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Lead-acid Batteries

The Pb/PbO2 battery is the most well-known, low cost, secondary battery system. Some laboratory cells have delivered characteristically 50 Wh/kg, and 435 W/kg for a 1 second discharge (pulse power). Until recently, the system was not available below a D-size cell.

The lead-acid battery is familiar as the automobile starting-lighting-ignition (SLI) battery, but it is also important for silent watch (e.g. Coyote LAV) power supplies, emergency lighting, uninterruptible power supplies (UPS) and vehicle propulsion. The following comments refer mainly to the modern sealed, recombination batteries that are now widely available. If a lead-acid battery is: left in the discharged state for only a very few days (open-circuit voltage less than about 1.85 V/cell); or used at too high a temperature; or used with too high an acid concentration; or if a small discharge current is drawn for a long time, the lead sulfate may recrystallize into a dense coarse-grained form which has high resistivity. This is known as sulfation and it inhibits recharging or discharging and reduces the effective capacity of the battery. Sulfation can be reversed in some cases, although this is time-consuming and the CF has been closing down battery maintenance shops.

During discharging, the lead di and lead at the positive electrode (cathode) and negative electrode (anode), respectively, revert to lead sulfate, releasing water and diluting the electrolyte in the process. A discharged lead-acid battery is, therefore, more prone to damage caused by freezing of the electrolyte, which can distort the plates or cause active material (paste) to be shed from the plates. Hence, if batteries are to be stored where there is a risk of low temperatures, they should be charged periodically.

Maintenance-free and sealed lead acid batteries: Because lead is a soft metal, alloys are employed to make the grids that hold the electrode active material. Historically, antimony was the main alloying metal. One drawback of this metal is hydrogen gassing at the negative electrode. This was much reduced when calcium recently replaced antimony. A further refinement is to seal the cell under pressure and to use a "starved electrolyte", or a gel, held in a highly porous microfibre separator. When gassing occurs, oxygen evolved at the positive electrode diffuses through the separator to recombine with hydrogen at the negative electrode, thus maintaining the water content of the electrolyte. Hence there is no loss of water, no gas build up, and no maintenance is needed.

The large surface area of the thin plates used in typical sealed cells gives a low current density, thus allowing rapid discharging and recharging - within 1 hour, if necessary. The main disadvantage of the sealed lead acid cell is that regular maintenance is needed to avoid the onset of sulfation.