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.
My experience is that they are not good anymore.
I made the mistake of replacing mine that was working fine but the battery didn’t last too long after 7 or 8 years (didn’t think to replace the battery) and I had to replace the new model 3 times already in 3 years (under warranty but still). I’m noticing more plaque build up too, which never happened with my previous model
I’m waiting to finish my stockpile of replacement heads and I’ll try oral B or whatever else Costco has that is not Phillips