The battery is securely adhesed to the housing; use a hairdryer or hot air gun at moderate heat for 10 seconds to loosen the adhesive.

Use a plastic guitar pick or spudger to separate the adhesive from the plastic housing. There should be less resistance; if it's still not budging, re-warm the area and try again.

Once the adhesive is freed, separate the controller apart more to handle the battery, and remove it from the main handle.