The CR2 and 123 batteries are both lithium batteries designed for use in cameras and other similar applications. Shaped roughly like a short AA battery, they have a much higher power density and excel at powering the LCD screens and high-power flashes that are present in most digital camera equipment.
The 123 cell is larger than the CR2. The CR2 is 27 mm long from tip to base while a 123 battery measures 34.5 mm long. The CR2 is also narrower, with a maximum diameter of 16 mm to the 123's 17 mm. For comparison, an AA battery is 50 mm long but 14.2 mm in diameter.
Given their long lives, both the CR2 and 123 battery cells are relatively light. While the CR2 weighs 11 g, the more powerful 123 cell weighs 17 g. Their high energy density relative to their weight is made possible by their lithium chemistry. For a comparison, a typical alkaline AA battery weighs 24 g while a rechargeable Ni-MH AA battery weighs 27 g..
Both the CR2 and 123 batteries have a nominal voltage rating of 3 volts, as opposed to a AA cell's 1.5 volt rating. At a 100 milliamp draw, a CR2 battery lasts between 8 and 9 hours before its power output drops below 2 volts. A 123 cell lasts between 10 and 11 hours. At a one watt draw, the CR2 will last just under two hours while the 123 cell will hold on for approximately 3.5 hours.
At room temperature in a 35 millimeter point-and-shoot camera, a CR2 cell will last for over 700 photos. The 123 cell should provide 1400 shots. In a single-lens reflex, or SLR, camera, the CR2 falls a few shots shy of 400 photos, while the 123 provides power for a full 800 exposures.