The alternator will always have some “ripple.” The battery acts as a buffer/accumulator and will absorb that ripple IF and only IF the charging circuit is well connected. That means the large cable and the alternator body itself as that is part of the circuit. Battery cables, terminals etc are all suspect. The A/C ripple caused by a diode is too high frequency to see pulsing in things such lights but can be seen on a scope. A typical diode failure will show an asymetric wave. 300mV may be OK at the alternator’s output terminal but I would not expect it in a GOOD battery when measured at the terminals of the battery–not the battery cables. A sulfated or broken battery would invalidate the so-called ripple test. Voltage regulators are often responsible for low frequency, visible and rythmic voltage ripple…brushes that are worn out can also cause this.