Regenerative Charging while Flat Towing

[BigB]

I really want an electric vehicle that I can flat tow behind my motorhome while I'm traveling that will regenerate and charge the battery and disconnect when full via clutch like traction control 😉

Here is the updated engineering proposal. I have added a section on "Serviceability & Lifecycle Alignment" to address your point about syncing the clutch maintenance with the brake pads.
Since the Slate truck relies on brakes for its differential action, the pads are already a high-wear, critical component. Tying the clutch lifecycle to them makes perfect logical sense.
Engineering Proposal: Active Drivetrain Decoupling (ADD) for Permanent Magnet PMDU
Subject: Solving the "Full-Battery Regeneration" Risk during Flat Towing via Integration with TCS Architecture.
The Problem:
Permanent Magnet Synchronous Motors (PMSM) generate Back-EMF when rotated externally (towing). If the HV battery is at 100% SOC, there is no sink for this energy, leading to thermal runaway or inverter failure. Currently, this prevents safe flat-towing without manual disassembly.
The Solution: Electromechanical Dog Clutch Integration
Implement a normally-open electromagnetic dog clutch on the motor output shaft, specifically integrated into the existing Traction Control / Open Differential architecture.
1. Mechanical Implementation: The "Jaw" Clutch
Instead of a friction clutch (which slips and generates heat), we utilize a Dog Clutch (Jaw Clutch) design.
* Placement: Located between the PM Motor output and the reduction gear/differential input.
* Actuation: A high-torque 12V solenoid.
* Engagement: The motor controller RPM-matches the dog teeth for seamless engagement (zero wear).
* Safety: The system is Normally Open (Spring-Retracted). If the vehicle is completely dead or the low-voltage system fails, the spring forces the clutch to disengage. This guarantees the truck is safe to tow even with a dead 12V battery.
2. Integration with Brake-Based Torque Vectoring
We utilize the vehicle's existing ABS/Traction Control Module to manage the clutch state, treating the "Disconnect" as a logical extension of the Brake-Lock Differential system.
* "Tow Mode" Logic: The TCU cuts current to the solenoid. The spring retracts the clutch. The wheels turn the open differential, but the connection to the motor is physically severed. Zero Back-EMF.
* "Drive Mode" Logic: Upon startup, the TCU energizes the solenoid, locking the motor to the differential. From here, the Slate’s standard Open Differential strategy takes over (using brakes to vector torque across the spider gears).
3. Maintenance Strategy: The "Brake-Synced" Service Interval
Since the Slate architecture relies on "Brake-Actuated Limited Slip" functionality, brake pads are a primary consumable and a critical drivetrain component. We propose aligning the clutch actuator lifecycle with the brake pad service interval.
* Co-Terminus Maintenance: The clutch solenoid and return spring should be designed with a Mean Time Between Failure (MTBF) that matches the severe-duty cycle of the brake pads.
* Service Workflow: When the vehicle alerts the user that brake pads are due for replacement (due to torque vectoring wear), the clutch actuator mechanism is inspected or replaced as a modular unit.
* Benefit: This prevents a separate "clutch service" appointment. By treating the clutch actuator as a consumable paired with the brakes, we reduce perceived maintenance complexity and ensure the safety-critical "spring retract" function is regularly refreshed.
Summary for the Engineers:
"We solve the regen-fire issue by placing a solenoid-actuated dog clutch on the motor output. By wiring this into the existing ABS logic, we ensure the motor is isolated during towing. Crucially, we treat the clutch actuator as a maintenance item paired with the brake pads. Since the truck eats brake pads to manage traction, we simply swap the clutch module during that standard service interval, ensuring reliability without adding new logistical headaches."
How does that look? If you are ready, you can copy and paste that directly to their team.

 

[Paul]

Few things come to mind: You're actually going to be dragging quite a bit and using fuel in the motorhome to generate the battery, Wear and tear and tear on the EV tires. And the biggest one is that you are not in the vehicle If something starts going wrong and then you're just towing a ball of fire. It'd be interesting to figure out how much fuel it would cost you versus just using the mains, Which is actually pretty cheap.

 

[Thutchis]

My Hyundai Ioniq 5 SEL has a mechanical decoupler in its front motor. Munro Associates did a tear-down of the motor and explains how it works in the YouTube video below. They estimated the cost to add the decoupler was about $100.

 

[Thutchis]

Few things come to mind: You're actually going to be dragging quite a bit and using fuel in the motorhome to generate the battery, Wear and tear and tear on the EV tires. And the biggest one is that you are not in the vehicle If something starts going wrong and then you're just towing a ball of fire. It'd be interesting to figure out how much fuel it would cost you versus just using the mains, Which is actually pretty cheap.

I agree, personally I don't think it would be worth the engineering cost and adds unnecessary complexity and points of failure. I just want to flat tow a slate behind my MH and have some way of braking assistance for the towed.

 

[KevinRS]

My Hyundai Ioniq 5 SEL has a mechanical decoupler in its front motor. Munro Associates did a tear-down of the motor and explains how it works in the YouTube video below. They estimated the cost to add the decoupler was about $100.

The issue there is can slate get something like that with that cost difference, as a stock item, without designing their own motor. A big part of how slate is keeping costs down is by using stock off the shelf parts.

 

[Paul]

I agree, personally I don't think it would be worth the engineering cost and adds unnecessary complexity and points of failure. I just want to flat tow a slate behind my MH and have some way of braking assistance for the towed.

What would probably make more sense if you reeeealy wanted to do this was to come up with some way the flat tow vehicle charges the flat towed EV either by trickle charging it constantly Or maybe only load the alternator while braking Or decelerating or come up with regen braking on the tow vehicle. That would eliminate any redesign to the Slate. Tow vehicle is just a source of charging you plug into the slate. Something like Fords onboard power but with a regenerative braking mode for flat towed EVs. This reminds me of about 40 years ago. I had a 52 horsepower VW Diesel with air conditioning. I put it inline switch on the compressor clutch where it was handy down by the e-brake. The engine had so little horsepower but it would drag the AC compressor very noticeably on any acceleration On hilly terrain I would just flip off the compressor uphill and then engage running down long downhills. There really was enough left in the coil that it would continue cooling on the uphill until the cycle was repeated. You could actually feel the compressor engine braking the vehicle on the downhill. So in a sense I was regenerative breaking with a freon pump to reclaim energy as cooling rather than engine braking using a lower gear or mechanical braking. They're already was a switch on the bottom of the accelerator peddle to do this, but it was just for almost full throttle.