Can I use the older NP-W126 batteries in my Fuji X-T2?

Q. Can I use the older NP-W126 batteries in my Fuji X-T2?

A. Yes, they will work, both the old NP-W126 and the new NP-W126s batteries can be used. 

The new ones (126S) are better at distributing the heat, so if you are doing 4K video, then the new ones are a must. The new ones and the old ones are exactly the same in terms of capacity.