I had intended to do a test from a full charge with nothing running except the cellular modem, but the phone seemed to keep “forgetting” my saved network name after the screen was off for a while, so I finally just left it off. I imagine this is a Modem Manager software issue. (Maybe @dos could comment on that.)
I started the test at 2pm from a full charge with the phone sitting on the table. I checked the charge level every two hours:
2pm - 100%
4pm - 88%
6pm - 77%
8pm - 65%
10pm - 53%
12am - dead
I discovered that the battery had apparently drained completely sometime between 10pm and 12am. I tried switching the phone on, but it wouldn’t, so I plugged it in for a recharge and went to sleep.
So that gave me an idle time of between 8 and 10 hours. I’m not sure if the modem might have been searching for my cellular provider during that time, or literally doing nothing. For a “computer” I suppose that 8 to 10 hours is not too bad. For a phone, of course, that will have to improve.
I also can’t say whether the charge indicator is precise or not.
After I recharged the phone fully, it still wouldn’t turn on, so guess what? I REMOVED, THEN REINSERTED THE BATTERY. Then I was able to turn it on again. Removable battery, folks! Enough said.
Edit: By the way, the phone stayed quite cool during the entire test.