120-127 F degrees is when the phones battery will overheat causing the phone to automatically shutdown to protect itself.
Thanks for this info. I wondered what temp it would take for the phone to shutdown.
I check my temp and have seen it at 116F a number of times with no outward adverse effects. Admittedly, over time, heat is always the enemy of batteries. But it is a catch-22 situation. When your battery works it gets hotter. When it doesn't work it gets cooler. That's life. Choose your poison
As far as the OP's 104F - heck, that's only 5-6F over body temperature. That could happen by keeping it in your pocket and standing in the sun.
One thing I have noticed is that if you are using your battery for a heavy duty task (i.e. Mobile Hotspot, Nav, etc) it is usually better NOT to keep it plugged into the charger during that time. When I'm on my MHS, if I leave it plugged into the charger while I have the MHS radio on, it will get very warm (112+F), but if I don't leave it plugged in it only gets to maybe 100F. I now choose to not plug it in and just resign myself to charge again later. If the battery is plugged in and working hard at the same time it is charging, it does get very hot. Hot from charging combined with hot from working. Too much sometimes.
For anyone who needs a quick and dirty battery monitoring tool, there is an HTC widget included with the ThunderBolt. Just long press on screen | widgets | Get more HTC widgets (if you don't see the battery widget) and then use that HTC battery widget. It will show you the temp.
-Frank