My Lumia 950 phone is great and with a 3000mAh battery it lasts quite a while. But sometimes I like to carry around a spare battery. Rather than buy a spare, I had some old Lumia 635 batteries (1830mAh) which are a bit smaller and thought they might work in my 950. The electrical contacts certainly line up:

But the notches in the plastic battery casing didn’t match:

With a Dremel tool I cut away a bit of the plastic in about 30 seconds. No danger of rupturing the Li-ion cell since the plastic case is a few mm thick:

So now the smaller battery fits:

Its held in place with tape which is OK, but I plan to design and 3D-print a little case/spacer for it so it doesn’t dislodge from the electrical contacts. I’ll report back on battery life with only 1830mAh.

Happy modding!