Thanks again Chad. I agree that checking the battery level is something I shall now do on a regular basis. Using the Stages Power App gives a pretty good estimate of remaining battery life, plus allows a regular check of the status of the Firmware version.
Stages say this about the battery life indicator: “This battery indicator will display green from the time the battery is new until it starts to run low, then changes to yellow when the battery has only 15-20%, and red at 5% or less power remaining. Keep in mind that battery life is being estimated and that expected remaining time can vary slightly.”
My Garmin Edge 810 / Garmin Edge 130 head units only give a warning at 5% battery life remaining. If I’d checked before I set out, I could have swapped out the old battery. Lesson learnt for the future!