Method for ascertaining the fill level of a tank unit, and tank unit for storing a gaseous fuel

A tank unit (1) for storing a gaseous fuel, comprising:

• a plurality of tank vessels (TB) into which the gaseous fuel can be filled;
• a gas pipe system;
• at least one valve mechanism (2) per tank vessel, which connects the respective tank vessel to the gas pipe system (GL), wherein the at least one valve mechanism (2) is switchable between an open state, in which it connects the tank vessel in a fluidically conductive manner to the gas pipe system (GL), and a closed state, in which it separates the respective tank vessel from the gas pipe system;
• a sensor device
• (Si) which is designed to capture a temperature in each tank and a gas pressure in the gas pipe system;
• and
• a control device (SE) which is connected to the sensor device (Si) and is configured to
• read a fill level of all tank vessels out of a data memory, said fill level having been ascertained and stored during a preceding operation of a vehicle;
• control switching of the valve mechanisms from the closed state to the open state such that gas can flow from the tank vessels into the gas pipe system;
• ascertain a mass of gaseous fuel dispensed from the gas pipe system (GL) during a specified time interval (t) during operation;
• and
• ascertain a current fill level of all tank vessels during the specified time interval by subtracting the ascertained dispensed mass of gaseous fuel from the stored fill level.