That laptop, I believe it's built into the motherboard. There is a seperate board that comes off the motherboard that connects to the battery, but do not mistake that for having anything to do with the DC-DC circuitry. It's just serving to connect the battery to the motherboard (and i believe that daughterboard also has your CMOS battery, your card reader, and a USB port).
One thing that I just remembered (that might not apply to your particular problem) is that I saw a few Thinkpads back in the day that you had to clear their CMOS settings by pulling the battery to get them to charge a battery. I dont know why, and it probably doesn't apply to your case, but it never hurts to try.
But if all else fails, and your're sure your...