Ultra bay battery

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Have an ultra bay battery & it works fine in my t400 & T420s. Just wondering why it wont fit/work in my ultrabase 3? It would have been handy as currently waiting for batteries for x220 tablet. Perhaps a design issue but wonder why lenovo did this if so as i can quite happily swap dvdrw between them all?
Sorry if i appear to be spamming the board at present. . I was Just curious.

EDIT: Thinking about it maybe it is just not possible to run a thinkpad with ultrabase not attached to mains? Actually scrap that im sure ive used ultrabases with just a normal battery attached, Guess it maybe an issue with the ultrabase battery?