WHAT IS IT WITH BATTERIES ??????????

Older digital compacts and bridge cameras often take 'AA' batteries. Nickel-Metal-Hydride (NiMH) rechargeable batteries are the best choice for these cameras, giving good power storage with modest self-discharge.

Most DSLRs however, use dedicated battery packs, almost always using Lithium-Ion technology. This chemistry gives a high power density and low self-discharge, and for these reasons is also the preferred technology for power hungry devices such as Cell Phones, Lap Tops etc. The downside is that there are safety issues which require protection circuitry to be included inside the battery pack, as already stated in other posts. 

It doesn't seem to matter how often a Li-Ion Battery is used, it will still fail after 3 to 5 years. It is not worth buying too many spare Li-Ion batteries because they will wear out even if left on the shelf. Similarly, out-of-date batteries are likely to be useless even before you put them in the camera.

Newer cameras use 'Smart Batteries', which incorporate a fuel-gauge charge monitoring system which gives an indication of the recharge performance. Taking notice of the recharge efficiency figure displayed on the camera menu will help avoid the situation of suddenly being stuck with a useless battery. Often though, the manufacturer's own brand of Smart Battery will also have some form of authentication which means that cheaper third-party batteries cannot be used.