First of all, you should check the battery voltage readings with a voltmeter and then start the car to see what voltage reading is comparing the two. If the voltage reading goes up between 13.5-14.8, it would be a bad battery, if no change it would be the alt. not charging. And also others things to check is the voltage regulator and make sure the gen bulb or resistor is still functional behind the speedometer case. Good luck.
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