Using a phone battery as a power source possible? Use
I am assuming that this is possible for any phone model's battery. I am making this vague on purpose (this isn't for a specific project). The goal is to have the most compact and long-lasting battery possible for my arduino projects.

Note
I don't care about the cost. This would be for personal use (not selling this or using it for school), so I don't care about patenting or copyright issues.

Question
I want to know if this even makes sense for a portable arduino power source option. I want to benefit from newer phones' long battery life, as well as their compact size.

Also:
Are there better alternatives to this using different batteries? (I have looked at Nick Gammon's posts over setting sleep cycles to extend battery life (http://www.gammon.com.au/power); If utilizing his methods on different batteries are better than using a phone battery please let me know...)

If Possible...
Does it make sense for something like a:


LED suit (lasting 12 hours)
Portable gaming system (lasting 8 hours) - similar to: (https://www.adafruit.com/products/2510)
I Know this is for Raspberry Pi
BlueTooth Speaker (lasting 6 hours)


etc...
Sounds like a good solution. 

Just note that most cellphones use Lithium batteries. Those are typically around 3.7 volt. They require some special way of charging, or they may explode or catch on fire. They also get damaged if discharged too much.

These batteries may have some protection circuitry inside them. This would be ideal in your case, as it protects against over-discharge, and accidental shorts.

An alternative would be the cells used in notebooks called 18650s. These are available with protection and without. The advantage is that you have a standard size. You can buy battery holders for them. That way you can easily exchange batteries. You can also combine cells to create higher voltages.

As for the runtime; you can make a rough calculation of it.