Standard Dry Cell Rechargeable Batteries
NiCad - Nickel-Cadmium battery
NiCad's are obsolete rechargeables. They've been replaced by NiMH's which last longer, don't suffer from the memory effect, and aren't toxic.
NiMH - Nickel Metal Hydride battery
These replace obsolete NiCad rechargeables. NiMH's have much higher capacity than NiCd's and don't suffer from memory effect. NiMH's are great all-around batteries but they have a high self-discharge rate (they go dead after a few months even if you don't use them) so they're not good for things like emergency flashlights or smoke detectors.
Li-Ion - Lithium Ion battery
Lithium-ion batteries have a higher energy density than most other types of rechargeables. This means that for their size or weight they can store more energy than other rechargeable batteries. They also operate at higher voltages than other rechargeables, typically about 3.7 volts for lithium-ion vs. 1.2 volts for NiMH or NiCd. This means a single cell can often be used rather than multiple NiMH or NiCd cells.
Lithium-ion batteries also have a lower self discharge rate than other types of rechargeable batteries. This means that once they are charged they will retain their charge for a longer time than other types of rechargeable batteries. NiMH and NiCd batteries can lose anywhere from 1-5% of their charge per day, (depending on the storage temperature) even if they are not installed in a device. Lithium-ion batteries will retain most of their charge even after months of storage.
Lithium-ion batteries are more expensive than similar capacity NiMH or NiCd batteries. This is because they are much more complex to manufacture. Li-ion batteries actually include special circuitry to protect the battery from damage due to overcharging or undercharging.
Lithium ion batteries are not available in standard cells sizes (AA, C and D) like NiMH and NiCd batteries. This may be due to safety reasons since a lithium-ion battery put into a NiCd or NiMH charger not designed for lithium-ion, might ignite. Also, because Li-ion batteries operate at much higher voltage (typically 3.7V per cell) than the 1.2 to 1.5V of most cell batteries, designing a 1.5V lithium-ion cell would be expensive.
