I’m not really sure where to post this, but wanted to share that screwing a 1/4-20 bolt into the bottom hole of my otherwise-sealed Philips Sonicare 4100 and just pulling on it successfully opens the toothbrush. An M6 screw might work too; M5 is slightly too small. Philips says to use a hammer to open it, which is absurd. The battery is still soldered, but in my case that wasn’t what I needed to repair anyway.

Apparently other brands are designed to have replaceable batteries. Don’t buy Philips Sonicare.