The voltage depends upon the charge state of the battery.
Fully charged is about 12.5 Volts but it can read as high as 13 volts if the battery is freshly charged and has had no load applied.
So, charge the battery with a battery tender until the green light is illuminated, turn the key on and then back off to apply a load. Then measure the battery voltage at the battery.
If the voltage is ok, then let the battery sit for a day and measure again. It should read the same.
If the voltage is below 12.2 then the battery should be tested under load. Auto parts stores and some garages will test the battery for free. Make sure that they have a load tester appropriate for the size of the scooter battery.
The scooter's charging system should supply voltages greater than 12.5 but less than 15 volts.
14.5 is the usual max voltage .
The supplied voltage is dependent upon the engine RPM to a degree. At idle the voltage may be below 12.5 but the voltage should increase with RPMs until it reaches the max, which (modern systems) should be on the order of 3000 RPMs.
Be sure that the scooter is on the center stand and that the rear wheel is free to rotate if you make this RPM / voltage measurement. And be sure to measure at the battery terminals.
If the voltage never gets above 13, then the charging system is suspect.
If it rises above 14.5 then you need to replace the voltage regulator and not run the engine until it is replaced.
Have a good winter,
