Your Nikon Digital SLR is like a mini-computer with a lens. The battery must power the LCD monitor the motorized zoom autofocus mechanism metering system electronic flash and more. A spare rechargeable battery is an essential accessory. No power no pictures! Genuine Nikon Brand Battery Pack. With constant shooting flash usage image review and playback especially when you have access to additional memory your standard battery alone will not usually be sufficient. Providing an extra 1080mAh of power this spare EN-EL9a will not put constraints on how you want to shoot. A battery loses power in cold weather and can become too cold to operate the camera. Keeping a spare battery warm in your pocket to maximize battery power and periodically switching the cold battery for the warm one can significantly enhance battery performance. Memory Free! This EN-EL9a can be charged at any time without harm even if it already has a partial charge and can be typically charged over 1000 times. Fully compatible with any Nikon Digtal Camera that use the EN-EL9a battery including the Nikon D40 D40x and D60.