Tow charging!

[pictsidhe]

From before I got my SE, I suspected the regen braking could be used to tow-charge it.

I needed to move a dead car for someone today with my truck. So while we were in a towing mood, we tried tow charging my SE.

Yes, the SE will tow-charge! I started it up as normal and left it in max regen. At speeds up to around 35mph, I was looking at -0.9mi/kWh. Trip computer did not go negative and pegged at 20mi/kWh. That was despite finishing the short test with an extra 4% in the battery.

I suspect maximum charge rate is the same as DcFC, 49kW. So over maybe 45mph, you won't charge faster and you will need to do more miles. At some point, the motor may well get warm and throttle charging. 49kW is 65hp. I doubt that it is rated for that continuously. Teslas M3s aren't...
Depending on tow vehicle and road speeds, you may want to use low regen. Low regen may allow faster motor limited charging. Current is the main cause of heat and is approximately proportional to torque.

My 10' strap was scary short. I'll get a longer one to add to my charging kit.

 

[AndysComputer]

Is there any issue re cooling we might need to be concerned about?

 

[pictsidhe]

Is there any issue re cooling we might need to be concerned about?

I'm certain that the car will limit charge rate if things get warm. Much like if you take a Mini to a track, it's not long before power is severely limited. I saw one video where the driver reported 30% power after some fun laps.

 

[Jim In Tucson]

Tow-charging is the same as going downhill, right? Except using a tow vehicle instead of gravity.


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[pictsidhe]

Tow-charging is the same as going downhill, right? Except using a tow vehicle instead of gravity.


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Yes.

 

[carrrl]

With a 135kw motor and 0.19G regen, it should be able to exceed DCFC rates. Many slow charging EVs do since regen is meant to be short bursts not a sustained event. SE is also really opaque about when and how it’s using its friction brakes. No way to guarantee it’s all regen all the time. Temperature, SOC, speed, and traction control play a role.

 

[insightman]

With a 135kw motor and 0.19G regen, it should be able to exceed DCFC rates. Many slow charging EVs do since regen is meant to be short bursts not a sustained event. SE is also really opaque about when and how it’s using its friction brakes. No way to guarantee it’s all regen all the time. Temperature, SOC, speed, and traction control play a role.

I love the way my SE uses the brakes to simulate regen deceleration when the battery is fully charged. Some EVs instead surprise the driver with zero regen-like deceleration if the battery is fully charged.

 

[pictsidhe]

Hmmm. If mine is fully charged, I need the brake pedal to stop at my first set of lights in the morning. So no it's not giving it full 0.19g brakes
I did check brake temp the other day, they weren't used. I started at 36% charge, so the battery should have taken everything the motor would give it. I have noticed a lower kWh/mi regen rate at higher speed. DCFC connects the charger directly to the battery. No real need to skimp on power there. The battery will have a maximum charge rate.
More research could obviously be done. I just thought others might like to know that tow-charging is possible.

 

[fishbert]

Some EVs instead surprise the driver with zero regen-like deceleration if the battery is fully charged.

I rented an SE for a couple days to test drive before replacing my F56 Cooper S. First time getting back in my old car it felt like I had a brake problem, because that extra regen drag wasn't there. It was quite alarming. So glad I discovered that on my quiet neighborhood streets instead of in traffic.

 

[pictsidhe]

I rented an SE for a couple days to test drive before replacing my F56 Cooper S. First time getting back in my old car it felt like I had a brake problem, because that extra regen drag wasn't there. It was quite alarming. So glad I discovered that on my quiet neighborhood streets instead of in traffic.

My R53 does that to me, too. Whoah!

 

[insightman]

Hmmm. If mine is fully charged, I need the brake pedal to stop at my first set of lights in the morning. So no it's not giving it full 0.19g brakes
I did check brake temp the other day, they weren't used. I started at 36% charge, so the battery should have taken everything the motor would give it. I have noticed a lower kWh/mi regen rate at higher speed. DCFC connects the charger directly to the battery. No real need to skimp on power there. The battery will have a maximum charge rate.
More research could obviously be done. I just thought others might like to know that tow-charging is possible.

I was going by feel only. Because it always feels the same to me when I lift off the accelerator pedal, I was amazed when I realized that there can be no regen braking with a 100% charge.

 

[F14Scott]

How do EVs stop regenerating power (and therefore braking) after the battery is full? I suspect it is the same way one can toggle from heavy regen to light regen, but I don't understand that mechanism, either.

If I leave my house at 100%, accelerate to 90 MPH, and let off the throttle, the motor/generator has a lot of force that wants to spin it, but there's no place for the juice to go. How does the EV *not* make electricity? Is the motor clutched, somehow? Or are the rotor and stator moved apart?

 

[SameGuy]

If you accelerate to 90 mph and then slow down with regeneration, the car uses more juice to get to that 90 mph than it can possibly regenerate bringing it back to zero. You already created enough room in the battery. The system is far from 100% efficient, otherwise you’d have a perpetual motion machine.

 

[F14Scott]

If you accelerate to 90 mph and then slow down with regeneration, the car uses more juice to get to that 90 mph than it can possibly regenerate bringing it back to zero. You already created enough room in the battery. The system is far from 100% efficient, otherwise you’d have a perpetual motion machine.

OK, fair point. But, the car can't accept charge at the top end of the batteries' charge as fast as it can discharge.

Let me alter the scenario. I charge to 100% at the top of Pike's Peak. I unplug and start rolling downhill, regenerating immediately. What happens?

 

[Puppethead]

The SE must have a way to disengage regen and substitute the friction brakes (using traction control?), because when I coast to my first stop sign with 99-100% SoC still showing the car automatically brakes like normal when I take my foot off the accelerator, although maybe not quite as aggressively as actual regen.

 

[F14Scott]

The SE must have a way to disengage regen and substitute the friction brakes (using traction control?), because when I coast to my first stop sign with 99-100% SoC still showing the car automatically brakes like normal when I take my foot off the accelerator, although maybe not quite as aggressively as actual regen.

Yes. It's that disengagement that I don't understand. Either it doesn't make excess juice, or it vents the juice to some kind of sink. I don't think it's the latter.

 

[teslarati97]

Yes. It's that disengagement that I don't understand. Either it doesn't make excess juice, or it vents the juice to some kind of sink. I don't think it's the latter.

FWD = front regenerative brakes + rear brakes activated at 95%+ SoC.

If it was AWD you would need to have some disengagement clutch like the Koreans do with their E-GMP platform.

 

[Carsten Haase]

Yes. It's that disengagement that I don't understand. Either it doesn't make excess juice, or it vents the juice to some kind of sink. I don't think it's the latter.

I'm not 100% on all the details but I believe you just apply a small amount of power to the motor to "coast", at least for permanent magnet motors.

Normally, regen creates voltage/current due to the spinning magnet field but if you apply the same voltage from the battery, no current flows and no power is generated/used* because the voltages are matched.

*The inverter is not 100% efficient so some power is used to match the voltage.

For those who don't understand voltage/current, think of it like water: if you have a bucket of water with an open drain (an EV going down a hill), the water will flow out of the bucket (water flow = current). If you attached another bucket that has the same water level (voltage), no water will flow (aka no current, and no regen power)

This is why permanent magnet AWD cars are less efficient, the second motor will always take some current even when it's just "coasting" along.

None of this applies to induction motors since they have no magnetic field when no power is applied.

 

[AndysComputer]

Tow-charging is the same as going downhill, right? Except using a tow vehicle instead of gravity.


Sent from my iPhone using Inside EVs

At first glance, yes, but…
Heat is generated in the motor and the inverter (AC from motor back to DC in battery surely?) and the cooling system isn’t active while towing. Surely neither is the inverter for that matter, but I guess it must be if it works.
With regards to a recent post on this thread about balancing the power from the motor with an equal amount from the battery to cancel it out, ok, but that requires electronics to monitor and achieve so again; while towing that isn’t happening.
Unless the car is towed while powered on?

 

[teslarati97]

None of this applies to induction motors since they have no magnetic field when no power is applied.

One thing to add is that the Gen5 BMW motors have changed to a brushed electrically excited synchronous 3-phase AC motor. The BMW i3/i8 and the MINI SE uses the Gen4 motor (with magnets). This whole regen thing at 95%+ SoC won't be a problem for Neue Klasse platform.