The stock battery damps system noise and voltage variation effectively. The Shorai battery is physically smaller with considerably less plate area. The plates are what absorb system transient changes. When loads drop out and then on the DC level changes, same with when the charging system switches on and off. The battery is expected even this out, or at least tolerate the transient voltage changes. Using ACV, you can measure the transient changes which ride on a the DC level in the system. When you transition into the realm of AC, however, you also have to deal with time periods.
In the real world, our bikes aren't strictly DC in operation, they have variations in system voltage, which can be described as an AC component within a DC system. What IS required is that the AC variation either be at very low amplitudes or have extremely long time time components.
Most hand held meters are calibrated to accurately measure in the 50-60Hz range. AC potentials ocuring over, say hour time periods, won't register, unless some sort of peak hold circuit is employed in the test device.
The alternator is an AC device delivering 3 phases of Sine waves at frequencies variable with RPM. While the phases do have some overlap, the rectifier does nothing to smooth out the DC output. It's the battery that is expected to make the DC level smooth. If it is NOT doing that job adequately, it will show up as excessive AC voltage riding on the DC average level. Of course if the rectifier has shorted diodes, the AC would be impressed onto the DC system as well.
A full 24 VAC at the battery would lead me to scrutinize the rectifier, at least.
Cheers,
