So here's a thing, having the 1D MK IV I have LP-E4 batteries of which I have the original Canon ( 7 years old now) at 2300mAh @ 11.1v and 26Wh
The slightly revamped LP-E4N has 2450mAh
The new 1DX MKii has a new LP-E19 at 2750mAh etc so larger capacity but not size.
The older and newer batteries work in each camera as long as you use the correct charger.
Hmm... well here's the thing, I just realised that for some time now well over two years I have been using along with the original battery 3rd party batteries.
These are get this 4400mAh
I tested them way back when I first got them via an electrical mate and they are indeed 4400mAh @ 11.1v but 48.8Wh
And the main thing is these charge fully with out issue via my original 1D MKIV dual charger.
Hmm I wonder if the new LP-E19 will charge in the old charger and vice versa???
And of course if my 4400mAh batteries outlast the new LP-E19, they certainly do outlast my original.
Blimey they're still available,
http://www.amazon.co.uk/DSTE-3-Pack...59458957&sr=8-15&keywords=canon+lp+e4+battery