A secondary battery is a battery that can be repeatedly charged and discharged, and it is for primary batteries.

Traditionally, representative examples of chargeable and dischargeable batteries are lead storage batteries that have long history.

Although it is still widely used for automobiles, uninterruptible power supplies and so on, the reasons why it is used in large quantities despite the disadvantages such as size, weight, and poor temperature performance are that the cost per capacity is overwhelmingly cheap compared to other rechargeable batteries and that large current discharge is possible.

As a new secondary battery that appeared in recent years, it had characteristics in Ni-Cd batteries, Ni-MH batteries, and lithium-ion batteries and so on. However, as a result, it seems that it is in the direction of being converged to lithium-ion batteries.

Originally, Ni-Cd batteries appeared for shavers, and as rechargeable batteries capable of high current discharge, they were used in large quantities in electric tools such as electric drills.

Compared with Ni-MH batteries or lithium-ion batteries that appeared afterwards, it has been rapidly declining due to heavy weight per capacity and remarkable memory effect.

Later, what became the mainstream were Ni-MH batteries, but in the 1990s lithium-ion batteries that are now the mainstream appeared.

Ni-MH batteries have characteristics similar to Ni-Cd batteries, and regarding the memory effect that became a major issue in Ni-Cd batteries, as per Ni-MH batteries it is eliminated once they are deeply discharged.

Capacity density is the same as lithium-ion batteries and it was used for inexpensive batteries because it is cheaper than lithium-ion batteries. However, since the memory effect became an issue in practical use and the weight capacity density is also inferior to lithium-ion batteries, lithium-ion is now becoming the mainstream


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