Start by checking the serpentine belt tension. (Please do that with the engine not running!) Next, suspect a defective diode in the generator. That will show up as very high ripple during a load test with a professional tester. You can get a head start on testing the output by using an inexpensive digital voltmeter to measure battery voltage while the engine is running. It must be between 13.75 and 14.75 volts.
If the generator is producing very high ripple, a diode has been damaged by the very high voltage spikes these generators produce. To reduce the number of repeat failures, replace the perfectly good battery at the same time unless it is less than about two years old. As they age, they can still start the engine just fine but they lose their ability to dampen and absorb those voltage spikes. That leads to shorted diodes, destroyed voltage regulators, and engine running problems when those spikes interfere with computer sensor signals.