When faced with such a question I usually reach for my DigiKey catalog.
All their batteries have the mAH and voltage ratings.

First you need to give us an idea of what your power requirments are going
to be and whether you have any preferences as to battery type and number.
Would you like to use 4 AA cells?  Two lithium watch batteries? An N or
other camera battery?

Are you planning on using a regulator?  What current will your circuit
require?  Is there a voltage requirement as well, or can we assume the
3-5.5V range of a PIC?

For instance,
http://rocky.digikey.com/USCatalog/V3/554.PDF
shows some carbon-zinc and alkaline batteries.  The alkaline AA is rated
as lasting 170 hours on a 100ohm load.  This is 15mA for 170H, or 2.55AH
(ie, if you could safely draw one amp from it - it would last for 2.55
hours).  Its highest rated drain is 130mA, so you shouldn't draw more than
that.  This is your first check - find your current drain, and then look
at batteries that can deliver that rate.

Once you've done that you need to determine how long you want the product
to last.  If it takes 20mA, and you need it to last for 5 days, then you
need 20mA for 120Hrs.  This is 2.4AH, so 4 AA batteries would be able to
meet your needs.

-Adam

Andrea Aizza wrote:
>
> Hi,
>
> I am going to use a PIC on a battery supplied card. I do not know what are
> the possible batteries to choose, what are the differences and what is mean
> life time of the battery (the order of magnitude). I imagine there are so
> many possible batteries and that the lifetime of  the battery heavily
> depends on the application. In this application the PIC needs to ''know''
> the time thus there will be either a RTC chip or the PIC will be used as
> clock. Do you have an idea of what is the order of duration for some
> ''standard'' batteries for this problem?
>
> Thanks and Best Regards,
>
> Andrea.
>
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