If it is the batt that is dead, I know you can rig a power adaptor to the battrie in slot like this guy said on lem

Godwin says, "If the acid battery dies, one can bypass it and convert the Portable into a luggable. One needs 2 AC adapters - the original one and a 9V DC adapter. Take out the 9V DC battery and replace it with a 9V 1A power adapter. Then remove the battery and cover (there is a switch at the battery cover which we need to disable). Plug the two adapters in and viola! one of the sleekest Macs on anyone's desktop!"

also this is what some one elts said

You can use a PowerBook 100-series AC adapter to run the Portable without the internal lead-acid battery. However, you will lose PRAM settings if you unplug the power (date, time, cache size, etc.)

Try It