When ordering Lenovo replacement batteries be sure to check the numbers on the battery itself. You can not simply order a T400 replacement and expect it to fit.
Our T400 on one is
11242T4752Z1ZHFE12VF5L (42T4752 should be enough to identify)
Google results
Our T400S is 42T4691 Google results
If needed for spec., Lenovo site for batteries.