They tend to charge by induction through the base of the toothbrush. No idea if this is just mains fed to a coil in the base, or if there's high-frequency switching going on.
Someone must molish a USB-powered toothbrush, Shirley? That way you could side-step the mains inverter entirely.
I bought a USB powered charging base for my toothbrush. It lives in the bag of usb charging stuff for travelling.
Ah, one of these perhaps:
https://www.amazon.co.uk/Electric-Toothbrush-Toothbrushes-Universal-Connection/dp/B07FH7M2P1
But why not from a car charger port? 5v 500mA requirement.
It's one of those sweeping generalisations.
USB chargers always change the voltage from something to something else. A laptop will be stepping down from the 12 - 20 V (approx) of the laptop battery. The mains chargers will step down from 240 V ac, and have to manage the gaps 100 times a second, and the power bank will have to step up from 3.4 - 4.2 V, whatever the LiIon battery happens to be at. A car USB charger will have to handle the 9 - 16 V, plus cranking dips, plus all the other noisy things on cars.
Any of those can be well engineered, or they can be terrible. See Big Clive for critiques of various USB power supplies. Generally, I would trust a car charger that comes with a modern car more than various chargers from eBay or the pound shop.
