Measure the battery voltage first. If it's 12.6 volts, it's fully charged. Check the smaller red positive battery cable where it attaches to the under-hood fuse box. Be sure that connection is clean and tight.
If the battery voltage is around 12.0 volts, it is discharged. Suspect the generator. Charge the battery, then measure the battery voltage again with the engine running. It must be between 13.75 and 14.75 volts.