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Here is a silly technical question that I was not able to figure out for myself, nor did I find much enlightenment by googling around a bit. I found no indication at all that the on-board charger of an EV is being used during regenerative braking, but I cannot see how it can be avoided.

I am making a few assumptions about which I am not absolutely sure, so please correct me if I am mistaken here. Most importantly, I think most EVs use AC motors (one of various types in existence), which is why they need an inverter to operate. Using such a motor as a regenerative braking system would produce AC power, wouldn't it? Now charging batteries from an AC power source requires a charger, which is essentially a rectifier, to convert to the DC current needed by the batteries. It seems like the conclusion is that the charger needs to be used during regenerative braking.

But if that were true, it would often place a severe limit on the efficiency of regen braking. Often the power which the charger can handle is much more limited than the power the batteries can absorb while charging; for instance my Leaf has a 6.6kW charger but its 24kWh batteries can handle upward of 40kW of power during DC charging. Other cars will have different figures, but the general picture I think is more or less the same (with the exception of cars like Zoe which can (currently) only accept AC charging). Now a back of envelope calculation for the amount of power that could be produced by regen braking while descending a steep hill gives me something near 20kW, in other words much more than my charger can handle, though well within the power limits of the battery. Rapid deceleration probably even releases more power than that. I know the friction brakes do kick in when braking hard, but using less than a third of the absorbed power for regeneration seems like a large waste.

So what is it? Maybe there is a clever way to use the inverter (which obviously can handle much more power than the charger can) in reverse in such a way that it actually makes the motor generate DC power that can go directly to the battery. But if this is so, then why can the inverter not be used as a charger too, making a separate charger unnecessary? Is there something about the AC current from the motor that is fundamentally different from the AC power from the grid?
 

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Hi,

Its not so much a difference in the supply, its all about what electrical noise is introduced back to the source. I'm sure the Leaf can regen at greater than 30KW, but in doing so, a lot of noise is generated, which is absorbed in the motor, no problem.

Connected to a mains supply, that noise would cause untold misery.

In the Zoe the mains power is first fed through the motor windings, which wrap around a lot of iron, this dampens noise, for most not a problem, though some TV's and microwaves can play a tune to it!
 

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Discussion Starter #3
Its not so much a difference in the supply, its all about what electrical noise is introduced back to the source. I'm sure the Leaf can regen at greater than 30KW, but in doing so, a lot of noise is generated, which is absorbed in the motor, no problem.

Connected to a mains supply, that noise would cause untold misery.
That is quite interesting. So electrical noise avoidance is really the only reason one needs a charger at all! I never realised that.
 

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I think "regenerative braking" is a term dreamed up by the marketing department.

If you put my e-nv200 into brake mode going down a steep hill you can increase the battery SOC by quite a few percent. It now seems strange to drive an ICE car and just loose that energy.

If I apply the footbrake going down a hill, and then select neutral, the change in the retardation of the van seems very small when neutral is selected. Perhaps the braking system has a way of applying the friction brakes harder when I do this, but somehow I doubt that level of sophistication has been included. I am not convinced that a lot of energy is recovered when I just use the footbrake.

I think the term "regenerative braking" is possibly miss leading, making you think that most of the energy needed to stop a car is returned to the battery.

Not sure what a better phrase would be tho.

If my van uses 20kwh of energy in an hour (which it would on the motorway easily) then the average power fed to the motor is ....... err 20kw.

This does make regeneration at 30kw seem very likely, but if I am achieving this then 30kw of stopping power doesn't seem to slow you down very quickly.

Am I missing something here?

Perhaps I should take some tools out of my van?
 

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I'm my 30kWh Nissan Leaf the regen braking (particularly when in B mode) is noticeably less when the car has 98-100% charge.
 

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It is quite clever. Whether it can Regen the full 30 kW on the Leaf by just using the brakes when in Neutral I don't know, but don't see why not.

Here is an article about the Leaf EDIB: Electric Driven Intelligent Brake system.

Electric Driven Intelligent Brake

Sent from my iPhone using Tapatalk
 

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I'm my 30kWh Nissan Leaf the regen braking (particularly when in B mode) is noticeably less when the car has 98-100% charge.
At above 95 percent charge the battery can accept only a trickle charge, less than 2KW, and reducing as it fills up. Its another good reason to not charge to 100%, otherwise mechanical brakes actually get used, and wear out.
 

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Hi @Marc van Leeuwen - I think you're getting confused here. The on board charger is completely separate from the drive system and only charges the HV battery up from an external AC supply. When the powertrain is in regenerative mode the drive motor inverter operates in reverse and converts the rotary motion to DC current which is stored in the battery - this is totally independent of the on board charger, in fact you could disconnect it.

The way the regenerative energy is handled differs from EV to EV. In the Leaf I believe a blended braking system is employed so in the main the regen level is controlled mostly by the brake pedal position and as the regenerative energy capability drops off (eg: as the HV battery fills up or gets too hot) the friction brakes are fed in at an ever higher level to compensate. There is a low level of regen applied when the accelerator pedal is released but the main regen comes in when the brake pedal is depressed. This gives you a very consistent braking feel whatever the state of the HV battery.

The other approach is to control the regen braking purely using the accelerator pedal position. As you lift your foot the powertrain goes from applying positive traction power to negative (regen) and reaches full regen when the accelerator pedal is released. This is the way that the i3 and the Tesla (unless overriden int he menus) operates. This means you get full regen without any intereference from the friction brakes and allows one pedal driving but has the disadvantage that as the battery fills or heats up the regen power drops off so can feel a bit inconsistent. However the effect is similar to the car being on a varying slope so in practise I think it works well and maximises regen.

Hope that helps.
 

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The mains charger inside my Ampera has just broken, but the regen still works perfectly. It's simple to transform 250V AC up to 400V, rectify it and charge the 400V battery. But I would love to know just how the regen amount is made proportional to throttle position; when regenning the reverse voltage generated by the coils must be less than 400V usually, so maybe there's a charge-pump circuit to boost this to battery voltage levels? Hmm...
 

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@Marc van Leeuwen - your question is actually quite a sensible one, but alas this is not how modern EVs are constructed.

It would not be impossible to configure the traction inverter to double up as a three phase charger, capable of delivering charge current at the same rate as its maximum regen during driving. This is some serious charge current!!!

However, this is not the currently accepted 'topology' of EVs. In all EVs to date, except for a few exceptions, the charger is a totally separate module from the traction electronics.

The reasons are down to electromagnetic compatibility issues, and manufacturability. The traction electronics and power system are not designed to create nice smooth back EMF to the motor during the traction inverter operation, instead whatever noise is generated in the switching of the inverter bridges flows around the high voltage power lines and the motor isn't going to suffer from this.

In other words, during regen, the electrical 'noise' flowing back to the motor is considerable which is immaterial while it is a closed electrical system on the car, but would be unacceptable noise back into the mains supply.

The other side of this is the manufacturability side of it, in which it is seen as easier to have two separate modules because if you try to do both functions with one set of electronics, you probably get little advantage in cost, but you would get two compromised systems rather than highest performance out of each. You'd have to encumber it with lots of EMC mitigation, big capacitors and inductors, to keep the noise back into the mains to a minimum.
 

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Discussion Starter #11
OK, thank you all very much for enlightening me. I'll recap the pertinent answers: yes, regen braking generates AC power, but no the on-board charger is not used to rectify it, since yes the inverter is capable of converting the generated AC (back) to DC for storage in the battery. The maximal power that can be recovered during regen braking is therefore limited only by what the batteries can handle, which is typically the power used during DC fast charging (and like DC charging, that power reduces significantly when the batteries are near 100%).

And although the inverter which is usually running as a DC to AC converter can thus double as an AC to DC converter (which is something I as an electronics ignoramus had not expected was possible), doing so during charging from the mains would be unacceptable, because of the (high frequency?) electric noise this would send back to the grid. But during braking that is a non-issue.

And my conclusion is that rectifying current using power electronics is not very hard in itself, but what makes chargers so expensive (and limited in their power rating) is that it is quite hard to do so while keeping the electric noise produced at the input within reasonable limits.
 

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And my conclusion is that rectifying current using power electronics is not very hard in itself, but what makes chargers so expensive (and limited in their power rating) is that it is quite hard to do so while keeping the electric noise produced at the input within reasonable limits.
Is this really where the expense is?

I can imagine a charger based on a rectifier draws very heavy current at the top of a sine wave and next to nothing at the centre of it, and I can imagine this producing a load of harmonics on the supply. Is suppressing this rubbish where the expense of the system is?

I assumed there was also something else clever going on with the charger that was expensive to do, but not sure what.
 

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Hi guys I no tek
I think "regenerative braking" is a term dreamed up by the marketing department.

If you put my e-nv200 into brake mode going down a steep hill you can increase the battery SOC by quite a few percent. It now seems strange to drive an ICE car and just loose that energy.

If I apply the footbrake going down a hill, and then select neutral, the change in the retardation of the van seems very small when neutral is selected. Perhaps the braking system has a way of applying the friction brakes harder when I do this, but somehow I doubt that level of sophistication has been included. I am not convinced that a lot of energy is recovered when I just use the footbrake.

I think the term "regenerative braking" is possibly miss leading, making you think that most of the energy needed to stop a car is returned to the battery.

Not sure what a better phrase would be tho.

If my van uses 20kwh of energy in an hour (which it would on the motorway easily) then the average power fed to the motor is ....... err 20kw.

This does make regeneration at 30kw seem very likely, but if I am achieving this then 30kw of stopping power doesn't seem to slow you down very quickly.

Am I missing something here?

Perhaps I should take some tools out of my van?
Hi guys am no teck guy but as fas as I know the best regen from any car is 80%
You read it right just 80% that with 4x motors
So one would assume you could actually do the maths but need to do a lot of calculations
But I recon & this is my guess for the leaf best kWh sayvwhen the car says 30 kWh its actually putting in about 50% max of the 30kwh .
So at best the leaf will regent at 15kwh & that's in night braking sorry I know people won't believe that's fine but
Its my poor guess then :)
 

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... because of the (high frequency?) electric noise .....
High and low frequency - you are also meant to create a charger that does not put a huge distortion on the power supply.

Is this really where the expense is?
Not so much 'expense' as 'unnecessary cost' when it could be done in two modules in a 'neater' way. The auto industry likes modularisation because it is 'easier' to diagnose and swap out parts that fail, and if so it makes sense to try to keep less value in any 'one' component and spread the cost around a bit.

You also have to think about co-co-ordinating the supply voltage to the operating range of the motor. This is an additional 'restriction' on optimising the two parts. It is OK if designed in from the get-go but the battery voltage needs to match the supply voltage if you are going to lump the two functions into the one module. Easier just to build them separately.
 

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Just one clarification wrt i3 -- the one pedal driving does mean regen generally occurs on lift-off, at a decent rate -- but where the battery is too full to accept this charge rate the brakes are *automatically* applied -- the rationale both of this system AND the blended brake system is to maintain the same feel for the driver regardless of what's actually happening under the covers

The only time regen is suspended is if you lose traction.. momentarily, this IMO makes sense too.
 

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Hi guys I no tek

Hi guys am no teck guy but as fas as I know the best regen from any car is 80%
You read it right just 80% that with 4x motors
So one would assume you could actually do the maths but need to do a lot of calculations
But I recon & this is my guess for the leaf best kWh sayvwhen the car says 30 kWh its actually putting in about 50% max of the 30kwh .
So at best the leaf will regent at 15kwh & that's in night braking sorry I know people won't believe that's fine but
Its my poor guess then :)
You think the regen is only 50% efficient? I've see a lot of people speaking about >90% apparently confirmed with Leafspy too.
 

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Just one clarification wrt i3 -- the one pedal driving does mean regen generally occurs on lift-off, at a decent rate -- but where the battery is too full to accept this charge rate the brakes are *automatically* applied -- the rationale both of this system AND the blended brake system is to maintain the same feel for the driver regardless of what's actually happening under the covers
For the Nissan Leaf (at least for a 24kWh Gen2 Leaf) if the battery is unable to accept the full regen output, then the amount of regen braking is reduced, and no blended friction brakes are applied. This gives a different feel with a fully charged battery - something I have to be aware of when leaving home with a 100% charged battery (compared with when I leave home with an 80% charged battery) when the foot off the accelerator regen braking is missing on the approach to the first right turn!

For a Gen2 24kWh Leaf, maximum regen is 30kW, which for B-mode Eco occurs 'foot off' at 40mph or higher, and with the brake pedal touched occurs at 20mph or higher.
 

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It was my understanding that the Zoe uses charger circuitry for regenerative braking. I occasionally see an indicated 43kW going back into the battery when slowing down from high speed. The charger circuit is a switched mode power supply, the incoming supply (mains or from the motor) is rectified then chopped up at high frequency and passed through a transformer. the output of the transformer is then rectified and passed onto the batteries. The switching is usually done at a fixed frequency which is the resonant frequency of the transformer circuit as this gives the highest efficiency and the least waste heat. The power level is set by the mark space ratio of the pulses fed into the transformer (pulses that are on for longer allow more current to flow through the transformer primary and give a higher output).With the European supply at 50Hz when charging the coils of the motor help to smooth the power consumption. When regenerative braking the frequency the motor makes will be significantly higher and vary with speed, the voltage that it generates will also vary with speed, the regenerative braking charging circuitry needs to cope with this variation.

Can anyone with a Q240 Zoe confirm the maximum regen power?

As virtually every bit of electronics we use now have rectifiers in them they all slice off the top of the mains, this results in distorted mains but there isn't much we can do about it. This can be seen with an oscilloscope. Even LED and compact fluorescent light bulbs have rectifiers in them. The main exception to this is incandescent bulbs, heaters and motors.
 

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So what is it? Maybe there is a clever way to use the inverter (which obviously can handle much more power than the charger can) in reverse in such a way that it actually makes the motor generate DC power that can go directly to the battery. But if this is so, then why can the inverter not be used as a charger too, making a separate charger unnecessary? Is there something about the AC current from the motor that is fundamentally different from the AC power from the grid?



On the 2nd Generation LEAF, the charger and the inverter are integrated into a single unit but they only share some components.



On Zoe the charger / inverter / electric machine are integrated. Zoe uses all three when charging from AC. Zoe can charge at 43kW from AC.


Tesla and 1st generation LEAF use separate chargers.
 
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