>?>     When sleeping, the PIC16F628@4MHz consumes about 42uA @ 5V. If I
suply
>> it with some AA Alkaline batteries, it would mean rougly:
>>     2850mAh / 42uA = 67857 hours = 2827 days = 7.74 years without use.
>
>I know that this was only a calculation to determine how heavy the load was
>compared to the battery's capacity but do note that an Alkaline battery has
>a shelf life of around 3 years (some manufacturers claim longer). If you
>produce equipment with very low current drains then the reduction in
battery
>capacity with time can make serious inroads into the actual operating time.
>When that happens, Lithium batteries are a possible solution. In this
>particular case it's not an issue as your expected  operating lifetime is
>far shorter.

but you probably should put some sort of disclaimer in your instruction
manual recommending replacement of the batteries every 12 months (or
whatever lesser period you feel you need) "to ensure reliable operation of
the product".

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