hi,
A good quality charger should last a long time.

As you know, even with careful charge/discharge a rechargeable battery has a limited life and you have to buy replacements in the future.

Rechargeable batteries are not cheap, so its a good long term financial investment to make a charger that will give you the longest battery life.

I would be interested to see your final design.

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

I think you should research the voltage that is required to fully charge a lithium battery because 4.0V is too low.
Also you should see the max allowed charging current spec'd for your battery so it doesn't catch on fire or explode.
Research lithium chargers in general so you can see that the charging must be shut off when the battery reduces its own charging current to a certain low current.
Or use a battery charger IC made for charging a lithium battery.

__________________
Uncle $crooge