There should be the same Voltage at the charger port, or at the controller side of the charger port connector, as the battery pack has.
Please let us know if you have any questions.
Another good test to perform would be to test the Voltage of the battery charger to make sure that it is working. This way you will know that it will recharge the new batteries. On the Razor E100 scooter this is very easy to do and only requires inserting the multimeter probes into the holes in the battery chargers output plug. The polarity of the charger's output plug is shown below.