Remove the battery.

Don't reuse the battery if it's creased or even slightly bent, as doing so is a potential safety hazard. Replace it with a new battery.

Attach your replacement battery with some double-sided tape such as Tesa tape or stretch-release adhesive strips.

Be careful where you position the stretch-release adhesive strips. On the right side of the battery well, the Portal has a plastic lip. Position the strips such that they're not blocked by the lip.