I have a (genuine) T400 myself, and while shopping for batteries on Amazon, I read some tales of woe from people who tried saving money and bought cheap OEM "Thinkpad-compatible" batteries and ran into the same issue.
Apparently, this is Lenovo's way to block competition. For their side, I read a claim that cheap, no-QA batteries may cause fire hazard and they are just trying to protect their users. I tended to scoff at that (just like I scoff at HP for blocking non-original inkjet cartridges), until I bought a cheapo power adapter for my T400, to replace the original. Boy, that thing heats up - even when the laptop is not connected.
All this long story just to say: take the battery out and make sure it's an original Lenovo. If it isn't - get an original one. If it is, there may be a Power Manager upgrade that will support it.