Uhh, khlam, you're a little bit off there. As a US 453 ed/eq owner, the HV battery is 7.2 kW, not 17.6. Also the 120v power charger cord supplied with the car comes with 2 settings, an 8a and a 12a. My 240v power charge cord, however, allows 30 amps. The car can be recharged from empty with 240 volts at 30 amps (i.e. 7200 watts) in 2.5 hours. Using the 120v power charger cord the smart car can be recharged from empty in 10 hours. But nobody usually allows the car to go to empty, so charging times are less.
Ha, sorry about the amp rating on 240v plug and I got the EV battery number from here: Smart electric drive - Wikipedia
While the number may be off, the math behind it is not.
If your 120v charger says it has 2 settings (8a and 12a), then charging time with 8a setting will take longer to charge from "empty to full" than the 12a setting.
(120v x 8a = 0.95kW vs 120v x 12a = 1.44kW). If you divide the "capacity of your HV battery" by this, it should give you the "time need to fully charged" number.
If you have a 240v charger (30a max. but you did not say what the actual amp it is actually drawing from wall plug), then it will charge much faster from the same "empty to full" state.
(240v x 30a = 7.2kW, which is the max power you can draw from the wall plug)
If the HV battery is 7.2kW (wrong unit, shouldn't it be kWh?) as you said, then in theory, you can fully charge it in 1h (7.2kW x 1h = 7.2kWh).
Since you said, in practice, the HV battery fully charged in 2.5h, then I would guess the 240v charger is drawing 12a from the wall during the charge cycle.
(240v x 12a = 2.88kW, with 2.5h charging time, then 2.88kW x 2.5h = 7.2kWh)
By the way, "kW" and "kWh" is 2 different measuring unit.
kW -- power rating in Watt (kilowatt)
kWh -- power used in 1 hour (kilowatt-hour)