Nikon D300 Complete Manual page 87

The design of the EN-EL3e battery makes it impossible to
insert it incorrectly into the D300's battery compartment, so
never force it. The same is true of putting the EN-EL3e into the
charger. The MH-18a Quick Charger can fully charge a fully
depleted EN-EL3e battery in a little over two hours (two hours
and fifteen minutes to be exact). The MH-18a is fully
compatible with 120 or 240 volt, and 50 or 60Hz outlets.
Another point of confusion for D300 purchasers coming from
older consumer Nikon DSLRs will be the MH-18 versus MH-
18a charger. It shouldn't be. Technically, the MH-18a is the
charger designed to work with the 1500mAh batteries (EN-
EL3a and EN-EL3e) while the MH-18 is designed to work with
the older 1300mAh batteries (EN-EL3). But either will charge
a D300 battery. The only real difference between them is that
the older MH-18 is a little bit bigger than the MH-18a
supplied with the D300. For those of us who travel a lot, that
was a welcome change.
The fact that the charger only has two connection terminals
while the battery has three also confuses some users. The
charger just charges the battery—more sophisticated battery
systems sometimes use extra connections to tune or balance
cells within the battery, which the MH-18a doesn't do—so
the MH-18a only needs the two power connections. That
Thom Hogan's Complete Guide to the Nikon D300
The charger is light (3.6
ounces, or 100g) and
modestly sized. The battery
"docks" in the charger by
sliding it into the charging
position (don't worry, you
can't do it wrong). The AC
power cable is removable.
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