Drop by an Auto Zone or Advance Auto. Usually they'll test the battery and alternator charging function for free.

Before even doing that you should check that the battery connections and especially the -(neg) ground cable connection to the body are clean and tight.

Has it been starting OK (without having to jump it from another car) every day?

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'81 300CD - 180K, cannot be killed by any conventional means
'99 Ford Escort - good MPG