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.
The models with the oval bottom just have that cap pressed into the end with two small clips on the long side of the oval. Deforming the oval into a circle and using a small flat blade screwdriver to pry it out is usually all it takes.
I’ve successfully desoldered the battery for recharging before. We had one go so completely flat the wireless charging wouldn’t work. Putting the battery in an external charger brought it back to life. It was in service for another couple years till a drop in the brush end ruined it.