How MAN makes its e-Buses safe

Safety even in the event of a bus rollover

With the help of simulations, extensive crash tests with vehicles and batteries as well as bus rollover tests in accordance with ECE-R66, the passive safety of the e-vehicles is assured. Of course, special attention is paid to the battery packs. When electrifying its vehicles, MAN uses mature battery cell technology from the Group’s modular system, which ensures the reliability and safety of the batteries.

The battery modules of the MAN Lion’s City E electric bus are specially developed for commercial vehicles and optimised, tested and approved with a specific regard to reliability, robustness and safety. “Not only do we check the safety aspects of our own production, but we also check components from suppliers,” says Kienast. In the process, the components are tested three times, at different points in the production process. For the first step, strict internal controls ensure the quality of the battery cells. “This is followed by a second start-of-line test of the individual modules – and finally an end-of-line test for each individual battery pack,” says Kienast. They are then securely installed in the top of the roof and are thereby protected from the forces of common rear-end collisions.

Continuous improvements to eBus safety

The relevant software and hardware components are protected with appropriate safety concepts. Various safety standards were used for the Lion’s City E electric bus, including the development in accordance with ISO 26262. One example is the protection of the charging process. “Early warning systems immediately switch off all relevant high-voltage functions, including charging, in the event of a rise in temperature or a system-related anomaly,” says Kienast. Safety comes first here too. “The shutdown of all high-voltage components is secured for the relevant fault cases with appropriate safety concepts,” says Kienast.

From model year 2023, an additional safety level will also apply. A warning will be transmitted to the customer’s backend so that they can react and take action as quickly as possible, i.e. alert the fire brigade and drive more buses out of the depot. “In the future, we want to enable our customers to have the eBuses transmit a warning signal to the responsible fire brigade automatically,” says Kienast. The eBuses will then use warning lights and horns to draw attention to a hazardous situation. By doing so, it will be immediately apparent to the employees and the rescue services which bus has a problem.

“Our goal is to avoid a thermal runaway,” says Kienast. That is the worst possible outcome in electromobility. Thermal runaway refers to a rare chain reaction that can theoretically occur during a fire in the battery of an electric vehicle. A worst-case scenario of thermal runaway would see it eat its way from cell to cell over a very long period of time and lead to the vehicle catching fire. “This has never happened with a MAN eBus,” Kienast emphasises. “To ensure that this remains the case, the potential risks or causes of faults have been analysed in detail and safeguarded with appropriate safety concepts.”

Close cooperation with customers and fire brigades

eMobility expert Kienast therefore considers the degassing of batteries to be the absolute worst case. In order to protect the passengers as best as possible in the unlikely event of a thermal runaway, the batteries are degassed in a controlled manner and additional measures are taken to ensure that the gas is discharged to the outside of the bus. “Close cooperation with our customers is important here, who need a carbon monoxide detection system in their bus depots,” says the expert. Thanks to MAN’s continuous and close cooperation with the fire brigade in the creation of rescue guidelines, rescue workers also know exactly what to do in the event of an accident.

3 questions for ...

Is charging eBuses more dangerous than cars?

No, electric cars and buses use up to 800 volts. Therefore, there is no increased fire hazard from higher voltages. The currents, especially the current rates (C-rate) seen from the cell perspective, are significantly lower in our buses than in passenger cars. This also leads to significantly lower temperatures during charging. This tends to result in a reduced fire hazard compared with passenger cars.

Do eBuses burn faster than electric cars?

A single battery pack in the Lion’s City E has about the same energy content as in an electric car. The battery packs in our electric buses are individually fused, so the fire hazard posed by the batteries is even lower than in a car. Of course, the amount of energy stored in a bus is higher, so more energy could be released in the event of an accident. But this is already the case when comparing cars and buses: the stored energy in buses, whether in the form of diesel, hydrogen or an electrochemical battery, always brings with it a potential danger.

What precautions have you taken at MAN?

In preparation for bus production in Starachowice, new equipment was purchased or expanded, processes were rethought and around 3,500 employees were intensively trained in the handling of batteries and high-voltage components. We have also worked out emergency tactics with the plant’s fire brigade. In our test workshop in Munich, we have successfully tested the parking of vehicles on so-called boards. This allows them to be moved away as quickly as possible, even if the parking brake is applied.