A battery-changing vehicle includes a mobile carriage, a frame disposed above the mobile carriage, and a boom assembly connected to the frame. The swirl generator also has elements that are used to variably influence the pressure prevailing at a predetermined point in the cavity. In some embodiments, the hold limit is based at least partially on an account balance associated with the payee account, and one or more other characteristics associated with the payee account. Therefore, the flat surfaces on the cover member decrease, thereby reducing sound emission, and thus noises from the cover member.