Momentary Braking Loss

[AlanD]

There is a ¼-mile hill I go down often. Its slope is such that I can just “coast” with my foot off the pedal in “B” mode and maintain a speed of about 40 MPH. On three occasions, as I was braking to slow for the signal at the bottom, the brake pedal would suddenly drop an inch or more but then would catch and continue slowing the car. It is all over in an instant, but, of course, I am wondering what is happening. Most of the times going down that hill, there is no problem.

Is the regenerative braking momentarily cutting out?

 

[knightmb]

There is a ¼-mile hill I go down often. Its slope is such that I can just “coast” with my foot off the pedal in “B” mode and maintain a speed of about 40 MPH. On three occasions, as I was braking to slow for the signal at the bottom, the brake pedal would suddenly drop an inch or more but then would catch and continue slowing the car. It is all over in an instant, but, of course, I am wondering what is happening. Most of the times going down that hill, there is no problem.

Is the regenerative braking momentarily cutting out?

If ABS kicks in, regen is cut-off instantly until some traction returns. The problem is, when it happens, it feels exactly what you described, a loss of braking power, but it means you have to mash in the brake pedal more to make up, which when you are accustomed to coasting with regen and only needing to feather the brakes to stop, can be surprising.
It does have a logical explanation though, so you are prepared should it happen again.

 

[AlanD]

That is interesting, but I don't know why ABS (Anti-lock Braking System) would do that when the road is dry and I am braking only moderately to slow down and not stoping quite yet.

 

[91040]

What was the traction battery’s State of Charge at those times?
What capacity loss does your battery have?

 

[AlanD]

State of charge was around 50%. Capacity is about 87% on a 40KWHr battery.

 

[91040]

That rules out the battery’s condition limiting regeneration.

 

[Lothsahn]

12V battery can get weak and cause problems with the brake controller (up to and including complete loss of brakes). I'm not saying the 12V battery is your problem, but if you haven't changed it in a long time, it might be good to get it tested and/or replaced.

I strongly recommend AGM batteries for the Leaf instead of standard batteries.

 

[AlanD]

What advantages do you see with an AGM battery? Mi 12V is fairly new, but will eventually have to be replaced.

Thanks.

 

[LeftieBiker]

AGM batteries are more tolerant of being partially discharged most of the time, and will also accept a higher charging current from the car, making their demise from low charge less likely.

 

[kaihripentiki]

In my prior 2013 leaf, i noticed that on long downhill regens with light brake pedal pressure that occasionally, especially on steep grade, the regen would get suddenly cut (indicated as 0 on the in-dash energy meter and LEAFSpy) with the described sudden fall back to hydraulic braking.
I would suspect something similar is happening here. Though, in the cases i noted, it seemed that the only common factor for when this happened was that the car was going down-hill but still slightly accelerating.
I would echo the ABS related answer as well. If the traction control or stability control is activated during regen, regen is unceremoniously cut off by the EV system.

 

[AlanD]

OK, so it doesn't sound like a problem, just an unexpected quirk.

Thanks to everyone for their helpful comments.

Alan