It is a fan--it's a circulation fan located inside the right rear of the battery pack. It runs it for a brief time to ensure that the system is working properly. If the fan doesn't work then charging may be limited, and a warning light and trouble code will be stored in the EV-ECU. If the battery temperature rises during charging, then the fan comes on to pull air thru the pack to help cool it off.
There is also a water circulation pump motor that runs to push coolant thru the baseplate of the onboard charger.
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