As probably many agree, having a spare battery is one of the major benefits of having a Fairphone. However, batteries should be treated with care. They are complex and fragile products that could cause fires or damage to your device when treated poorly. Just having one in your pocked might not be a great idea.
I use the box below to safely carry it in my backpack/jacket. Unfortunately I couldn’t find a nice wooden alternative, so I got this plastic one. It’s for regular batteries, but the FP4 battery fits nicely, especially if you keep the anti-static wrap around the battery. It doesn’t move at all. So no sounds, and also no chance of damage with regular use.
Just sharing some ideas here