Just think - your telephone is always on. If with special BTC miner
you can mine some BTC in your telephone - why not ? But the monthly BTC profit can be very low, because telephone can not mine a lot.
Lets look at the math. A typically mobile phone battery stores about 6 Watt Hours.
Say you use a third for mining. That means you can mine with 1 Watt power for 2 hours.
Phone with ASIC mining chipSay the phone has the latest and most efficient mining chip running at 0.1J/GH, you will do 10 GH for 2 hours.
This will earn you about 0.2 of a cent for the day. Say 500 Satoshi per day.
If you did it every day for a year, and difficulty did, not rise you would earn about 70 cents per year.
This would not even cover the cost of adding the mining chip to the phone.
Using a standard phone without any special mining chipsAssuming a standard mobile phone can mine about about 10 Megahash/s and it mines for 2 hours its mining speed will be about a thousandth of the phone with an mining chip.
So expect to make about .0002 cents per day or 0.07 cents over a year. Say 0.5 Satoshi per day or 200 Satoshi over a year. It would take 50 years to make the equivalent of a single transaction fee (10,000 Satoshi).
In both mobile phone cases you will need to cover the extra cost of the electricity to recharge the now more frequently discharged phone, any extra data cost, as well as the reduced battery and phone life.
Mining in your phone charger.Every now and again someone suggest mining using a phone charger, as if a phone charger has free electricity. A typical mobile phone charger is very electrically efficient and extremely cheap. It draws only about 0.1 Watt when not charging a phone. Adding mining to a charger increases the chargers power draw and you need to add all the bits required for a miner, ASIC chips, internet access etc.
You no longer have a $5 phone charger but an expensive, power hungry free standing miner with a USB port.
Feel free to correct or comment on any errors in the above.