Most alkaline batteries available are disposable, designed to be used until exhausted, then discarded. There are also rechargeable alkaline batteries, designed to be recharged, which are not covered by this article.
Disposable alkaline batteries are classed as primary batteries; manufacturers do not support recharging, and warn that it may be dangerous. Despite this advice, alkaline batteries have successfully been recharged, and suitable chargers have been available. The capacity of a recharged alkaline battery declines with number of recharges, until it becomes unusable after typically about ten cycles.
Low-ripple direct current is not suitable for charging disposable alkaline batteries; more suitable is a current pulsed at a rate of 40 to 200 pulses per second, with an 80% duty cycle. Pulsed charging appears to reduce the risk of electrolyte-usually potassium hydroxide (KOH)-leakage. The charging current is low to prevent rapid production of gases that can rupture the cell. Cells that have leaked electrolyte are a safety hazard and unsuitable for reuse. Fully discharged cells recharge less successfully than only partly depleted cells, particularly if they have been stored in a discharged state -battery charger manufacturers do not claim to recharge dead cells.
Attempting to recharge a discharged alkaline battery can cause the production of gas within the canister. As the canister is normally sealed, pressure generated by rapid accumulation of gas can open the pressure relief seal and cause leakage of electrolyte. Potassium hydroxide in the electrolyte is corrosive and may cause injury and damage, particularly corroding the battery contacts in the equipment.
Some materials used in batteries are damaging to the environment. In some jurisdictions it is illegal to dispose of batteries in ordinary waste streams. Free collection points for used portable batteries are often available where batteries are sold.
As an alkaline battery is discharged, chemicals inside the battery react to create an electric current. However, once the chemicals have reached chemical equilibrium, the reaction stops, and the battery is depleted. By driving a current through the battery in the reverse direction, the equilibrium can be shifted back towards the original reactants. Different batteries rely on different chemical reactions. Some reactions are readily reversible, some are not. The reactions used in most alkaline batteries fall into the latter category. In particular, the metallic zinc generated by driving a reverse current through the cell will generally not return to its original location in the cell, and may form crystals that damage the separator layer between battery anode and electrolyte.
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