the OEM alternator, cost effective, works only on the highway where rpm produces power. Generalizing, a Ford 120amp alternator supplies lights and air at 50-60mph. Add a stereo amp, way over limits. The negative threshold comes slowly with yellow lights after 2-3 hours of asking a very good battery for more discharge than charge.

A 200amp alternator (Summit Racing) claims production of 120amps at idle but give it 15-20 mph. The OEM needs 65mph to give 120amps.

The battery your using may not be quality or fresh. Charging takes power and voltage the system may not support but that the OEM alternator at 65mph does. The 200amp unit charges at 45 mph in abt 20 miles.

drivers 'charging' dead batteries at idle after jump starting are not getting their $$$ worth.