The reason for crunching to stop is not the lack of internet connection (if the device already has some workunits downloaded for crunching).
The device was chewing on seven at the time, and there were plenty of WUs available besides.

Connection is needed only when transferring completed workunits back and to get new workunits to crunch. Boinc can crunch offline with any device.
That's what I thought. In this case no WU was ready for upload.

Please make sure from Boinc settings you have allowed battery for being a power source for crunching, this setting is OFF by default, and marked red to avoid accidental selection.
Yes, I noticed that setting early on and changed it to allow running on all available power sources, and put the battery limit at 20%.

Beware of high battery drain when crunching with only battery power.
Yes, I have noticed, but it will suffice for almost 2 hours even as I listen to audio-books as well.

If crunching has stopped for any reason, Boinc should display a light blue box at the bottom of the screen explaining why it does not crunch.
I didn’t notice any light blue box, but will specifically look for it now you have brought it to my attention, should it happen again.

If it says "No wifi..." it means all workunits has been completed and Boinc needs internet connection only for getting more workunits.
That is understandable and what I would expect it to do.

Thank you very much for taking time out going through this with me, Halo Jones, even though it did not give the solution.
Yet.
I shall pay attention to any unwarranted stopping and possible light blue box and return with whatever findings may arise.
Twice you helped, Halo Jones, two roses it will be - soon it will be an entire bouquet for my favorite teacher.

The roses are well deserved. You have a knack for explaining so that I understand

I don't think any of the things you mentioned happened, but, you know, when something new happens, you tend to get baffled. If it happens once more, you are more ready to notice what the circumstances are. And that's what I will do, and if it happens again, I shall certainly be back and tell you, Halo Jones

The time counters may stop running without actual suspending happening (and no blue box appearing) if Boinc needs to run some other workunits with high priority mode due approaching deadlines. The battery heat problem also sounds like a possible explanation for the interruption you experienced earlier (but it should also trigger the blue box to appear).

If the CPU gets too hot it will automatically limit its calculation speed (MHz) to cool down which slows down crunching too. This feature is programmed into microchips themselves and it does not trigger any warning or notification on device's screen, so people don't usually even know it's happening. While crunching the CPU gets easily overheated if the screen is on for some time, then later the battery may get overheated too.

I'm sorry I don't have any experience of good practice crunching on battery power. My offline devices crunch only indoors and they have constantly plugged in power. They have been automated to have few minutes of wifi connection every day to perform workunit transfers at morning and late at evening, so they are not actually outside of the wifi coverage. I only occasionally check they are working as planned and do not get overheated. Small application is showing battery temperature and actual processor speed. I also periodically dig out the battery too see it is in good shape and even measure it with micrometer gauge and record any bulging it may have formed.

I hope someone can give you more information, or you can just make few tests running on fewer cores or allowing less CPU computing time for crunching by using CPU limitations available on settings page of the Boinc manager. With desktop and laptop computers I have had the best results against overheating by limiting CPU time and still crunching with all available cores, but I don't know if this applies to mobile devices too.

I'm glad I could help, and many thanks for the beautiful flower bouquet, which I share with every kind and helpful people on these forums, and with the wonderful crunchers worldwide! So pick your's and smile! :)

You are always welcome to ask questions and share opinions, I guess that's the purpose of these forums... :)

I have detected only minimal bulging on batteries so far, aroud 0.10 to 0.30mm in the middle, but I have seen an extreme case where the swollen battery actually popped off my friend's phone's back cover and the battery looked more like a pillow. Such bad batteries must NOT be used at all and properly disposed off immediately.

I think someone really should compile a comprehensive collection of frequently asked questions for Android crunchers. (I'm happy to share my knowledge if needed).

A year ago when I started crunching with Android devices (so I'm still learning all the time...) I didn't know absolutely anything about them, I didn't own any such device. WCG's FAQ told you need an Android device with ARM processor to crunch, and so I searched for hours for such devices. I found only Snapdragon, Cortex, MediaTek or Whatever processors but no mention of ARM. How difficult, I thought. Then I learned that ARM isn't a processor name or manufacturer at all but a processor type, and all of those I had found were actually ARM processors. I thought WCG did give a bit misleading or inadequate information for the beginners, and there are several questions left unanswared on the subject, answears scattered aroud the internet as tiny bits of information, not always so useful.

Well, yes I feel certain commitment on crunching, and helping others, whatever species they may represent. It is a duty and it would be passive violence to deny help from a being who really needs it.