Clearly breaking down is not something makers have given much thought to, nor their advertising depts care to mention. But it ought to be given some thought.

It should be technically possible to tow an electric car, at least at a low speed, and even at the risk of increasing the drag on the towing vehicle, to have an option to trickle charge the batteries from the regenerative braking while doing so. That or 800volt jump leads;-)
Mike

The Renault Zoe for example seems to need its front wheels lifted.

It seems you can tow charge a nissan, just it is way off what the manual suggests is possible, let alone advisable.. Or at least a few  enthusiastic Dutchmen have done it.

Mike

Not that I'm an expert by any stretch of the imagination (I'll leave that to Deborah-Claire McKenzie and the volunteers in the Automotive and road transport systems Network) but it seems that you can push an EV to the side of the road if needs be, they just can't be towed. 

My 2001 Audi can't be towed more than 30 miles at 30mph either, which made breaking down on a busy dual carriageway in Stevenage and wanting to be towed up the motorway to my trusted mechanic some 15 miles away quite fun 

I'll bring this post to the attention of some of the ARTS TN volunteers, as we do have a few EV experts on the team who should be able to contribute to the discussion.  

Hopefully I can add some clarity, but unfortunately it isn't a straightforward or simple answer!

Firstly, yes I think the problem has been exaggerated but also that it is a real issue. The trouble is not all EVs use the same batteries, motors etc and then the manufacturers also have their own ways of control, protection and safety.

The majority of EVs on the road today use a Permanent Magnet (PM) motor, and the motors are usually directly connected to the wheels, via some gearing but no clutch, this means that the motors can't be disconnected from the wheels. Being a PM motor, if it is being mechanically driven, the magnets will enduce a voltage in the windings of the motor (back emf) and if the motor control electronics (inverter) isn't operational (vehicle has broken down) then this voltage is uncontrolled and increases with speed, hence the vehicle manufacturers not wanting the vehicles to be towed or at least towed while the driven wheels are on the ground. But the vehicle should be able to move a very slow speeds, so pushing off the carriageway onto the hard shoulder shouldn't be an issue.

However, if there is an electrical load connected to the inverter, the electricity generated by the motor when the vehicle is moved will drive current into the load and the result is that there will be a resistance in the motor making it more difficult to move the vehicle. This would be down to how the vehicle manufacturer has designed and implemented the EV powertrain.

Regarding the vehicle being immobilised, all EVs are fitted with a similar 'park' function to that on a regular automatic transmission. This consists of a small pin which engages in a hole in the powertrain and 'locks' the powertrain. It has the same safety features and control as the one on an automatic transmisson, so it should be able to be released if the vehicle needs to be moved. For example the Jaguar I-Pace has a parking release handle under the bonnet which if the vehicle is completely dead, the gearbox lock can be manually released by pulling on this handle. But I'm not sure if this is in the user manual or widely known by rescue services. Electric handbrakes can usually be released in the same way.

I hope that makes sense and you can see how what you experienced can occur. In real terms it shouldn't be any more of a risk than an ICE vehicle breaking down.

Regards
Pete

Lisa, it's off topic but I presume your Audi is an automatic? If so, it will probably have a wet multiplate clutch in the gearbox. Wet being wet with oil which is used to cool the clutch plates and lubricate the gears. The clutches aren't designed to have a high speed difference between the plates for long times, just pull away from standstill and shifting gears. When the vehicle is being towed the one side of the clutch is stationary (engine not running) and the other side is spinning due to being towed down the road. This causes friction and the oil to heat up and can damage the oil or clutch plates if it gets too hot for too long. Hence the manufacturer limiting the speed and duration of the tow.

Regards
Pete

It’s not an auto Pete,️ it’s manual and a quattro. It’s something to do with it being a 4wd and not damaging the haldex controller or so I believe️ ️

Ah OK, I think it is the same reason the oil heating up, but rather than clutches in the transmission the haldex uses a differential pump, which I would have thought would have been OK to tow if all four wheels are on the ground and therefore going at the same speed. It would be a problem if two wheels were lifted.

Thank you all for the information. There seem to be multiple problems here:

Failure of the 12V system so that the steering cannot be unlocked. This seems to apply to IC vehicles as well but shouldn't normaly happen when underway, only after a full stop and locking of the car.

Failure of the 12V system so that the handbrake or parking lock cannot be released. This would also apply to IC vehicles but again shouldn't happen when underway but maybe if stopped in a queue. There appear to be manual release options but most people do not know about them.

Failure of the EV traction system so the vehicle is hard to move/imobilised. This would be dependant on the individual system. Putting a short circuit accross a PM motor will make it hard to rotate but is this a feasible fault? What other mechanisms might there be? 

 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

Hi Roger,

A short across the motor is very unlikely but the transistors in the inverter (motor drive) can fail 'on' so can cause a short. It is more likely that there is some load on the DC bus like the heaters, air con or DC-DC converter which will cause the motor to be hard to rotate.

Regards
Pete