Swap-ability of batteries?

[Marine03]

My 6D should arrive today, which comes with 1 LP-E6   but I also have several LP-E5 from my old Xsi(450D)   

LP-E5   Lithium-Ion rechargeable battery (7.4 V, 1050 mAh) new for EOS 450D/XSi and 1000D
LP-E6   Lithium-Ion rechargeable battery (7.4 V, 1800 mAh) new for EOS 5D Mark 2

Does anyone know if they size etc are the same and the way I read this is that they are the same power the LP-E6 is just capable of storing more charge, but as a backup could I use the old batteries?

[untitled10]

My 6D should arrive today, which comes with 1 LP-E6   but I also have several LP-E5 from my old Xsi(450D)   

LP-E5   Lithium-Ion rechargeable battery (7.4 V, 1050 mAh) new for EOS 450D/XSi and 1000D
LP-E6   Lithium-Ion rechargeable battery (7.4 V, 1800 mAh) new for EOS 5D Mark 2

Does anyone know if they size etc are the same and the way I read this is that they are the same power the LP-E6 is just capable of storing more charge, but as a backup could I use the old batteries?

only the ones from the 5d will fit.

[neuroanatomist]

...as a backup could I use the old batteries?

When you unpack your 6D and look at the battery, it will immediately become apparent that the answer is no.