Primary cell
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A primary cell is any kind of electrolytic cell in which the electrochemical reaction of interest is not reversible. The most common primary cells today are found in alkaline batteries; earlier carbon-zinc cells, with a carbon post as cathode and a zinc shell as anode were prevalent. Unlike a secondary cell, attempting to reverse the reaction in a primary cell via recharging is dangerous and can lead to a battery explosion. A related difference is that primary batteries use up the materials in one or both of their electrodes, while, ideally, the reversibility of the reactions in a secondary cell allows them to be restored to almost the same fully charged condition on each recharging.
[edit] Obsolescence
Primary cells are considered to some extent obsolete. It has been believed that even though rechargeable batteries have a higher MSRP than disposable batteries with equivalent voltages and shapes, the rechargeable batteries would be much cheaper if the main price is divided with the full number of recharge cycles, even including a battery charger, compared to the total cost of number of primary cells equivalent to recharge cycles of NiMH, NiCd and Li-Ion batteries.
However, there are some battery uses that require long dormancy periods and few replacements, so major issue is charge retention. In these circumstances, certain rechargeable battery technologies may not be appropriate, as they may have a high self-discharge rate compared to equivalent non-rechargeable batteries. For example, a flashlight used for emergency purposes must work when needed, even if it has sat on a shelf for an extended period of time. Primary cells are also more cost-efficient in this case, as rechargeable batteries would use only a small fraction of available recharge cycles.
