Cordless vacumn charger rating calculation
Probably "never."  I doubt it is charging at all.

The voltage on the "new" charger is lower than on the old one.  

You need to match the voltage of the new charger to the voltage of the original one.

If the voltage is lower than it expects, then the regulating circuit that controls the charging process won't even start.



Your vacuum cleaner may have a simple "charge regulator" that is nothing more than a current limiter (like a series resistor.)

It might seem to do something, but will also never fully charge the battery.