From mobile phone to dedicated BOINC cruncher

One of the problems of making an old mobile phone into a dedicated BOINC cruncher, is power. Another is heat. And the third is connectivity.

These can be tackled in reverse order. Connectivity - well, being an old mobile it no longer has a SIM and so no mobile network. But its WIFI and Bluetooth still work. It is of course a very simple matter to just switch on the WIFI, connect to the local network, and start crunching! As the thing is no longer going to be mobile that issue can be put to bed.

OK then, we come to heat. Crunching the numbers for BOINC work units (WU's) at 100% CPU duty, is clearly going to make it hot. Again, solving this is relatively easy - just give it a fan!

Test Set-up

The test set-up shown above is being powered by USB and the fan is a 5V USB fan. Its enough to keep the phone and its battery below 40C. USB isnt really the most convenient source for this project, so later i'll be changing to a 12V source, and so to a 12V fan. But for now it does ok. You can see in the photo that all four CPU cores are in use.

A final aspect of the cooling will be to enclose the fan, phone, and any other electronics needed, so the whole project becomes a single small box of tricks.

Which leads us to the main problem - power.

The phone has a 1900mAh Li-ion battery at 3.7V. A few rough calculations based on the USB current readings, show that it would be flat in a little over 3h if used as the sole supply.  I also discovered that when crunching, the battery slowly drains even when on USB supply. Only if the battery is allowed to charge to 100% will it then remain so, I suspect due to the charge controller at this point switching the phone over to purely USB supply. But this ive found led to another issue - remove the USB, and the battery level drops alarmingly! That could be that the battery is on its way out of course. Also, even when supposedly charged to 100%, the USB current crept up close to the maximum 1A of the source. Again, probably due to the charge controllers attempt to handle a failing old battery.

So it makes sense then to plan to dispense with the battery entirely. Since I plan to move to a 12V source (as another future aspect of this project is to go green with a solar supply), then I can utilize a low cost adjustable Buck converter to give me a 3.7V 1A supply, and connect this directly to the phones battery contacts. Because I might need a back up phone, I dont want to do this permanently, so will do so using micro test clips. The only issue I envisage with this approach is that part of the BOINC clients control sets the maximum battery temperature for running the projects, and without a battery present there will be no data on this. I may need to either frig a suitable temperature setting using a resistor (a bit of a bodge), or else also add a suitable thermistor to give an actual measurement of the battery compartment temperature. This is preferable as it will be a good relative reading to the cooling system.