Under the skin: a crash course in making electric cars collision-safe
18 Mai 2020 - autocar
EVs have different structures and safety technology to provide protection in a collision. We take a closer look
How safe is an electric car in a crash and what are manufacturers doing to make them safe? These questions crop up a lot and are usually triggered by lurid videos of an EV being consumed in a battery-fuelled inferno, but that doesn't make electric cars any more dangerous than their fossil-fuelled equivalents when things go wrong.
To minimise damage to the high-voltage battery in an impact, manufacturers are rethinking their approach to vehicle architecture. Fully encased, floor-mounted batteries provide a protective environment but deflecting the energy of an impact away from the battery is also important.
Polestar has developed the 'SPOC Block' (severe partial offset crash) for its 2 EV. This consists of two aluminium blocks set in either side of the front bulkhead. The blocks prevent front wheels being driven backwards into the battery compartment during a partially offset frontal collision.
The EV also has a new front lower load path (FLLP – how the structure diverts the forces from an impact) to further protect the battery and occupants from frontal impacts, including one of the worst: from a pole.
Design can help push back the threshold of fatal injury to a car's structure but eventually the laws of physics have their way. All cars rely on one fundamental thing to do their job and that's energy – and lots of it. MPGe (miles per gallon gasoline equivalent) puts the energy contained in a single gallon of petrol at around 40kWh, equivalent to the battery capacity of many smaller EVs. Whether a car is powered by a battery, petrol or diesel, the energy it stores is likely to be released in a major crash.
EV batteries are a little more tricky than conventional fuels because a fire inside one is due to thermal runaway, when the heat produced causes damage that results in more heat being released. It's difficult to extinguish because the battery is made up of several thousand individual cells and there will be sections of the battery that are still active and still contain energy. If the damaged battery is disturbed, thermal runaway can resume until the battery is totally kaput.
The other major difference between conventional and all-electric cars is high voltage. EVs operate at around 400V, and while the Porsche Taycan is the first EV with an 800V system, it won't be the last. For a car's occupants or emergency services personnel, damaged high-voltage cabling could be lethal, so in a crash, the battery is disconnected at source.
This isn’t a new idea: it has been a regulation in motorsport for decades to prevent electrical sparks igniting fuel in a crash and all race and rally cars must be fitted with either manual or electronic battery disconnect systems.
The usual method for disconnecting an EV battery is the firing of explosive ‘pyrofuses’, which sever the high-voltage cables close to it. Bosch revealed a new device last year triggered by a silicon chip responding to the airbag system that drives a small sharp wedge through the cable.