Battery Storage; Clock Battery; Alternate Power Sources - Nikon D300 Complete Manual

V1.02

Battery Storage

If you're not going to use an EN-EL3e for a long period of time
(several weeks or more), you should make sure that the
battery is neither fully charged nor almost empty. In the case
of the former (fully charged), storing it for a long period may
cause it to lose some of its capacity (a couple of deep
charges—from <5% capacity to full—on the charger might
restore that, but not always). In the case of the latter (empty),
the battery could discharge too deeply and be damaged by
that.
Thus, always store the EN-EL3e battery with a mid-range
amount of charge left in it.

Clock Battery

The D300 also has a small, internal battery for keeping the
date and time. This battery has an expected charge life of
about three months. Keeping the camera powered for two
days (either with an EN-EL3e battery in it or by connecting the
camera to the AC Adapter) fully recharges the internal clock
battery.
When this battery depletes, a & symbol appears on the top
LCD and two other symptoms appear: the Interval shooting
method doesn't work properly, and image files no longer have
a date and time stamp.
To recharge the clock battery, simply make sure that the
camera has a fully charged EN-EL3e in it for two days.
Note that charging the clock battery is one of the reasons why
D300 users report that their battery life improves after the first
uses of the camera. Once the clock battery is fully charged, it
needs only trickle charge energy from the EN-EL3e to keep it
healthy.

Alternate Power Sources

As an alternative source of main camera power, you can use
the EH-5a AC Adapter, which plugs directly into the DC In
Thom Hogan's Complete Guide to the Nikon D300 Page 91