I am sorry to say that is very common especially with cheaper escs. Alot of fans on escs are really rated for max 2s lipo they usually say this in the manual. The fan is being run from the voltage of the battery and is not regulated by the esc so the stronger the battery the faster the fan spins. Some escs you can get away with it but run the risk of burning out the fan. The safest way to me is get a fan rated at 5v and run it off the receiver so it gets consistent power no matter what battery you use.