XT2 Slice Battery

Hey guys,
I recently traded my surface RT for an xt2 (with the SU9600 and 60GB SSD). I use it in all of my courses for notes, which has been super convenient. It came with a 30% worn 4-cell, which lasts somewhere between 1-5 and 2 hours, which doesn't cut it for a day with multiple classes. I was looking into getting the slice battery, although most of the ebay ones are listed for the XT rather than the XT2. I seem to remember seeing several slice model numbers. Are all the batteries inter-compatible?

 

I'm pretty confident both the primary and slice batteries are inter-compatible.

 

Technically they are, but practically they are not. Used to own an XT2, and tried out a slice battery. Seems about the firmware on the battery (!), since Dell is a bit picky about chargers and batteries.

I could only charge it externally directly via the power adapter, and my XT2 would only run exclusively from the slice, not in addition with the normal battery - in that case it'd use only the 1st battery and then shut down without even hibernating.

 

I guess it's safer to just go with a new 6 cell. I intended to get the 9 to compliment my 4 cell, which has about 2 hours of life left, but all of the 9s on Ebay are used. I'd rather just have a nice, new, fully functional 6 cell.