High or moderate speed brake pulsing

I've read the posts on grabby brakes, and I'll say I've seen this recently on my leaf. I have 9,200 miles, and it started about 200 miles ago. But, the "grabby" brake thing seems to be described as always a low speed phenomenon.

I also have an issue at high to moderate speeds where if I'm braking moderately as well, such as going down a hill or coming off a highway exit ramp, the brake systems seems to be hunting for a consistent brake level. I'd call this pulsing brakes.

It's not such an aggressive braking situation as to be enaging the ABS. I thought originally that the system was hunting between the regen and mechanical brake system, I've certainly had situations where this seemed to be the case. But, I think I've had cases as well where I was well with the regen bubbles and still got this behavior.

I have not gotten the firm ware update. Plan to go do this in a few weeks after a trip.

Anyone else see the "pulsing" and have information to share? :roll:

I've noticed the same issue

Ive had the update and it made no difference

It's seems intermittent and I'm not sure what conditions it occurs in

?

I just saw a post tonight by someone who had similar behavior but worse. His situation ended up with warning lights and a visit to the dealer. Turned out to be a loose connector in the ABS. Hum....

My braking is generally soft to moderate, steady, with a slow onset ...
and I experience only smooth, steady, "normal" braking.

I have not experienced panic Brake Assist,
or the low speed "grabby" brakes.

I hit 10k miles recently and I am feeling the same thing. It's something relatively new to me. Seems to only happen exactly when you described...coming off long downhill exit ramps. It's rather subtle.

Funny, in development of hybrid control systems I'm involved with, we've run into this problem before and I was surprised the leaf does such a good job at it. Even with parallel hybrids, it can be a very perceivable problem depending on the size and internal resistance of your energy storage system.

My theory..
The problem occurs when the battery has a moderate SOC. You are on the highway pulling lots of current. Then all of a sudden you get off the throttle and on the brake in full regen. For me I notice it more when I got the brake pressed enough to do full regen, but not enough to be in the friction brake zone..

In regen, the motor drive/controller will apply negative torque, this in turn produces energy in exchange for mechanical force. The energy goes back into the battery. The battery can only take so much current before the voltage rises to the cells operating limit (this is why you can't jam 80kw back in to the battery). At that point, the controller has to back off the regen so that the voltage comes down. Once the voltage drops, and you are still regening, the controller will re-apply the neg torque and the voltage will shoot back up. There is a oscillation as the controller constantly tries to extract the most current out of the regen, while not overvoltaging the batteries. This 'hunting' can be perceived as a slight surging as the controller adjusts the regen torque.

turbo2ltr said:

The battery can only take so much current before the voltage rises to the cells operating limit (this is why you can't jam 80kw back in to the battery). At that point, the controller has to back off the regen so that the voltage comes down. Once the voltage drops, and you are still regening, the controller will re-apply the neg torque and the voltage will shoot back up. There is a oscillation as the controller constantly tries to extract the most current out of the regen, while not overvoltaging the batteries. This 'hunting' can be perceived as a slight surging as the controller adjusts the regen torque.

Click to expand...

Interesting explanation for the reported behavior, but is the regen level on the Leaf actually powerful enough to generate the kind of "over-voltage" scenario you describe as necessary to create this "oscillation" effect? Even full (5-bubble) Leaf regen seems extremely anemic to me, especially compared to the Tesla roadster regen I have experienced, that it seems unlikely that it would be pouring such massive current back into the traction pack that it exceeds cell voltage limits. The Nissan engineers seem to have intentionally limited the amount of regen available to what--30% of the motor's capacity? If 24kW is all the regen energy that can be created, is that really sufficient to trigger this "overcharge oscillation" scenario?
Just wondering,
TT
ps- I haven't yet experienced the described "pulsing" braking behavior in our car as yet, except when ABS has been triggered, which is another matter.

Note the tesla is quite a bit lighter than the leaf so you will feel regen more, even if it both vehicles were applying the same amount of negative torque. The tesla also has a pack thats double the capacity of the leaf, so you'd be able to put more into without hitting limits.. The battery technologies used in each car are different and thus may act differently. That's not to say nissan isn't being conservative, or that it's definitely hitting the voltage limit.

But again, it's only my theory based on my (limited) knowledge of the leaf, combined with experiencing the same feeling in other (hybrid) vehicles for which I do have intimate systems knowledge.