Was working with oder Version!!
There is a simple workaround to this known issue. To me it happens when I let the Edge charge. By itself it goes to sleep mode when changing is completed. If you use it after that it will have the battery stuck at 100%. However wake it up, do a complete shutdown (red Power Off button on thouch screen). Once this proper shutdown is done, the battery % will work just fine after reboot. I got into the habit of, after charging, unplugging the device, waking it up, doing a shutdown. After that all is good.
I purchased an Edge 1030 this spring, and I think it came with firmware 7.50. The battery percentage worked fine until I upgraded to 8.00, then it got stuck at 100%. I'm not sure if it started working correctly (i.e. decreasing from 100% without a full power cycle) after it all of a sudden powered off during a ride. After upgrading to 9.00 it was stuck back at 100% until I power cycled it tonight. My point is the battery percentage did work for me after firmware 3.90 but before 8.00. Is the solution that one must fully power cycle between rides or just after a firmware update?