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Types of Rechargeable Battery
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Lead Acid (SLA) - Lead Acid batteries fall into the "transportable" class - they are heavy and not particularly safe. The newer Sealed Lead Acid batteries are much safer, but have even lower capacity for their weight. These are good for applications where occasional hefty bursts of power are needed and can be trickle charged for long periods.
Lithium Ion (Li-Ion) - These have the highest theoretical capacity of the common phone and laptop/notebook batteries, but recent NiMH cells come close. However, they require special circuits built into the battery for their complicated charge regime and thus are much more expensive. Moreover they respond very badly to frequent deep discharge (flattening the phone) and to frequent overcharge (leaving the phone on charge continuously).
Nickel Cadmium (NiCd) - These are the oldest, lowest capacity and most commonly available rechargeable replacements for AA, C and D cells and also used in older mobile phones and other rechargeable devices. You may have these in your camera or diskman, your portable drill or car vacuum. However they are notorious for their memory effect - that is their capacity and life is diminished if they are not discharged till the point the device turns itself off. Cadmium is a poison so these batteries should be disposed of responsibly. Upgrade to NiMH if possible.
Nickel Metal Hydride (NiMH) - These have in large measure replaced the NiCd cells, having up to three or four times the capacity for the same size and weight. They are also less sensitive to the memory effect and work well in mobile phones when recharged each night. They are the most economical both in terms of capacity and in terms of the life of the battery for a typical phone user.
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