If a PowerBook of the 100 series has been unused and off its AC adapter for a long time (eg years), not only will the main battery be feeling proper poorly but so also will the backup battery. The difficulty then lies in getting the backup battery charged as well as the main battery.
Before they will catch on to the idea of recharging, backup batteries need the main battery to be at full terminal voltage. (NOTE: not at full capacity.) Jan indicated the adapter's output voltage, but a main battery may be in the range 6.9 to 7.4V at the end of charge. Less than 6.9V may barely allow the PB to operate in concert with an adequately charged backup, but a failing or failed main battery may not be capable of reaching a voltage that encourages a
backup to charge.
Backup batteries are of sterner stuff than main batteries, however. I recently successfully revived a PB 160 that had, by conservative estimate, been out of use for 12 years. I did this by cycling the main battery's charge first, but it is sometimes possible to achieve the same effect by allowing the PB, with its main battery inside, to remain attached to its powered AC adapter for up to a week before resetting the PMU in line with
Apple IIe; 68K: 11DT + 4PB; PPC: 5DT + 3PB; G3: 6DT System 6.0.8 to OS 10.4.x