Regen and friction brakes - do/how do they work together?

[Cocijo]

This has been discussed in other threads But as far as I can tell no dedicated post.
My main queries are around the relationship between application of the footbrake and regen.
Does the first part of depressing the pedal always engage regen first and then if further depressed the friction brakes also?
We know that we get regen when we release the accelerator and that this is incremental.
If in regen3 we lift fully off, does then applying the footbrake give even more regen braking? A sort of extra boost that is more than fully lifted off regen3? I don’t really notice this when driving if it does.
I’ve read in a few posts that using the footbrake engages more regen - I think someone has measured this? Is there a direct link between the footbrake and regen or do the two systems work completely independently of each other? I also believe Miles mentioned this on one of the podcasts but can’t find his commnents.
It makes sense to use regen instead of friction brakes where possible but how does this work on the MG when the footbrake is applied?
Regen is definitely speed and momentum related and I wonder if applying the footbrake and reducing the speed gives the impression of additional regen? I’m just surmising here and giving my experiences - I don’t know the answer - i’m interesting to hear what members think.

 

[Jake]

I seem to recall something in the i3 handbook saying that when applying the footbrake it uses regen first and then friction brakes as necessary. I hardly ever used the footbrake so can't remember if that was true in practice. I certainly use the footbrake now in the ZS but haven't done any decent journeys during lockdown to ascertain if any regen is applied first. Looking forward to resuming longer trips soon especially with warmer weather.

 

[JodyS21]

I've tried:
Lifting straight fully off accelerator and feeling the slowing force/watching the -Amps
Lifting straight off accelerator but then pressing the brake pedal slightly, I think this generates more -Amps and slows it down more but I can't "feel" the friction brakes kicking in.

There could well be all sorts of parameters that go into the system deciding how much regen to use - constantly calculating every hundredths of a second etc and when to use friction brakes.
F1 for instance with KERs if a highly complex and adjustable calculation that makes a lot of difference.
I'd have thought quite a bit goes into setting the system up on the car.

 

[Cocijo]

I've tried:
Lifting straight fully off accelerator and feeling the slowing force/watching the -Amps
Lifting straight off accelerator but then pressing the brake pedal slightly, I think this generates more -Amps and slows it down more but I can't "feel" the friction brakes kicking in.

There could well be all sorts of parameters that go into the system deciding how much regen to use - constantly calculating every hundredths of a second etc and when to use friction brakes.
F1 for instance with KERs if a highly complex and adjustable calculation that makes a lot of difference.
I'd have thought quite a bit goes into setting the system up on the car.

Good points as always.

 

[Cocijo]

I've tried:
Lifting straight fully off accelerator and feeling the slowing force/watching the -Amps
Lifting straight off accelerator but then pressing the brake pedal slightly, I think this generates more -Amps and slows it down more but I can't "feel" the friction brakes kicking in.

There could well be all sorts of parameters that go into the system deciding how much regen to use - constantly calculating every hundredths of a second etc and when to use friction brakes.
F1 for instance with KERs if a highly complex and adjustable calculation that makes a lot of difference.
I'd have thought quite a bit goes into setting the system up on the car.

Do you think the brake pedal will have some form of potentiometer to activate regen? It seems too complex to be generated from say the brake light switch.

 

[JodyS21]

Do you think the brake pedal will have some form of potentiometer to activate regen? It seems too complex to be generated from say the brake light switch.

Yes it must have.
On my 10 year old BMW when I had laptop connected up to it ages ago, I think it shows what percentage the pedals are pressed. I'd be shocked that an EV car nowadays doesn't measure it & use it.

 

[Steptoe]

I live down a very short cul-de-sac so need to brake for the junction by the time the car is doing about 5mph. The regen never comes in at this point (but does for the next junction after getting up to 20mph) and I've always assumed this is down to the car prompting me to give the footbrake an active running test as well as the passive one on start-up.

However this also helps in cleaning up the discs after parking up overnight......

 

[Cocijo]

Yes it must have.
On my 10 year old BMW when I had laptop connected up to it ages ago, I think it shows what percentage the pedals are pressed. I'd be shocked that an EV car nowadays doesn't measure it & use it.

I’m thinking this could be linked to acc to produce regen on acc... or at least the system is there ready.

 

[Deleted member 2439]

My view is the complete opposite.

Taking your foot off the throttle will invoke regen - in the same way as engine braking in an ICE car.

The brakes then operate mechanically as normal, with the possibility that activation of the brake pedal may cause regen to move from whatever it’s set to on KERS to KERS3 to give maximum braking.

The actual braking system is exactly the same as on an ICE car - right down to being vacuum assisted, despite that vacuum having to be made by a pump - which is what you can feel “hardening” the brake pedal when you press the start button.

Essentially the braking system is totally separate - as it must be for safety reasons when you think about it. Apparently the Audi E-tron has some whizzo clever system that differs, but that’s just VW being a bit too clever, again, IMHO - of course.

Rob

 

[Cocijo]

I did feel that the friction brakes were separate to the regen but others differ. I’m not sure.

 

[JodyS21]

I did feel that the friction brakes were separate to the regen but others differ. I’m not sure.

Yeah I’m pretty sure it’s not brake by wire, not sure that’s allowed (yet) is it?
It’s a question of how much KERS does it try to generate at different positions of the accelerator and brake pedals (and other conditions like speed)

 

[Deleted member 2439]

Yeah I’m pretty sure it’s not brake by wire, not sure that’s allowed (yet) is it?
It’s a question of how much KERS does it try to generate at different positions of the accelerator and brake pedals (and other conditions like speed)

The Audi system has an over-complex system which has brake-by-wire AND conventional brakes in case of failure. I’d wager that our brakes are exactly the same as on the ICE ZS. Why over complicate it? Cost effective & cheap

 

[JodyS21]

The Audi system has an over-complex system which has brake-by-wire AND conventional brakes in case of failure. I’d wager that our brakes are exactly the same as on the ICE ZS. Why over complicate it? Cost effective & cheap

Agreed. It is an MG not an Audi!!!

 

[SCB86]

It comes down to how much retardation you as the driver are demanding.
Any EV will first use the motor in reverse to replenish the battery when you press the brake, however once the level of retardation from regen alone isn't sufficient to match the braking force the driver is requesting, then the friction brakes are then activated.

 

[Deleted member 2439]

It comes down to how much retardation you as the driver are demanding.
Any EV will first use the motor in reverse to replenish the battery when you press the brake, however once the level of retardation from regen alone isn't sufficient to match the braking force the driver is requesting, then the friction brakes are then activated.

Incorrect. Braking via pressing the pedal is a simple mechanical process which is then supported by some regen effort. Just think what would happen in an emergency stop.

The brake system is the same as in an ICE car - regen is an overlay to that, the two systems essentially operate independently from each other.

 

[JodyS21]

Incorrect. Braking via pressing the pedal is a simple mechanical process which is then supported by some regen effort. Just think what would happen in an emergency stop.

The brake system is the same as in an ICE car - regen is an overlay to that, the two systems essentially operate independently from each other.

Very true.

Realistically, the brake pedal will not cause the friction brake pads to engage until it's been pressed more than lets say 20%, but it probably causes the regen process to kick in to a higher degree when it's just been pressed a little bit lets say less than 20%.
So from a drivers perspective the brake pedal is doing regen first before the friction brakes, in reality it's doing both in tandom to different degrees.

 

[Deleted member 2439]

Very true.

Realistically, the brake pedal will not cause the friction brake pads to engage until it's been pressed more than lets say 20%, but it probably causes the regen process to kick in to a higher degree when it's just been pressed a little bit lets say less than 20%.
So from a drivers perspective the brake pedal is doing regen first before the friction brakes, in reality it's doing both in tandom to different degrees.

I‘ll go along with that. The initial touch on the pedal operates the switch which then initiates further regen activity. I say further as the act of lifting off the throttle will have started the process.

 

[Kithmo]

If it's like the Prius, which I suspect is very likely, regen does all the normal braking, except for emergency stops or heavy braking (EBA), ABS activation and when the car is rolling slow, like 5 mph. Activation of the friction brakes is controlled by the ABS ECU.

 

[SCB86]

Incorrect. Braking via pressing the pedal is a simple mechanical process which is then supported by some regen effort. Just think what would happen in an emergency stop.

The brake system is the same as in an ICE car - regen is an overlay to that, the two systems essentially operate independently from each other.

I'm not incorrect;

How Regenerative Braking Works

Vehicles equipped with regenerative braking systems are able to recapture some of the vehicle's kinetic energy and convert it into electricity. This electricity is then used to charge the car's batteries. How is this possible?

auto.howstuffworks.com

 

[Tim Green]

I suspect the regen level is increased when the brake pedal is used. However, it will not use the brake light switch, it will use the rate of chang of speed as measured by the speedo system. As this is all in software it is easy to do.