AA battery cradle that charges as well as powering devices when not charging?

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Hi,

I am an absolute beginner when it comes to electronics but have had an
idea to provide two power options when using my PDA:

When I am using my PDA I would like to have the option of using the
built in battery that comes with the PDA or in other instances use
batteries (AA) purchased from a shop when I am not near a mains power
to plug it in. So, I was thinking about getting my hands on a AA
battery charger. Now, here comes the clever part (ha) - instead of
lugging about a AA battery charger, my PDA charging cradle (both with
their own power leads) I was thinking about simply using a AA battery
charger that also could be used as a power out supply where by plugging
it into my PDA could be the sole power supply for the PDA. When Im out
in the wilderness and come across a shop I can buy batteries, plug them
into the AA charging cradle and plug that into the PDA ... or when I do
have access to a mains power supply I can charge my rechargeable AA
batteries with the AA charger for another run.

So, my question would be does such a battery charging device exist that
can charge batteries as well as output to an electronic device when not
charging (or even both at once)?

Cheers

Burnsy

Ps. If it can also power AAA batteries too that would be a bonus