Nikon D4 - Manual (Page 437)

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• Turning the camera on or off repeatedly when the battery is fully
discharged will shorten battery life. Batteries that have been fully
discharged must be charged before use.
• The internal temperature of the battery may rise while the battery is in
use. Attempting to charge the battery while the internal temperature is
elevated will impair battery performance, and the battery may not
charge or charge only partially. Wait for the battery to cool before
charging.
• Continuing to charge the battery after it is fully charged can impair
battery performance.
• A marked drop in the time a fully charged battery retains its charge when
used at room temperature indicates that it requires replacement.
Purchase a new EN-EL18 battery.
• Charge the battery before use. When taking photographs on important
occasions, ready a spare EN-EL18 battery and keep it fully charged.
Depending on your location, it may be difficult to purchase replacement
batteries on short notice. Note that on cold days, the capacity of
batteries tends to decrease. Be sure the battery is fully charged before
taking photographs outside in cold weather. Keep a spare battery in a
warm place and exchange the two as necessary. Once warmed, a cold
battery may recover some of its charge.
• Used batteries are a valuable resource; recycle in accord with local
regulations.