I have them in a fairly accessible location, so if needed I just go take a look. I also monitor the device statistics page, to make sure each of them are phoning home to WCG to return work. If a device fails to post a result in a day, I am not too worried. Sometimes the wus can be challenging, especially for the slower clocks on these. But if it goes over two days, that is when I start checking. I just wish there was a way to rename them. My device manager has a ton of devices all named "android". Very annoying. I know there is some workaround by using some other software, but I don't want to mess with that and want BOINC to be able to do something to fix it.

From what i see:
If diodes are used, they are becoming like a heat source of 0.25-1W heating the phone, plus there is the possible ripple effect influence.
If 5V are fed directly, then there is the risk of overvoltage and possible damaging the phone.
If batteries are used, then you have a small ticking bomb and a heat source.

So I guess the best option would be like I did in my original instructable with separate power source and constant current of 3.8-4.2V. My friend should be able to print the PCB next month and I will make it available

My feeling is that the solution is not so easy since it involves finding the most efficient way to have:
max cpu cores
max cpu frequency
min temperature
min voltage

If someone can make an excel shared gdrive file, we could all add our findings for better transparency, since I do not have time

I have an old Motorola razr max.
It has an internal battery and is probably I think 5 years old, I forget really.
No idea if it even powers on anymore - think I need to throw a custom rom onto it.
It's 2 days of runtime, I guess. Unless the battery explodes. I don't have the knowledge to take it apart and run it off USB.

I had a razr maxx as well, several years ago. It was a dual core phone. I ran it on wcg after upgrading to a newer phone, so it was a dedicated cruncher. I dont remember how long it lasted before the battery bloated out and actually started popping the back off, but I just tossed it. I did not have it actively cooled, which I think is important for these devices. They need airflow to keep the batteries cooled.

I also have a motorola droid turbo as one of my dedicated crunchers, which is actually pretty powerful (quad core 2.7GHz I think it is). That one recently bloated out and has an internal battery. I ordered a new battery from ebay for cheap, and it just arrived. I plan on keeping the back of the phone off, and will mount the battery up so it can stay connected, yet have some space between the battery and the phone for airflow and cooling. I have not yet figured out how I will do this, but I'll figure something out this weekend.

Wow. They took a lot more steps than I did. I didn't remove the board from the screen, although I kind of wish I had at this point. Would have been nice to remove the wireless charging, since I don't use it.