In my experience, when unused Librem 5 idles at 25-30°C depending on ambient temperature; can get a bit lower with suspend and higher with screen on. When under active use (like, web browsing for a while), it gets up to 50°C where CPU frequency gets reduced; under prolonged intense use (like compiling big stuff and using GPU a lot) it can get up to 60°C; it generally doesn’t go much higher than that since at that point intense throttling would pretty much make the device unusable.
The above was about the SoC temps. When it comes to battery temperature (marked as bq25890-charger-0), it shouldn’t ever get much higher than 40°C. If it does, you’re likely dealing with some abnormal situation, like CPU cores being pegged to 100% for some reason in the background; or intense use in high ambient temp not allowing the heat to dissipate. Battery charging stops automatically at around ~42°C IIRC.