Thinkpad 600E (2645-8AU):
I got my serial port working!
The trick, was to not follow the fragmented How-To's completely.
I had to use a dos until by IBM called ps2.exe
Now my battery lasts twice as long.

I had to use a dos until by ibm called ps2.exe
I had to deactivate the internal winmodem, the IR, and the serial port
ps2 imodem disable
ps2 ir disable
ps2 sera disable

Then I had to set the serial to 1, and the IR to 2.
ps2 ir address 2 <== all the docs say this should be 1
ps2 sera address 1 <== all the docs say this should be 2

Finally I had to enable serial, and leave the others off.
ps2 sera enable


Perfectly straight forward.
Only took a year and a half to discover that.
Now my battery lasts twice as long.