Apple PowerBook (Duo 270C) - Manual (Page 109)

Recharging in both the computer and a recharger
You can use a single power adapter to run both the
PowerBook Duo and the battery recharger at the same time.
If the PowerBook is on and contains a battery that is not fully
charged, the computer charges the internal battery first. The
external battery does not begin charging until the internal battery
is fully charged. If you are still using the computer at that point, it
takes a little over 2 hours for a fully depleted external battery to
charge.
If the PowerBook is in sleep or shut down, internal and external
batteries charge simultaneously. If both batteries are fully
depleted, it takes about 4
1 ⁄
2
hours to charge both. If the internal
battery is fully charged or absent, the external battery takes a little
over 2 hours to charge.
Chapter 6: Power Management 93