I presume by "convertible", that you are writing of the IBM Convertible, the IBM 5140.
I do not own an IBM 5140, but here are some of my thoughts:
There are three power options for the 5140:
1. AC Adapter (100-240 Vac), 40W
2. Battery Charger, 3W (charges the battery only - does not power 5140 itself)
3. Automobile Adapter, 38W
Is it option 1 that you have?
Assuming so, does the machine power up without the battery? By the way; I do not know if that is a valid thing to do on a 5140. To me, it just sounds like a useful thing to try.
However, the following earlier post by billdeg suggests that a dead battery should not stop the 5140 from at least displaying something:
"The reason the clock with the question marks appears when you boot is that your battery is dead or needs to be charged up and the time/date are not saved in the battery save state memory. Just hit F1 to bypass. If you're not also seeing the "battery dead" icon on the same screen, you're in luck, you can probably recharge your unit enough to hold the date and time if you reboot. Either way, it does not matter. As long as your power supply works, the system will be fully functional you just won't be able to save the date. "