Quote Originally Posted by lerameur View Post
I want to keep the price as low as possible. Lithium batteries are very expensive compared to round alkaline battery. but even at 3v, it does not leave much space for its lifespan , no ? when it falls just .5v then its game over.. i guess, I could be wrong.
Many chips minimum operating voltage is at 1.8V, so I don't see any problem. Check out this application note about the coin cell lifetime.

http://www.maxim-ic.com/app-notes/index.mvp/id/505

Robert