Volvo Buses
Global
Volvo Buses
Global
The introduction of electric buses means a technology shift. Electric buses and conventional buses have mostly the same safety-related features. But important differences include the driveline and the energy storage. This is an integral part of Volvo’s safety architecture, which includes all aspects of the vehicle’s electrical systems and components.
Already in each battery cell there is preventive functionality, autonomous functions that prevent escalation in the event of damage. And on vehicle level, all functions are monitored so that the driver, and the traffic control, can be alerted if a risk is detected.
The safety system in a Volvo electric bus will instantly alert or warn the driver, but only when a direct driver intervention is required. Avoiding redundant information helps the driver to stay focused on the principal assignment – to take the passengers to their destination, safely and on time.
Vehicle safety is subject to a wide range of regulations. For electric powertrains, the R100 standard is central. The standard describes a multitude of parameters to be considered and tested. All electric vehicles from Volvo Buses are R100 compliant.
There are no indications that electric buses should bring more risks than conventional buses. But risks exist with all vehicles and for electric vehicles they are different than for diesel and gas buses. The risks mainly fall into the categories operational, collisions and fire. situation.
Li-Ion batteries are sensitive to operational misuse such as overcharging and excessive discharge. Overcharging can occur if faulty or unsuitable charging equipment is used, if safety procedures are neglected or if onboard safety functions are overridden.
In the event of a collision, a massive impact could damage high-voltage cables and floor mounted batteries. If the vehicle rolls over, roof-mounted batteries could be damaged. And if this damage is sufficiently serious, there could be leakage, and, at worst, a fire caused by internal shortcuts.
It’s a fact that a severely damaged battery means a risk of fire. But electric vehicles are not more prone to catch fire than ICE vehicles. Reported data* indicates that the fire incident frequencies for EVs are generally 8-20 times lower than for ICE vehicles.
* Report from RISE: Electric Trucks – Fire Safety Aspects Jonna Hynynen, 2023-06-26.
Battery health and performance are crucial factors for safe operation of electric buses. The battery management system regulates charging and discharge current, and controls the cooling and heating of the battery installations, in order to let the batteries function under optimal conditions.
In a Volvo electric bus, all relevant parameters are continuously monitored in order to maintain safe operation. To keep the cell in optimal condition, and well within the safety window, the central unit and the battery management system continuously:
The safety window visualizes the battery’s defined range for temperature and voltage, within which operation is safe. Deviations from these values can affect performance and lifetime.
If the temperature remains above the safety limit, or frequently exceeds it, a cell internal short might occur which in turn is leading to gas venting or thermal runaway. Furthermore, voltage outside the safety window will slowly degrade the cell, and could eventually cause a safety concern.
Energy storage must always be protected from all kinds of abuse. Damage to a battery can lead to several risk scenarios and escalation of events. Ultimately a thermal runaway, which could lead to fire. This is why Volvo electric buses have several levels of safety functions that will stop the progression of problems.
In Volvo batteries, the cells are assembled in modules, which provides mechanical protection, vibration damping and heat insulation. The modules are clustered in the battery pack and the robust steel casing of the battery pack is an integral part of the mechanical protection.
| Robust casing prevents physical impact and deformation |
| Sealed modules prevent water immersion |
| Barriers between modules reduce and distribute mechanical impact forces |
The batteries and all electrical components are continuously monitored by the battery management system and the central unit. If an anomaly is detected, cells, packs and the entire traction system can be disconnected. The high-voltage cabling is monitored by the HVIL (Hazardous Voltage Interlock Loop) circuitry. If the circuit is broken, the battery will be cut off at the source.
| Monitoring keeps all electrical parameters under control |
| The HVIL will cut off the traction voltage at detected risk. |
| Fuses disconnect parts or all of the traction voltage circuitry. |
| Vents reduce the build-up of internal pressure. |
The entire energy storage system is temperature controlled by a separate cooling/heating liquid-based system. If the temperature rises, adjacent cells and modules are protected by heat-resistant materials.
| Heat barriers prevent temperature rise in adjacent cells and modules |
| Active heating and cooling keep battery temperature within the safety window |
| Ventilation reduces the build-up of internal heat |
The temperature control of batteries actively reduces the risk of malfunction and will disconnect batteries to remove further abusive conditions. The system also actively detects signs of thermal runway, and activates warnings to the driver.
| Temperature control |
| Battery disconnect |
| Thermal Runaway Detection |
| Driver Warnings |
Maintenance and repair of an electric bus has a lot in common with working with conventional buses. The difference is the driveline, the batteries and the internal energy management. Working with high-voltage systems requires special training and certification.
Several components in the electric traction system cannot be repaired, but should be replaced and returned to Volvo. All kinds of work on the traction system requires following of a decommissioning procedure, and after completed work the commissioning procedure puts the vehicle back into operational state.
The strict procedures ensure that the service technician cannot be exposed to hazardous voltage. Only authorized persons, i.e. the “technician responsible for work activity”, are allowed to do the decommissioning and commissioning. This person must also have the delegated responsibility to be allowed to do the work.
Learn more about Volvo’s holistic commitment to electrical safety.
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