Regen braking questions for the knowledgeable Clarity drivers

What triggers the brake lights? On a traditional vehicle it’s a small switch that closes when the brake pedal moves a certain distance. ACC may apply the friction brakes. It does on our Jeep which does not have regenerative braking.

Does the brake pedal move when we use the paddles on a Clarity? I’ve never looked. Is there another sensor that detects deceleration, which illuminates the brake lights when when a sufficient amount of inertia is present, regardless of the method being used to decelerate?

The Jake brake on our motorhome can provide substantial braking force, yet the brake lights never come on. On other coaches the brake lights do come on when the Jake is used. I believe it is something that can be programmed in the ECU.

Our 2019 Touring has never started the engine with a fully charged battery, while going downhill using paddles or braking. And that occurs almost daily. I’d just like to hear an explanation on how the car is capable of using more friction braking with a fully charged battery, yet still it starts the engine due to excessive regenerative energy.

 

Yes, for sure. I haven't logged enough experiences to understand all the rules that trigger the lights. But, one example from a recent drive:

Driving country roads on a longer recreational outing, I was in HV mode with battery definitely not full. I was navigating to an unfamiliar county airport, going at a pretty good clip. Realizing, a little late, that the left turn was coming up, I quickly tapped to 4 chevrons and was surprised to see brake lights illuminating. Paddles only, for sure. (I then had to brake with the pedal to make the turn, but even before that braking, the brakes lights were definitely illuminated with paddles only.)

In this case, it may be that I was cruising along with the engine generating power for HV, and my rapid application of paddles did not allow the car to turn off the engine so that the system could absorb enough energy from the regeneration system. So instead, I'm guessing, it used friction brakes. Worded another way, from the car's perspective, it was a similar scenario to having the battery full: I had asked for max paddle slowing and it could not do it using only regen because, I'm guessing, current to the battery (coming from the engine/generator) was already too high to allow for more. This is speculation, and the observation of the brake lights is the only thing I can guarantee.

Your question, and then writing out this response, is leading me to question whether the ONLY time the brake lights activate with paddles is when the car has to employ friction braking to meet the paddle request? I cannot explain why the car "rejects" chevrons at times, if it truly has the option using friction brakes. To put speculation upon speculation, maybe the paddle system has only a limited amount of friction braking that it can use, perhaps programmed to ensure smooth, coasting-like performance? And so there may be cases in which even that allowable friction braking supplementation cannot give the deceleration requested by the paddles and the system therefore rejects those higher level chevrons?

 

It is a good question. If I were designing it, I would want a physical trigger that illuminates the brake lights whenever there is pressure at the caliper, whether generated by a human pushing the pedal or a computer-controlled pump. But, I really don't know.

I was just out driving and did some testing, and I feel certain that the brake pedal itself does not move when the brake lights are actuated by the car.

 

I live atop a gentle hill and I coast downhill for half a mile until the first stop sign, which is a the bottom of a steeper hill. With a partial charge, I observe substantial regen on the meter when braking to the stop sign and no brake noise until the last moment. With a full charge, I see much less regen on the meter and I can hear the friction brakes as soon as I apply them on the same stretch. Generally, the engine does not start at this point. After the stop sign, it flattens out some and the speed is faster. If the coming stop light turns red and I have brake, the engine turns on. If I hit the green light so i can continue driving for a bit, the engine stays off because the road starts going uphill and using up some charge. My understanding is that the front wheels cannot completely disengage itself from the electric motors so it will generate electricity no matter what. What the car can do is modulate regen braking so it can rely completely on the friction brakes to minimize how much power is returned to the battery. Thus, if I leave my home on a full battery, I'm overcharging my traction battery until after the first stop light. There seems to be some tolerance for it (like to the first stop sign) but any more (braking to the first stop light) and the engine goes on. Quite frankly, I have no idea how running the engine absorbs this power because the energy flow diagram shows that the running engine is actually charging the battery (and potentially causing damage). It stays on for about 5 min. I really wonder if this a flaw in Honda's software. That's one of the reasons I don't charge to full typically.

There is a long thread on the topic of the engine starting in EV mode. One former poster (Bob Cub's fan) was so unhappy with this phenomenon that he got rid of his Clarity after a short time. As I recall, he lived atop a hill and the engine always started as he descended (on a full charge).

 

I'm thinking about when the engine is started in normal operation. The starter/generator motor receives electricity from the battery, and acting in starter mode it uses that electricity to spin the engine crankshaft (whether directly or through gears I don't remember). As we know the rotating engine then begins to compress the fuel mixture, spark is provided and the engine begins running. At that point the starter/generator motor switches into generator mode, and begins receiving mechanical power from the engine instead of giving it. This causes the motor to generate electricity, which it then sends either to the battery or to the traction motor. Or if the engine goes into direct drive mode I think a clutch disengages the starter/generator motor from the engine, although I'm not sure of this.

But what if the starter/generator remained in starter mode even after the engine started? It would continue to use electricity to help the engine spin, similar to how some of the early mild hybrids worked, as well as Honda's original IMA system that was used in their hybrids for several years before they switched to the current i-MMD system. Although once the engine is running, instead of starter mode they referred to it as assist mode (@insightman do I have this right or am I out in left field about how IMA worked?)

Of course in the case of Clarity it would not be used for this purpose during normal driving, I'm just saying mechanically it could do this. So my thinking is that in the full battery regen situation, the electricity coming from the traction motor is sent to the starter/generator motor, which goes into starter mode to spin the engine, then after engine ignition it continues to provide mechanical power to the engine in assist mode. Of course all of this engine rotation would do nothing in this case other than make heat and noise, similar to a regular gasoline car engine being revved up while in neutral. The engine itself would be providing some of the mechanical power to spin itself, but probably only using a small amount of fuel similar to idling. The rest of the mechanical power to spin the engine comes from the starter/generator motor acting in assist mode, spinning the engine at whatever RPM is needed to dissipate the unwanted electricity.

One possible problem with my theory is that it doesn't take a whole lot of power to spin an engine that isn't connected to anything at that moment, so I don't know if this would be enough to deal with the excess energy that is beyond what the friction brakes are (presumably) taking care of.

But even if my theory is correct, why do they introduce fuel and spark instead of just letting the starter/generator motor continue to spin a cold engine? Oil pressure would be no problem. The only issue I can think of is that it's probably not good to spin a cold engine for too long, and the system has no idea how long the excess regen situation will last, since as far as it knows you just started down from the top of Pikes Peak. By the way that is one theory why it doesn't use friction brakes exclusively in this situation, because the brakes could start overheating if the need for braking goes on for a long time.

So proactively when braking is needed with a full battery, it spins the engine and activates fuel and spark. In some cases immediately depending on the circumstances, although you seem to have observed that it will accept a certain amount of regen into the upper unused battery buffer. But at some point it says no more and begins to spin the engine and starts it up. Unfortunately even if the regen situation ends two seconds later, once the engine is started it has to run through the full warmup cycle in order to get the catalytic converted hot enough to burn off any unburnt fuel (I think I read somewhere that this is an EPA requirement because it shortens the life of the converter when cold fuel is left in it). The same thing happens when you press the pedal to the floor even for just a second, it starts up ICE then keeps it running even though it is no longer needed so that it can get the catalytic converter hot enough.

My theory may be as full of holes as swiss cheese. But if my theory is correct, then the software flaw in my opinion would be that it overreacts too quickly to the situation, instead of considering the possibility that the regen situation may not last very long.

I.e. it should start out using just friction brakes alone, which I'm sure they are capable of. Then after a predetermined time limit (based on a combination of time elapsed and brake pressure) to avoid brake overheating it would start putting electricity into the unused buffer. Then when that limit is reached it would start spinning the cold engine. Then only after some predetermined limit to the amount of RPMs on a cold engine would it start adding fuel and spark so that the engine can begin warming up. In case you really are going down Pikes Peak.

My guess is that for whatever reason they didn't feel confident in setting the required thresholds for all of this, or they couldn't come to an agreement on the thresholds. So they went the safe and easy route of starting up the engine almost immediately.

 

Default regen is one chevron. We all know that isn’t very powerful. The precise amperage that is sent to the battery when we remove our foot from the accelerator may have been posted previously on this forum. I would agree that regenerative braking is reduced when the battery is fully charged. My efforts to engage 4 chevrons, while going downhill with a full battery, frequently results in flashing chevrons after which the displays shows 3. Even with 3 chevrons visible, applying the brake pedal produces more regen. Friction brakes do not produce regenerative energy. I’ve concluded that the car rarely relies completely on the friction brakes. If the car had the ability to rely completely on the friction brakes with a full battery, applying the brakes would not start the engine.

 

I experience the same thing about once a week. There is a freeway offramp that I take at 70 mph. The offramp is long and straight, and it has an uphill towards the end as it reaches the signal light. And plenty of shoulder on each side. So it's quite safe to enter the offramp at full speed. However when I attempt four chevron regen it always flashes and drops down to three. It will do that until I get down to around 45 mph or so. Someone suggested it's because of the high current load, too much for the small Clarity battery. Perhaps combined with the battery already likely being pretty warm from several miles of 70 mph driving. However as you experienced pressing the brake pedal in that situation is capable of quite a lot of regen. So there is no real explanation for this other than my usual suspicion that two different people (or groups of people) wrote the two sections of software, and they never talked to each other.

I remember someone saying that by regulation the friction brakes have to be fully capable of providing all needed braking, whether or not the car always uses it. How true that is I don't know.

 

When Sport is not engaged, speed may be a factor in determining the number of chevrons allowed. See attachment below. I can only image that the designers didn’t want the driver to be startled by the massive amount of deceleration from 4 chevrons, unless the driver had consciously selected Sport. To elaborate on that, it would seem that if full paddle regen is acceptable at high speeds in Sport, then we may be able to rule out the theory of excess regen from 4 chevrons, when Sport is not engaged.

The question I had was not about the friction brakes being capable of stopping the car, surely they are. The question was if the car was capable of using only the friction brakes, particularly when the battery is fully charged, to prevent the engine from starting due to excessive regenerative braking energy. Based on the number of owners who have reported the engine starting to dump excess regen, I do not believe the car has that capability. The car does appear to reduce regenerative braking energy when the battery is fully charged. However, it seems obvious that the car does not eliminate regen and use strictly the friction brakes, other than when coming to a complete stop. It would probably take a complete meltdown of the braking system, as designed, in order for the car to slow and stop using only the friction brakes.

 

Here's a short video on the Honda Fit EV "Electric Servo Brake System". The Clarity blended braking is an evolution of that system.

 

Thanks for your thoughts and insights. I had previously thought that the Clarity expended the energy by using one of the motors to spin the cold engine like the Insight does in the same situation. I was corrected by others in this forum that it's actually running. I confirmed it by observing energy diagram show power flowing from the engine to the traction battery. I do believe it is overcharging the battery because on a couple of occasions where I hit subsequent stop lights on red, the engine would spin again a few more times while stopped such that I ended up at my destination with a full battery in EV mode. I know there is a buffer of 10-15% on top so when I say "overcharging", I mean charging into the buffer. As I mentioned, this phenomenon is completely mitigated if I crank the heater or AC during the first 5 miles or so. A better method is simply to charge to 70-80% like I do most of the time. I typically use 15-20% of the battery after each errand before I can charge again so I suppose charging to 60% (thus keeping the SOC between 40-60%) would do a better job of preserving the health of the battery but I like having the buffer in case I can't charge right away.

 

I run into the same issue as you, living on a hill. If you put the car in Neutral, there will be no regen since only the friction brakes are used to slow/stop the car. Regardless what the energy flow diagram shows, when the engine starts and runs with a full battery, both HV and EV miles are consumed, so obviously there is no power from the ICE being sent to the battery.

I'm starting to see a pattern. Landshark and others who have 2019 and newer models seem to have never had the engine start with a full charge. Those of us with 2018s have it happen with great regularity.

 

My engine has started on occasion with a full battery and going downhill as I leave our house. It did it just a few days ago. I ignore it. 2018 Touring.

 

This screenshot from the Fit video indicates that friction braking is used, exclusively, under one of two conditions, emergency braking or slow speeds. If the Clarity system operates in the same or similar fashion, it would explain the audible alert of “Rusty Rotor Syndrome” and may offer another work around for the engine starting on regen issue. Just slam on the brakes at the last second.

It appears that all braking, outside of the 2 conditions, involves regen or a blend of regen and friction.

 

The engine starting off at the downhill after fully charging could also be contributed to charging while the battery was in a warmer day, and then the next morning your battery is cold and the battery still has same amount energy stored but the container is smaller. So charging to 100% while warm, the next morning it could be a 105% to the car, and the engine could kick in with slightest brakes even pulling back from a garage. (I had experienced this)

If you have a ODB monitor hooked up, you could monitor how much brake pressure is being applied in real time. If you have a close to full battery, then most of the brake will be a blend of friction and regen. I usually keep battery charge level lower than 80%. Going a steep downhill (from 500ft high) for about 3 minutes at 45mph will generate around 100A instantaneous at 4 chevrons without physical brakes used.

 

Yes, I believe that to be the case. I have a 2021 Clarity Touring. If I fully charge at night and don't run any environmental controls, at the first stop sign, I do notice a different brake "feel" and no regeneration. If I engage the paddles, there is no noticeable response either. This will continue to the second stop sign (as that is mainly downhill.) After that point, I have to accelerate and use battery and then regen begins like "normal." At no point in this scenario has it ever engaged the ICE, however. This is also the only portion of my driving where the brake response is "weak" or harder to engage.

 

Obviously there is a difference in models after the 2018 model year. You would think that if it was only a change of some kind in the software they would have made that an after-market tech campaign like they did with some of the other things that folks were complaining about. But then again, maybe there is a difference in brake system hardware and software by itself wouldn't fix it.

 

Can you try fully charge then next morning go on steep downhill with regen max plus medium brake? It won’t let you go all 4 chevrons and yes the brakes won’t feel normal but this would be when the engine kicks in, at least on my 2019 it does that.