Given the high power consumption of digital cameras, it is economically and environmentally unjustified to use disposable batteries other than in emergency situations when your rechargeables are depleted. Disposable Lithium AAs are more expensive than Alkalines, but having about three times the power packed in half the weight, they are ideal to carry with you as a backup.
Rechargeable AAs (NiCd and NiMH)
NiMH (Nickel Metal Hydride) rechargeable AA batteries are much better than the older NiCd (Nickel Cadmium) AAs. They have no “memory effect” (explained below) and are more than twice as powerful. Capacities are constantly improving and differ per brand.
Rechargeable Lithium-ion Batteries
Li-ion (Lithium-ion) rechargeable batteries are lighter, more compact, but more expensive than NiMH batteries. They have no memory effect and always come in proprietary formats (there are no rechargeable Li-ion AAs). Some cameras also accept disposable Lithium batteries, such as 2CR5s or CR2s via an adapter, ideal for backup purposes.
Fully charged batteries will gradually lose their charge, even when not used. So if you have not used your camera for a few weeks, make sure you bring a freshly charged battery along on your shootout. Charging NiCD batteries before they are fully discharged will reduce the maximum capacity of subsequent charges. As the effect gets stronger when repeated often, it is called “memory effect”. It is therefore recommended to recharge the batteries only after they are fully depleted. To a lesser extent, this is also useful for NiMH or Lithium-ion batteries, although they have virtually no memory effect. Doing so will also increase the life span of the battery which is determined by the number of “charge-discharge” cycles that depends on the type and brand.