Black & Decker NHT518 Instruction Manual page 8

LEAVING THE BATTERY PACK IN THE CHARGER
It is OK to leave battery packs on charge for long periods of time. The
charger has a maintenance mode which gives the battery pack
enough charge to stay "topped off". If however, the tool is not
expected to be needed for more than 30 days, unplug the charger
and store battery pack in a cool, dry location. NOTE: A battery pack
will lose its charge when kept off of the charger. If the battery pack
has not been kept on maintenance charge, it may need to be
recharged before use. A battery pack may also lose its charge if left in
a charger that is not plugged into an appropriate AC source.
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