lead oxides or other materials that have been applied to the grids.

Plante' would charge his cell and then discharge it, or alternatively, allow it to rest. During this time local action transformed the dioxide covering on the positive plate into lead sulfate. Periodically he would reverse the polarity applied to the cell and would repeat this process to increase the capacity of the cell. As Plante' had no generator until 1873 it took many primary batteries for the formation of the plates. The oxidation of 1 kilogram of lead (Pb) to lead dioxide (PbO2) requires 514 ampere hours of current. The difference between Plante' plates and modern ones is that the material is electrochemically formed from the lead of the plate itself in a Plante' cell and modern plates have their materials applied to them.

A typical lead-acid cell consists of:

A lead anode (negative plate)
A lead dioxide cathode (positive plate)
Grid structure plates made of various alloys
An electrolyte (sulfuric acid)
Microporous rubber separators between the plates
A container that is impervious to the electrolyte
The electrical capacity of the cell is proportional to the surface area of the plates, primarily the number of positive plates. In normal construction the outside plates are negative so they are the number of positive plates plus 1. Present types of lead-acid batteries are lead-antimony, lead-calcium and gelled electrolyte. Typical energy densities are 12 watt-hours per pound and 1 watt-hour per cubic inch. High capacity, long life batteries may be 25% larger than this. The capacity is expressed in ampere-hours since the cell voltage is known. The state of charge, the discharge current, the temperature of the battery and the cutoff or end voltage determines the capacity. Your battery should have sufficient ampere-hour capacity to carry momentary loads plus continuous or basic loads for a specified length of time before reaching its final or cutoff voltage, commonly referenced at 1.75 volts per cell.

Ampere hour capacities are typically specified at 20 hour rate for gelled electrolyte. Deep discharge utility cells are at 8 hour rate.

Due to electrochemical differences between cells it is not recommended practice to connect cells in parallel. If you need more capacity use a cell with more or larger plates rather than connecting two cells in parallel.

On daily or frequent charges it is common practice to charge slightly short of a fully charged condition. A complete charge must be made every 1 to 3 months (except gel cells). This complete charge is commonly referred to as an "equalizing charge" and is intended to be sufficient to equalize any minor differences among the cells. It should be continued until each cell of the battery reaches maximum voltage and specific gravity.

The commonest battery is the automotive starting battery. It is not designed for deep discharge and will quickly fail if placed in deep discharge service. It is constructed of many thin plates of lead sponge which provides a large surface area for chemical reaction enabling it to provide large currents while maintaining voltage level. This design is for short duration high currents followed by immediate recharge.

In deep discharge service the thin spongy plates will disintegrate from the chemical changes produced by charge and discharge. At 50% discharge they will often reach full failure by 200 cycles. In automotive service these batteries last 3 to 5 years. In deep discharge service they cost more than the expensive deep discharge cells due to their early failure.

The true deep discharge cell is designed for long life. They are large and heavy. The plates are more than 4 times thicker than standard batteries and the compartments are large with abundant electrolyte. They are rarely assembled into batteries of over 3 cells and are often individual cells because of their weight.