I have to agree Dave, I always check for ac when checking any charging system. Even small engine and motorcycle I have found it to be helpful. I usually grab my fluke 87 not the cheap meter.The concens
The consensus I have gained from Kubota tech's who work on these machines as a profession is it is a mistake to rely on cheap meters to do diagnostic tests.
Diodes fail in several ways. Keeping in mind the diode's main job is to block one half of an AC signal, I have seen unexpected signals when trying to determine if a battery is being charged.
If things do not make sense with voltage readings, the first thing I do is to change meter scales from DC to AC.
If I am reading AC at the battery my findings are generally there is charging system trouble.
There are forum members whose electronics knowledge is much greater than mine and they may have comments to provide.
Dave
Bill