Tow charging

Let’s say that you run down your EV battery to zero.  Or let’s say you run it down so far that you would be unable to reach your intended charging station.  What can you do?  Some people do “tow charging”, which makes use of the regenerative-braking feature of the vehicle. 

The fallback position for somebody who has run down their EV battery is, of course, to get the vehicle towed to a charging station.  This has many drawbacks, including:

  • it is embarrassing;
  • you have to sit around and wait until a tow truck is available;
  • it probably costs money.

It is possible to imagine setting up a mobile charging system that might fit into the back of a pickup truck or service vehicle.  One way to do this would be with a fossil-fuel-powered electrical generator, feeding 240 VAC to a Level 2 EVSE.  SUch an arrangement would probably be limited to about 11 kW, meaning that the fastest charging would be around 40 miles per hour.

Which brings us to “tow charging”.  The idea is that you use a powered vehicle with a tow rope or “come-along” to pull the EV along a road.  A driver sits in the driver’s seat of the EV, gently pressing the brake.  The goal is that the pressing of the brake leads to regenerative braking, which charges up the battery.

There are many reasons not to do tow charging:

  • It would be all too easy for things to get out of control.  The lead vehicle might need to stop suddenly and the driver of the towed vehicle might not be able to stop fast enough.
  • The regenerative braking system has some limit in the amount of power that it can absorb from the wheels.  It would be unfortunate if the people doing the “tow charging” were to pull the towed vehicle so fast that the regenerative braking system would risk getting too hot.
  • Related to the previous point is that if the amount of braking requested by the driver of the EV were to exceed the amount of braking that the regenerative braking system can provide, then the old-fashioned friction brakes would be called into service.  This would cause unnecessary wear and tear on the brake disks and the brake pads.
  • Likely as not, the user manual for the EV says not to do tow charging.  If the tow charging were to damage the EV in some way, the repair might not be covered under the vehicle warranty, and indeed there is the risk the tow charging could void the warranty.

Having said all of this, it seems to me that if the tow charging were done carefully and thoughtfully enough, it would be no more risky or harmful to the vehicle that simply driving downhill and using the downhill travel to accomplish regenerative braking.  One smart thing would be to ask the vehicle to display its detailed quantitative energy-flow graph, showing the number of kilowatts being pumped into the battery.  The driver of the EV could pay close attention to the graph, making sure to avoid pressing the brake pedal more than the amount needed to accomplish whatever the maximum regenerative power level is that displays on the graph.  It would also be important for the two drivers to be in constant hands-free radio or telephone contact during this tow charging activity.

Have you done tow charging?  Please post a comment below.

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