You can check the integrity of the coils on the stator at the bolt on cap at the Ebox, just unplug the connector once you unbolt the cap,
green & green are the battery charge coil, the manual will tell you which wires run the other two coils, the other wires are red, black & gray.
The coil itself, i believe you measure between the high tension leads, the ones that attach to the spark plugs, if the resistance is off by 10%+,
the coil itself is considered no go. (bad). Just use an accurate meter when you do this, you don't want to buy pieces you don't need.
Also, I don't understand how you are getting 20 volts, the system runs off a 12 volt DC battery, the most volts you could get should be
13+ after the charging coil topped off the battery. You may also have a bad voltage regulator, not uncommon failure on 550s. : (