Hello,
i can also offer some experience with running on BOINC devices. I was running Universe, WCG and yoyo on different devices in the last years. All of the got problems with the battery. In all devices the battery increased in volume. Sometime significantly and one it even broke the screen due to the tension.
The longest running device is Nexus 7 tablet with a Snapdragon 600. Here i soldered th USB power connection derectly to the charging logic and removed the battery. This tablet was contributing 5years 337 days 21 h of CPU time to WCG. It generated: 2,355,096 points with 9,348 returned results. Which means its roughly 1000 Points a CPU day. For comparison an Pentium J4205 makes about 3800 Points a CPU day. The best CPU makes about 8000 Points per CPU day (Core i5 with 65W).
The numbers are probably more an estimate, since BOINC is pushed into the background when other jobs are running. E.g. BOINC is running on my regulat laptop, but often this laptop has to run other calculations as well. This means that its hard to make a good Points per Watt comparison. Among the available X86 cpus the J4205 is among the best in terms of Points/Watt. Typical ARM CPUS are more efficient, but it will warn down your battery significantly. And those devises usually run only a few hours per day.
An other issues i encountered is that on some Android devices the threads are distributed stupidly amoung the CPU cores. E.g. on one device its always sending 4 jobs to the fastest 2 cores. Which makes it very ineffcient.
Thats my main experience from the last years, if you have questions iam happy to answer them.
Lars
You said that you have problems with all your batteries? At which temperature did you limit them?
I have 3 phones crunching atm and set them to 70% cpu and 32°c limits. Or is swelling inevitable?
I do try to cool them off. With various techniques, sometimes not worth the effort but it's fun doing it:
Placing usb fans, putting coins (hah it works slightly) and/or a glass of water on the waterproofed ones but laying them face down on window sills in the shadow side is fantastic, apparently natural stone (I got travertine ones) is exceptional at absorbing heat as I found out. Sometimes lowering temperature by almost 10 degrees celsius.