Battery is possibly bad - especially if more than a few years old and/or it hasn't been maintained properly. As per above, a cheap voltmeter will tell you present state of charge. But that won't tell you if the problem is with the battery or the charging system. Many places that sell batteries will check them for free. Also be on the look-out for parasitic loads when the car is turned off (eg, did you install a new stereo recently?).
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