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LEAD-ACID, EDISON, NICAD, & NIMH CELLS

The modern lead-acid battery is by far the most familiar rechargeable storage cell technology. The lead-acid cell was invented in 1859 by a French physicist, Gaston Plante. It uses dilute sulfuric acid for an electrolyte, lead for the anode, and lead oxide for the cathode.

The sulfuric acid dissociates into two hydrogen ions (protons) and a sulfate group. The sulfate group reacts with the lead anode to form lead sulfate and releases two electrons through the external circuit. This is the oxidation reaction, which can be summarized as:

Pb and HSO4- --> PbSO4 and H+ and 2 e-

At the cathode, the two electrons cause a reaction to create lead sulfate and water. This is the reduction reaction, which can be summarized as:

PbO2 and HSO4- and 3 H+ and 2 H+ --> PbSO4 and 2 H2O

At full discharge, both anode and cathode are covered with lead sulfate, and the electrolyte is mostly water. As the sulfuric acid solution is denser than water, a "densitometer", consisting of no more than a dropper with pellets of varying densities and different colors, can be used to examine the cell's charge level. Reversing the current flow reverses the reactions, recharging the cell.

A standard automotive battery consists of a box-shaped casing with internal divider walls to separate its series-connected cells. The electrodes in each cell are built as sets of interleaved plates to provide the maximum surface area for the electrochemical reaction.

Each cell in a lead-acid battery provides about two volts. Lead-acid batteries usually have large capacities, though they tend to run down quickly. They can be recharged hundreds of times until their electrodes are too eroded to allow the battery to hold a charge. They have indefinite shelf lives if stored without electrolyte.

Lead-acid batteries are cheap and effective, and at present are the sole practical choice where high power capacities are required at sensible cost, Ruggedized and sealed lead-acid storage batteries are in common use in portable equipment with large power requirements. However, lead-acid batteries are bulky, and their active materials are environmentally hazardous and so require recycling as a reasonable environmental safety measure.

A new type of lead-acid battery was introduced in the late 1990s that operates on the same chemical principles, but has a radically different construction. The electrodes are formed as thin plates, with the electrolyte stored in a separator sheet between the plates, and stored in a sealed can in a "wound" or "jelly-roll" configuration. The improved battery configuration provides a higher energy density, though the environmental issues remain much the same.

This is about the only significant innovation in lead-acid battery design in over a century of the technology's existence. To be sure, there have been improvements in packaging materials for lighter weight and greater reliability, but Gaston Plante would see little in a modern lead-acid battery that he didn't find familiar.

Trying to come up with a high-capacity rechargeable cell with a higher energy density at a reasonable cost has proven extremely difficult. This was frustrating even a century ago, and the well-known American inventor Thomas Alva Edison spent a fortune trying to build a rechargeable cell that could improve on Plante's invention.

The result was the "nickel-iron" cell, or "Edison cell", and though it still lives on in industrial uses, it never came close to displacing the lead-acid battery. The Edison cell uses an iron anode, a nickel oxide cathode, and a potassium hydroxide electrolyte.

The Edison cell provides a voltage of about 1.15 volts per cell. Its main virtue is that it is extremely rugged, tolerating discharge treatment that would ruin other types of storage cells, and has a very long service life.

The "nickel-cadmium" or "nicad" cell is similar to the Edison cell, but uses a cadmium rather than an iron anode. A nicad cell is generally a cylinder with layers of cadmium and nickel oxide separated by absorbent layers containing KOH electrolyte. The anode reaction is:

Cd and 2 OH- --> Cd(OH)2 and 2 e-

The cathode reaction is:

NiO2 and 2 H2O and 2 e- --> Ni(OH)2 and 2 OH-

These are reversible reactions. The nicad produces about 1.2 volts per cell. It has a low internal resistance and its cell voltage remains remarkably constant until the cell is almost discharged. While Edison batteries are generally built as large industrial units that physically resemble lead-acid batteries, nicads are built mostly for rechargeable consumer equipment and so have smaller form factors. Nicads were once predominant as rechargeable batteries in consumer gear, but they tended to be ruined by complete discharge, and the heavy-metal cadmium anode made them an environmental nuisance. Nicads are still in widespread use, particularly for portable power tools where their ability to provide large amounts of current on demand makes them particularly useful, but are now increasingly being replaced by improved rechargeable battery types.

One such improved rechargeable technology is the "nickel-metal hydride (NiMH)" cell. Most designs are similar to nicads, but replace the cadmium anode with a "metal hydride", based on complex metallic alloys that can store large quantities of hydrogen, The cathode is nickel oxide, the electrolyte is a solution of potassium hydroxide, stored in a polymer separator sheet. The anode reaction, with "(M)" representing the metal hydride, is:

(M)H and OH- --> (M) and H2O and e-

The cathode reaction is:

NiOOH and H2O and e- --> Ni(OH)2 and OH-

They have a typical cell voltage of 1.2 volts, which tends to remain flat through the cell discharge cycle. They tend to have a high self-discharge rate, but are relatively environmentally benign.

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