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.
I used to use electric toothbrushes religiously. Then I had one appointment with a hygienist who specializes in brushing technique (seriously). She used a dye to show where plaque was being left behind on my teeth and gums and taught me correct brushing technique to remove it all.
Now I use a very soft manual brush that I replace regularly (brushes always get stiff over time due to calcium deposits on the bristles). I have no issues with plaque or tartar buildup and no cavities in years. One 4-pack of manual brushes costs about the same as a single replacement brush head from my old electrics.