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. Comparison to other rechargeable batteries The rechargeable alkaline battery was, at one time, cheaper than other rechargeable types. Cells can be manufactured in the fully charged state and retain capacity well. Their capacity is about 2/3 that of primary cells. They are of dry-cell construction, completely sealed and not requiring maintenance. Cells have a limited cycle life, which is affected by deep discharge; the first cycle gives the greatest capacity, and if deeply discharged a cell may provide only 20 cycles. The available energy on each cycle decreases. Like primary alkaline cells, they have a relatively high internal resistance, making them unsuitable for high discharge current (for example, discharging their full capacity in one hour). Unlike rechargeable alkaline batteries, NiMH batteries can endure anywhere from a few hundred to a thousand (or more) deep discharge cycles, resulting in a long useful life; their limitation is now more usually by age rather than cycles. Capacity of NiMH batteries is close to that of alkaline batteries. Unlike all alkaline batteries (rechargeable or otherwise), internal resistance is low. This makes them well suited for high current capacity applications. Self-discharge rates are comparable, at least up to six months. Rechargeable alkaline batteries produce a voltage of about 1.5V, compared with NiCd and NiMH batteries which produce about 1.2V. For some applications, this can make a significant difference. In cases where resistance is not strongly dependent on voltage or current, since power varies as the square of voltage, rechargeable alkaline batteries provide about 50% more power. For example, incandescent lamps are much brighter when powered by rechargeable alkaline than by NiCd or NiMH batteries. Environmental notes Rechargeable alkaline batteries are developed from primary alkaline batteries, designed to resist leakage that a recharge could cause, so they can be safely recharged many times. According to the websites of EnviroCell, PureEnergy and old Rayovac packaging, these manufacturers' rechargeable alkaline batteries have no mercury or cadmium. See also List of battery types List of battery sizes Comparison of commercial battery types Battery recycling References External links Alkaline battery charging Potassium hydroxide MSDS from JTBaker Patent describing alkaline battery