Volt guages are a jokes. A battery can have around 12 to 14 volts and be deader than a doornail. A battery that reads as having 12 volts and 3 amps output is dead, but the sfb volt meter will tell you that--duh--the battery/system has 12 volts.

Simply, anything other than an amp meter, and your pizzin in the wind as far as trying to rate the charging system.