The PB100 can boot without problem from it's internal HDD, no working battery is needed. I guess nobody has a working battery for a PB100 anymore.
Of course the original Connor 20/40MB HDD is notorious for stiction of the heads so that needs to be repaired if you want the drive to work again.
However it can be done, I've got 3 fully restored and working PB100's.
Recap both the logic board as the LCD panel and invertor board. Make sure the battery switch at the back is in the up position.
It's best to cover to copper contacts of the battery with isolating tape so it doesn't connect to the circuit anymore as it can draw power or be shorted.
Also remove the small metal clip from the switch behind the battery: this disables the battery charging circuit as the system then things it's running without battery on the DC adapter only.
Works like a charm, I use my powerbooks 100 every week for multiple hours. The run quite well with OS7 if you have an additional memory module.