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Towing an electric vehicle

Towing an electric vehicle

Hi,

Could you just tow an electric vehicle behind another car?
I once heard it would upset the cars electronics but is that just a myth?
If not why isn't it possible to tow it?

Thanks,

Beau Ackx

Brian H | March 1, 2014

It's not approved, but towing (even rope-towing) an EV in gear will regen-recharge it.

Timo | March 1, 2014

If you have AC motor and don't power it it's just free-wheeling and does not produce anything. Problem probably is that it strains the reduction gear a bit because there is no neutral where wheels turn freely. But that's small problem, because if it can handle the strain from regen braking it can handle towing.

I see no reason why you could not tow BEV.

DTsea | March 1, 2014

@Timo, wrong. If it is in gear it will regen. that's how regen works when you go down a hill. Tesla says DON'T DO IT. So don't because you could void your warranty.

jkn | March 2, 2014

Motor needs either permanent magnets or current to produce magnetic fields or it is only free-wheeling. So no regen after battery is completely empty. Cars power train does not know if energy comes from rope or downhill.

Towing is considered dangerous and illegal in many places. Then people drive 80 mph so close to other car that there could be a towing rope. Towing with reasonable speed would be safer.

Brian H | March 2, 2014

"Completely empty" never happens. The battery accepts current except when too cold or too full.

Timo | March 2, 2014

Battery doesn't accept charge when it is absolutely full. When you charge the car in range mode there is a short period when you don't have regen. This means car can shut off the current to motor if needed. I think there is towing mode in fact. Model S owner could confirm (or deny).

What Tesla denies is to use regen when towing. Probably not because of electronics, but because strain it creates to frame of the car; a bit like towing with brakes on, not smart thing to do, might twist the frame slightly if done for prolonged period and towing rope is not absolute middle of the car.

jkn | March 4, 2014

Brian H,

By "Completely empty" I meant that it does not give any current out. In this case regen is impossible. 12 V battery might have enough energy to create magnetic fields in motor, but I don't think system can use it.

Timo,

60 kW regen at 60 km/h, + 10 kW losses, P=F*v ->
F = P/v = 70 kW/ 60 km/h = 4.2 kN

This force would accelerate 2100 kg with 2.0 m/s² = 14 s from 0 to 100 km/h. Not an enormous force. With an MS towing another, acceleration without regen could cause stronger force.

I don't believe that force would damage anything. Real risk: Towing car has to stop because of traffic. Towed car does not stop immediately, so rope gets loose. Towing car starts when there is gap in traffic, not noticing loose rope. Don't use too strong rope:)

Brian H | March 4, 2014

Wrong. EVs in Drive will generate regen current when towed. It's been done.

jkn | March 4, 2014

Brian H,

Do you even read what I write?

Don't change subject to all EVs. Permanent magnet motors are different.

I have not claimed that MS can never be charged with towing. It cannot be charged with regen if battery is empty (= no current available from battery).

Brian H | March 4, 2014

That makes no sense. The generation of current does not depend on existing current, only resistance in the circuit.

Timo | March 4, 2014

It requires magnetic field which requires some power to the induction motor. If that is absent nothing happens. It would just be lump of metal rolling inside other lump of metal.

Brian H | March 5, 2014

Which would require true bricking, even beyond the deepest failsafe TM has built in.

Timo | March 5, 2014

Correct. There is other situation where Model S doesn't regen also, that's when battery is full. In that case car lets motor free-wheel without power so that there's no feedback power. It's not long before it allows regen though.

jkn | March 5, 2014

Tesla has a safety limit to avoid 'true bricking'. When battery reaches that limit, it will not give out any current. In this situation car cannot be charged with regen. Of course TM could have added an exception into battery protection logic: Don't give out any current unless it goes to start regen.

I don't believe they did that because they don't even recommend towing.

jkn | March 5, 2014

I have learned here that 12 V system is needed to close relay to connect main battery to other systems. So logically thinking this relay is opened when battery is depleted to safety limit. No regen possible after that.