EVs immobilised by flat batteries or breakdowns

A few weeks ago while driving back from a weekend away we came across several long queues on the Autobahn apparently due to stuck vehicles. One, on our carriageway, was caused by a car half on the hard shoulder and half in the slow lane. The police were there and I couldn’t see why two police officers and the driver couldn’t move the car a few meters on the level. A little further on there was a car stopped in the middle lane of the other carriageway. Again the police were in attendance but seemed unable to move the vehicle to the hard shoulder. As there was a good 5km of queue behind this one there should have been enough people to pick it up and carry it off the road.

After some internet searching I came across this article:

https://www.motoringresearch.com/car-news/transport-minister-astonished-hazard-electric-car-motorway-breakdowns/

“When an electric vehicle ceases to function, it stops; it does not coast in the way that other vehicles do,” said Barnoness Randerson, Lib Dem transport spokesman, during the debate.

“Smart motorways are supposed to be the future, but the future is electric. Those vehicles stop very suddenly. They also cannot be towed; they have to be put on a low-loader, which is a much more complex and longer process that will put rescue teams in greater danger.

Indeed, during a test of real-world electric car range, Carwow found that some EVs were “difficult to move” when the batteries were run truly flat. Most cars in the test ‘locked up’ once they were out of juice. The tested included everything from the Tesla Model 3 and Audi E-tron, to the Kia e-Niro and Nissan Leaf.

 

Is this true/realistic or has the problem been somewhat exaggerated? I am used to manual cars with conventional handbrake that can easily be pushed. Are EVs such a problem to move when something goes wrong? What about conventional cars with electric handbrakes, can they get stuck as well?

Parents
  •  The implication is that if you pull the motor as a generator and the electronics is not energized, so the motor control fets/ IGBTs  are not commutating, then there is a possibility to generate over voltages that can damage them. This does seem like an oversight - some bosky bypass diodes ought to do it, even if a lack of cooling means the result requires you not to tow at more than 30mph or something. Failing that the ability to jumper 12V/24v or whatever it needs into somewhere to get basic  instruments and telemetry up .
    Mike

Reply
  •  The implication is that if you pull the motor as a generator and the electronics is not energized, so the motor control fets/ IGBTs  are not commutating, then there is a possibility to generate over voltages that can damage them. This does seem like an oversight - some bosky bypass diodes ought to do it, even if a lack of cooling means the result requires you not to tow at more than 30mph or something. Failing that the ability to jumper 12V/24v or whatever it needs into somewhere to get basic  instruments and telemetry up .
    Mike

Children
  • Hi Mike,

    Yes, it's actually the diodes in parallel with the transistors in the inverter that when not energised can't help themselves but conduct the energy generated by the motor to the DC Bus. You could use bypass diodes but where do you put the energy? And if you are taking energy out the motor it will cause a drag an make it difficult to move.

    I do know, as part of the safety systems, OEMs are looking to ensure the inverters can keep themselves energised, actually use the volts generated by the motor to power themselves so as to keep them operational even when the 12V/24V or HV bus have failed

    Regards
    Pete