Ok, so ordered this one from Amazon.
It’s labelled ‘fast wireless charger’. So I looked into this a bit – turns out that ‘fast’ isn’t just an adjective, it’s a different Qi standard, and includes handshaking and temperature monitoring, so it reduces power when the battery gets too warm. Apparently Li-ion batteries are ok if kept under 45C
It is a little plasticky, but it’s quite well designed – holds the phone very well although I haven’t fitted it in the car yet. The side clamps are well below both the power button and the Bixby button.
When I put the phone (an S9) in it, the phone displayed ‘fast wireless charging’ and said something like 1h52 to charge from 32%. So I left it in satnav mode for a bit, and checked the battery temperature with an app, it got warm but never seemed to go above 39C which seems ok. Just left it for about an hour doing satnav and it went up to 39% and the battery was still at 38C So it charges whilst using satnav, but it’s clearly holding back a bit to keep the temperature under control. Which is perfect.
So it looks like the ones labelled ‘fast wireless’ are decent. Let’s just hope this one stays on the windscreen or dash wherever I put it.