There might be a bad diode(output seems a little low) perhaps creating enough extra noise to make the meter jump around, but also think its the meter having sampling issues with a bit extra alternator noise ........If this measurement is from the B+ of the alt., then of course the battery is connected to this wire and with good/clean connections yet your meter is flipping around as low as 0.26V, 0.63V, 1.63V while connected to a 12V lead acid battery? Not entirely real readings, the 12V lead acid ain't going to drop like that, not that fast and esp not that low.