Do this first - make sure the clamps on battery poles are tight. If they're, then proceed below.
Charge the battery using a battery charger for a couple hours, disconnect the charger, and measure the voltage. If voltage declined and fell below 12V, it's likely bad.
If the battery tested good, start the car, and measure voltage again while the engine is running. If it's less than 12.8V, the alternator may be bad.
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