The lithium batteries form a backup battery, the notion behind which was common to older PowerBooks. In the PB100 (which was heavily based by Sony, with several improvements, on the Portable/Luggable), the backup battery maintains the contents of (pseudostatic) PSRAM during sleep and while the PB is off. This enables a user some latitude for safety of data when the main battery runs out, because time to reach a mains outlet is conferred by the backup battery, which therefore is much more than just a 'PRAM' battery.
Verify that the storage switch is in the ON position, and that contrast/brightness controls are not bottomed. Detach all peripheral devices. Make sure that at least System 7.0.1 is installed. If you have a known-good main battery ('system battery') measuring 7.5-7.9V, and an AC adapter, you can reset the Power Manager with the 4-fingered salute: simultaneously hold down Control, Command, Option, and Power On keys for 5-10 seconds after having the system battery and the AC adapter disconnected for at least 5min. This process may cause the loss of some of the parameters stored in PRAM (eg, time).
Now connect the AC adapter and try to restart the computer after a further 3-4 minutes. Install a charged main battery (>5.7V), and away you go. You will need to replace the backup batteries soon, for the reasons above.