First check and clean the battery terminals ^^^
Then check and clean the connections on the three grounding strap, if any strap is frayed, broken or corroded, replace it. Facing the engine, two are on the left and easily accessible. The third is on the right near the tranny linkages (manual tranny).
Easy charging system check: use a multi-meter to measure voltage at the battery terminals:
Engine off: about 12.5V or higher. Less than that and the battery is discharged, failing, or both.
Engine on: Should be well above 13V, closer to 14V. Less than 13V and the charge system has an issue
To get a good check on the battery condition, the best thing is to take the battery to FLAPS and let them put it on a tester. You can also buy an inexpensive tester like this...
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