We used 4 x TS-C slates on a 20 week show with one of the slates converted to 9v .
Used i-power 520mAh batteries,
Found it struggled to get a full day reliably as back lights were used on the slates usually.
Got a new Battery sled for that slate and converted it back to AA's.
Using powerex 2400mAh AA's now, guaranteed a full day but not much more.
Now 2 of the other slates are doing the powering off trick, so new sleds ordered, from my experience with them, the battery contact springs inside seem to get weak after regular use, its a little bit annoying as one of the slates is only a year old.