# Regen Braking. Is it sufficient for 4x4 downhill assist?



## major (Apr 4, 2008)

EV Peasy said:


> We’re thinking of converting a Land Rover Defender but am concerned that the regen braking may not be sufficient to replicate the compression braking from the original ICE. If this is to be a serious off road vehicle then this is a fundamental requirement. I’ll retain the original drivetrain.
> Are there any other ways to achieve this? What have everyone else done.
> Cheers
> Ed


Hi Ed,

Regenerative braking will be able to out perform engine compression braking. I see no problem with that.

Regards,

major


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## madderscience (Jun 28, 2008)

... especially if you keep a transmission.

Regen on my xB is set for a maximum of battery 100 amps (this is at 320v) which means about the equivalent of 40hp of braking, though I only see that much when the batteries are warm and at a low SOC, otherwise max voltage limits the braking to about 50A or 20hp.

This is pretty modest by regen standards (especially compared to a lot of the current generation OEM EV's) but still enough that much of the time on freeway off ramps and coming up to traffic lights I can do the single-pedal driving and get down to 10-15mph before I touch the friction brakes. Descending a hill, I can hold 25-30mph on a grade up to 10-12% before I need to start tapping the regular brakes.

This is all with a direct drive with 4.8:1 ratio. If you retain a gearbox you can get much stronger braking if the batteries you choose can absorb the charging current. (any OEM grade EV battery pack would be way better than the ~2C LiFePO4 pack I have)


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## EV Peasy (Nov 5, 2017)

That’s great news. We’re only interested in this project if it enhances the vehicle through conversion. 
Ed


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## Kevin Sharpe (Jul 4, 2011)

madderscience said:


> I only see that much when the batteries are warm and at a low SOC, otherwise max voltage limits the braking to about 50A or 20hp.


This is an important point... regenerative braking needs somewhere to put the energy it's recovered and if your battery is full (or at low temperature) you'll have nowhere to put the energy and therefore no braking 

I see this on my Tesla Roadster when I do a 'range' charge (i.e. charge battery until it's 'full')... for the first 50 miles or so I only have limited regenerative braking and on several occasions I've almost run into another vehicle because I'm so used to driving one pedal with a low SOC ('standard' charge mode stops at 80% SOC).

Basically, if you need regenerative braking from the get go you'll need to charge to a lower SOC and possibly use a larger battery.


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## RIPPERTON (Jan 26, 2010)

The biggest advantage of regen in your case (4x4) is you wont lock the wheels.
Regen by nature cant lock the wheel like a friction brake unless you get one wheel in the air then it might turn backwards through the diff and you will lose all rgen braking on that axle, theres the disadvantage !!!.
If you get a wheel in the air with friction brakes, the wheel still on the ground will continue braking so you will find you will be using both regen and friction in the rocks.


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## goingbush (Jun 20, 2017)

Im building a LandRover EV at present , I have installed a 5k pot on the dash so that I can adjust the level of regen braking required, In low range it will be certainly able to lock the wheels on a slippery slope , so can back off regen to regain traction level required . in high range or higher low range gears will be a useful hill decent control.


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## EV Peasy (Nov 5, 2017)

Kevin Sharpe said:


> This is an important point... regenerative braking needs somewhere to put the energy it's recovered and if your battery is full (or at low temperature) you'll have nowhere to put the energy and therefore no braking
> 
> I see this on my Tesla Roadster when I do a 'range' charge (i.e. charge battery until it's 'full')... for the first 50 miles or so I only have limited regenerative braking and on several occasions I've almost run into another vehicle because I'm so used to driving one pedal with a low SOC ('standard' charge mode stops at 80% SOC).
> 
> Basically, if you need regenerative braking from the get go you'll need to charge to a lower SOC and possibly use a larger battery.


So without reducing SOC is there any ‘parallel’ system that could be incorporated and left in a low SOC to be brought ‘online’ when temporarily needed when the battery bank is fully charged? Just trying to have my cake AND eat it.


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## EV Peasy (Nov 5, 2017)

goingbush said:


> Im building a LandRover EV at present , I have installed a 5k pot on the dash so that I can adjust the level of regen braking required, In low range it will be certainly able to lock the wheels on a slippery slope , so can back off regen to regain traction level required . in high range or higher low range gears will be a useful hill decent control.


Hi GB. I’m glad that I’m not the only one seeing the advantages of an EV 4x4. I know that Land Rover have already looked at this back in 2013 but things have moved on since then. Have you got a build thread or Facebook page going for the project?
Ed


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## goingbush (Jun 20, 2017)

Hey Ed, There is a build thread right here, 
http://www.diyelectriccar.com/forums/showthread.php/landrover-lightweight-ev-conversion-189274.html

once ive goit it finished I'll have time to document the build and put it on my website http://goingbush.com/ptev.html , yep I was going to convert the PT Cruiser but I'm glad that fell through, the LandRover will be so much better as an EV. 

cheers Don


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## Kevin Sharpe (Jul 4, 2011)

EV Peasy said:


> So without reducing SOC is there any ‘parallel’ system that could be incorporated and left in a low SOC to be brought ‘online’ when temporarily needed when the battery bank is fully charged?


You need to provide the motor with some work to do during regen if you wish to slow down the vehicle. If you're not charging the battery then providing a resistive load could work but I suspect it needs to be very large to absorb the energy generated by a heavy vehicle going down hill (iirc the Roadster generates 40kW and Model S 60kW during peak regen).

Do you have any idea what motor you'll be using and how much it can produce when used as a generator?


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## brian_ (Feb 7, 2017)

EV Peasy said:


> So without reducing SOC is there any ‘parallel’ system that could be incorporated and left in a low SOC to be brought ‘online’ when temporarily needed when the battery bank is fully charged? Just trying to have my cake AND eat it.


Yes: a really big resistor - always at zero SOC, never charges up. Seriously; they're a routine feature of diesel-electric locomotives, which have no batteries. Of course you would only use this to dump energy when you can't store it.


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## EV Peasy (Nov 5, 2017)

brian_ said:


> Yes: a really big resistor - always at zero SOC, never charges up. Seriously; they're a routine feature of diesel-electric locomotives, which have no batteries. Of course you would only use this to dump energy when you can't store it.


Thanks Brian. I was thinking of some way to temporarily recover the additional energy to be used for other systems. Could a large capacitor be used instead, maybe discharging through a heater element when needed?
Ed


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## major (Apr 4, 2008)

As far as overcharging the battery from regenerative braking, all it takes is some advance planning and common sense, not additional hardware. After all, the OP question was with respect to engine compression braking not a freight train on a 8 mile down grade. Yes, braking resistors work, very well. But are in fact redundant because the vehicle is outfitted with friction brakes which also convert kinetic energy to heat. 

A capacitor to store recovered kinetic energy (or potential energy due to elevation change) is also redundant. Modern batteries are fully capable.

major


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## EV Peasy (Nov 5, 2017)

major said:


> As far as overcharging the battery from regenerative braking, all it takes is some advance planning and common sense, not additional hardware. After all, the OP question was with respect to engine compression braking not a freight train on a 8 mile down grade. Yes, braking resistors work, very well. But are in fact redundant because the vehicle is outfitted with friction brakes which also convert kinetic energy to heat.
> 
> A capacitor to store recovered kinetic energy (or potential energy due to elevation change) is also redundant. Modern batteries are fully capable.
> 
> major


Thanks Major. Just overthinking things a little as it relates to off roaders. 
I’ll stick to the KISS principle.


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## Kevin Sharpe (Jul 4, 2011)

major said:


> Modern batteries are fully capable.


Correct but the OP asked for alternatives which is why we suggested some 



EV Peasy said:


> is there any ‘parallel’ system that could be incorporated and left in a low SOC to be brought ‘online’ when temporarily needed when the battery bank is fully charged?


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## brian_ (Feb 7, 2017)

major said:


> ... the OP question was with respect to engine compression braking not a freight train on a 8 mile down grade. Yes, braking resistors work, very well. But are in fact redundant because the vehicle is outfitted with friction brakes which also convert kinetic energy to heat.


Yes, the scale is very different, but the scenario is very similar: continuing regeneration without changing speed, in a vehicle which does not have energy storage available.

Trains (and trucks, and every vehicle using an electrical, electromagnetic, or hydrodynamic retarder) have friction brakes, too. People familiar with the intended sort of off-road use typically understand the issues with using friction brakes to control speed of descent.


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## brian_ (Feb 7, 2017)

EV Peasy said:


> I was thinking of some way to temporarily recover the additional energy to be used for other systems. Could a large capacitor be used instead, maybe discharging through a heater element when needed?


Capacitors work, and are often considered (but rarely actually implemented) for energy storage in electric and hybrid vehicles for situations where high power (but not large energy storage capacity) is desired. Since this isn't a high-power situation (compared to what the battery can already handle), and there are substantial control complications to using capacitors, this doesn't seem like a good fit.



EV Peasy said:


> Just overthinking things a little as it relates to off roaders.
> I’ll stick to the KISS principle.


I don't think that considering all of the conditions and possibilities, then making an informed selection between them (including the simplest solution) is "overthinking" anything.  It certainly beats *not* thinking about possible issues.


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## brian_ (Feb 7, 2017)

RIPPERTON said:


> Regen by nature cant lock the wheel like a friction brake unless you get one wheel in the air then it might turn backwards through the diff and you will lose all rgen braking on that axle, theres the disadvantage !!!.
> If you get a wheel in the air with friction brakes, the wheel still on the ground will continue braking so you will find you will be using both regen and friction in the rocks.


All true, and true of any form of engine braking - braking torque distribution between the wheels is determined by the drivetrain (so the conditions at one wheel affect the others if there are open differentials), rather than by the hydraulic pressure to the brake calipers (or slave cylinders).


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## GoElectric (Nov 15, 2015)

I don't understand where your original brakes have gone - that could be your parallel system if the batteries are fully charged and you cannot used regen (?)


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## p.devleugel (Jan 2, 2018)

Im not sure what you need to know, but i am interested in building electric (hybrid) 4x4's

Sent from my HTC One using Tapatalk


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## EV Peasy (Nov 5, 2017)

GoElectric said:


> I don't understand where your original brakes have gone - that could be your parallel system if the batteries are fully charged and you cannot used regen (?)


The whole idea here though is not to touch the brakes as it can upset the descent. The idea is to keep things smooth. I’m looking to achieve regen braking even on a full SOC, albeit temporarily, until the SOC reduces to a level that will allow regen. These are issues specific to this conversion. Is also like to be able to control the level of regen dynamically whilst descending. 

For those not familiar with standard practice here’s a guide to off road driving techniques http://www.drivingfast.net/descending-hills/

Plan so far is to use a parallel HPVES AC50 or AC51 with Curtis at 144V.


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## GoElectric (Nov 15, 2015)

Okay, go for it. Unless the charger is on top of the hill, you won't have a problem with a bit of regen going back down!


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## Kevin Sharpe (Jul 4, 2011)

EV Peasy said:


> These are issues specific to this conversion. Is also like to be able to control the level of regen dynamically whilst descending.
> 
> For those not familiar with standard practice here’s a guide to off road driving techniques http://www.drivingfast.net/descending-hills/
> 
> Plan so far is to use a parallel HPVES AC50 or AC51 with Curtis at 144V.


You'll need to check whether the chosen motor can generate enough power to give you a controlled decent. On a steep hill the Tesla Roadster will run away when generating 40kW (it weighs ~2727lbs / 1237kg).


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## brian_ (Feb 7, 2017)

Kevin Sharpe said:


> You'll need to check whether the chosen motor can generate enough power to give you a controlled decent. On a steep hill the Tesla Roadster will run away when generating 40kW (it weighs ~2727lbs / 1237kg).


A good point, but what is the speed for this Tesla scenario? For an example, a 1237 kg vehicle on a 12% grade rolling with zero drag and just balanced by 40 kW of regeneration would be moving at 98 km/h (60 mph). 12% is the steepest grade that I have encountered on a highway. With substantial drag the actual situation must be much steeper or (unlikely) much faster.

The off-road scenario would be at low speed, so power requirement will be moderate in comparison; the question will be if enough torque will be available, and there will be lots of multiplication in gearing to help. A Tesla only has one gear ratio (other than some early Roadsters which had two but failed to shift), so it's always in top gear.


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## Kevin Sharpe (Jul 4, 2011)

brian_ said:


> A good point, but what is the speed for this Tesla scenario?


If the hill is steep then the car will gain speed even when it says it's generating 40kW... given this is usually on mountain roads I've never waited to find out what the terminal velocity is


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## GoElectric (Nov 15, 2015)

I thought about how much reverse-torque the motor can provide, and although I'm not sure, it makes sense that it can adjust phase voltages to provide as much torque in reverse as it can forward? Motor-wise that makes sense, I'm not sure about the controller.

I know I've been able to get up to 60kW regen out of my little car with an 84kW motor, so I'm kind-of thinking "Yes." 

Of-course you will have to set-up adjustable regen.

I hope that helps.

Jim


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## brian_ (Feb 7, 2017)

GoElectric said:


> I thought about how much reverse-torque the motor can provide, and although I'm not sure, it makes sense that it can adjust phase voltages to provide as much torque in reverse as it can forward? Motor-wise that makes sense, I'm not sure about the controller.


Yes, that looks right, as long as the shaft speed is fast enough... faster than the slip speed for peak torque with an induction motor.



GoElectric said:


> I know I've been able to get up to 60kW regen out of my little car with an 84kW motor, so I'm kind-of thinking "Yes."


I agree, and I'll note that the limit on regeneration in a production EV may be set to limit the charging power to the battery, or may be the result of battery voltage (too high for optimal power from the motor). It seems unlikely to me that these factors will be important in the off-road descent control scenario.


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## brian_ (Feb 7, 2017)

Kevin Sharpe said:


> If the hill is steep then the car will gain speed even when it says it's generating 40kW... given this is usually on mountain roads I've never waited to find out what the terminal velocity is


But this is a high speed scenario, right? If you start from a standstill on the same slope, does it accelerate indefinitely, and what regen power is it running? 40 kW is not quite a sufficient rate of energy dissipation for the Roadster at highway speed, but would be far more than enough at one-tenth of that speed.

My point was just that regen power is directly proportional to speed and retarding torque, so at low speed a power limit is less of an issue.

For instance, 40 kW would be enough regen power to control the descent of a two-tonne vehicle vertically (infinite grade) at 2 m/s (7 km/h), or down a 100% grade (45 degree slope) at 3 m/s (11 km/h)... even with no rolling drag.


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## Kevin Sharpe (Jul 4, 2011)

brian_ said:


> If you start from a standstill on the same slope, does it accelerate indefinitely, and what regen power is it running?


On a 'steep' slope the Roadster will accelerate 'indefinitely' with regenerative braking peaking at 40kW.


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## piotrsko (Dec 9, 2007)

Actually somewhere past 180 mph. Aerodynamic drag tends to slow you down pretty good


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## brian_ (Feb 7, 2017)

piotrsko said:


> Actually somewhere past 180 mph. Aerodynamic drag tends to slow you down pretty good



The free-fall grade: vertical descent, no ground contact!

I don't think the plan is for the Land Rover to jump off a cliff, so it won't need to add propellers for regenerative braking.


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