Regenerative Braking and Driving Downhill

[jimmychopps]

So far, the only Leaf feature with which I am unsatisfied is the way mechanical and regenerative breaking work, especially as it relates to driving downhill. My complaints are these:

1. When the battery is fully charged, regenerative breaking doesn't slow the vehicle. Normally this is a non issue, as any amount of driving would deplete some of the battery capacity and regenerative breaking would return to normal. As it is, my daily comute (effectively the route from my house to anywhere) takes me downhill for the first 10-12 miles. If I ever charge my car to 100% regenerative breaking doesn't slow the car for these first 10-12 miles. I'm not concerned with the energy loss due to mechanical breaking, the issue is that I'm used to driving in "B"; during this first stretch my car behaves oddly. I have to push the breaks harder than normal to stop and cruise control stutters between accelerating and coasting.

2. Regenerative breaking cuts out while driving downhill and breaking. I always drive in "B". If I'm driving downhill and the hill is not to steep, I can control my speed by backing off on the accelerator until the regen breaking maintains my speed, this is ideal as most of the energy is going into my battery and the slowing is steady and controlled. If the hill gets steep enough to require additional breaking, then I get strange behavior. Regenerative breaking will suddenly cut back/out while I'm holding the breaks down. Speed does not appear to be a factor. I can't predict when this will happen, except to say that it happens almost every time I hit the breaks while coasting downhill.

The annoyance here is two fold: 1. When regenerative breaking cuts out, I can feel the breaking force of the car suddenly drop. I have to react to this by hitting the breaks harder to maintain stopping force. My car should behave predictably, especially w/ respect to breaking. 2. When regenerative breaking cuts out mechanical breaking has to do more work leading to energy loss, and break wear.

Its correct for regenerative breaking to scale w/ break pressure, it's reasonable for regenerative breaking to scale w/ velocity at slow speeds, if both of these factors remain constant then the stoping force of regenerative breaking should stay constant. This is not the case and is really annoying for people who drive downhill a lot.

I've seen some pretty detailed investigations around breaking energy and behavior, TickTock's thread is amazing. I've not seen anything about the issues I describe above, has anyone else experienced these behaviors?

 

[DanCar]

What about charging to 80%?

 

[LeftieBiker]

What about charging to 80%?

Or, better yet, 90%, even though this means ending charging manually. Why do you drive in "B" mode all the time? You get extra regen in Eco mode, with less squirrel-y behavior. The one point I do agree with is that the transition between regen and hydraulic brakes can be unsettling, although I mainly experience it when stopping - a brief moment in which the car seems to jump forward a little, and I still always overcompensate, making it jerk to a stop. Our Prius doesn't do this...

 

[Nubo]

If you generate power it has to go somewhere. If the battery is too full to accept the charge, what would you do with it? I suppose you could have some huge resistance heating elements to turn it into heat but that wouldn't be very elegant and there would be safety issues. You could power another motor to apply reverse torque... Not an ideal solution either.

 

[abasile]

On a 16 mile, 5000' descent from my home that I make regularly, I need to stop charging at 60% or less if I want full regen all the way down. Starting the descent at 80% can be a bit dicey. Starting at 100% is foolhardy and probably risky.

My biggest complaint is that I can't set a charging timer to end at 60% or 70%, so I have to check on the car multiple times each morning to ensure I unplug it with the desired charge percentage (these days, about 67% in GIDs). Nissan would do well to study the timer interface in the Tesla Model S, which allows any end percentage in the range 50% - 100%.

 

[cwerdna]

^^^
FWIW, the Model S didn't even ship with ANY charge timers. They only got added in an update in the last few months.

BTW, OP, can you please fix your original post and title? You're referring to braking, not breaking.

 

[DNAinaGoodWay]

Unless his braking is breaking.

I live atop a hill too. If I allow 1/2 hour charging per bar needed, set to charge to 100%, and set the timer to end an hour before I calculate I would need, it often comes in around 90% ('cause it always charges faster) and I still get regen, but I only have Eco, no B in a 12. Or, I just set it for 80% if I don't need the range, which is most of the time.

OP, would you mind listing your location in your profile please? It may be helpful in future. Thanks.

 

[jimmychopps]

Post title updated w/ correct spelling -> thanks cwerdna. I've added my location to my profile which is California, USA.

Most of the time I do charge to %80, I rarely charge to 100% at my house because I drive downhill when leaving my house. Doing so not only improves the behavior of my car, but also saves me some kwh.

My concern is not with energy efficiency, I'm talking about the behavior of the car. Its frustrating that the brakes behave differently depending on how full my battery is and/or how steep the hill is. Most cars put a lot of design work in to making sure that the brakes behave the same in as many driving conditions as possible. This design objective has lead to antilock breaking, traction control systems, all weather tires, etc.. For some reason the Leaf's breaking algorithm (how much mechanical and regenerative stopping force to apply as a function of how much pressure is being applied to the brake pedal) doesn't correctly handle full batteries or driving down steep hills. I wish it did.

 

[Stoaty]

Post title updated w/ correct spelling -> thanks cwerdna. I've added my location to my profile which is California, USA.

Including the city in your profile will be helpful anytime you ask a question about battery capacity and climate, or if you need help from a nearby member.

 

[DanCar]

On a 16 mile, 5000' descent from my home that I make regularly, I need to stop charging at 60% or less if I want full regen all the way down.

So you get like a 30% recharge going downhill?

 

[TickTock]

On a 16 mile, 5000' descent from my home that I make regularly, I need to stop charging at 60% or less if I want full regen all the way down.

So you get like a 30% recharge going downhill?

Let's see. 5000 ft * 3600 lbs = 18M ftlbs = 6.8kWh total potential energy. So, depending on what his full capacity is yes he could be getting a 30% charge (30% would equate to 22.7kWh full ignoring losses).

 

[abasile]

On a 16 mile, 5000' descent from my home that I make regularly, I need to stop charging at 60% or less if I want full regen all the way down.

So you get like a 30% recharge going downhill?

Let's see. 5000 ft * 3600 lbs = 18M ftlbs = 6.8kWh total potential energy. So, depending on what his full capacity is yes he could be getting a 30% charge (30% would equate to 22.7kWh full ignoring losses).

Don't forget, much of that potential energy is lost to aerodynamic drag, rolling resistance, and drivetrain and battery charging losses. At best, if I descend slowly, I might get back close to 15% measured in GIDs.

The higher the SOC, the more the car tends to limit continuous regen, just as L2 charging and DC fast charging taper at high SOC. Going down a big mountain, the car really won't let you fill the battery from regen. On my descent, I'm doing well if I can get the charge significantly above 80% (true SOC, which now equates to 74% in GIDs with my 2011 LEAF).

 

[DanCar]

I think a 2013 with B mode would help a lot in this situation .

 

[abasile]

I think a 2013 with B mode would help a lot in this situation .

Do you mean my situation? I'm not so sure whether B mode would help. ECO mode regen is more than enough to control speed on my 5000' descent, that is, until the car starts limiting regen, most likely to protect the battery pack.

Then again, I'm not aware of any data on this. If someone in SoCal with a 2013 LEAF with B mode would care to drive up to my home, charge to ~70% on my charging dock, and descend the mountain in B mode, we could see if it makes any difference. Thanks to the QC at Fontana Nissan, the San Bernardino Mountains are not hard to reach in a LEAF. And my neighborhood has nice access to hiking trails.

 

[DanCar]

Nissan said one of the tweaks of 2013 model was to allow more regen in more situations.

 

[DaveinOlyWA]

On a 16 mile, 5000' descent from my home that I make regularly, I need to stop charging at 60% or less if I want full regen all the way down. Starting the descent at 80% can be a bit dicey. Starting at 100% is foolhardy and probably risky.

My biggest complaint is that I can't set a charging timer to end at 60% or 70%, so I have to check on the car multiple times each morning to ensure I unplug it with the desired charge percentage (these days, about 67% in GIDs). Nissan would do well to study the timer interface in the Tesla Model S, which allows any end percentage in the range 50% - 100%.

not enough noise on this. there are several people who have hacked Carwings and putting in a notification for predetermined charge levels i suspect would only require an automatic polling of Carwings status at a preset interval of time.

problem is most people want the opposite. they need every mile they can get so charging to a lower SOC is simply not a priority with them.

when i first got my LEAF and my commute was short, i simply plugged it in for a few hours in the evening which was more than enough to boost me from 30-50% SOC to the 70ish % i wanted. i was not scientific about it simply because i was not all that particular where i ended up. but that was also in the days where i might only charge 3 times a week.

 

[dgpcolorado]

I think a 2013 with B mode would help a lot in this situation .

Do you mean my situation? I'm not so sure whether B mode would help. ECO mode regen is more than enough to control speed on my 5000' descent, that is, until the car starts limiting regen, most likely to protect the battery pack...

B mode would help a lot in my situation. The hill I have to descend is much steeper* than Eco regen can handle and keep the car at a safe speed.

...I live atop a hill too. If I allow 1/2 hour charging per bar needed, set to charge to 100%, and set the timer to end an hour before I calculate I would need, it often comes in around 90% ('cause it always charges faster) and I still get regen, but I only have Eco, no B in a 12. Or, I just set it for 80% if I don't need the range, which is most of the time.

I do much the same thing: when I need to push the range I tend to charge a half hour beyond 80%. That gives me some regen to help with controlling speed and I end up with about eleven fuel bars at the bottom. Like abasile said earlier, it isn't possible to regen a LEAF back to "100%" so choosing a charge level when descending a hill is something of a compromise.

If the OP doesn't need to squeeze every bit of range out of the LEAF for his route, I suggest doing an 80% charge (or less, as abasile does). Or, if the hill isn't so steep that the extra B mode regen is needed for speed control, don't use B mode.

Yes, electric cars are different from ICE cars and braking with regen isn't the same as engine braking plus friction brakes. It takes some getting used to, but I find it fun.


* My personal definition of "steep" is a hill that requires 18 kW or more to maintain a constant velocity of 25 mph going uphill.

 

[abasile]

My biggest complaint is that I can't set a charging timer to end at 60% or 70%, so I have to check on the car multiple times each morning to ensure I unplug it with the desired charge percentage (these days, about 67% in GIDs). Nissan would do well to study the timer interface in the Tesla Model S, which allows any end percentage in the range 50% - 100%.

not enough noise on this. there are several people who have hacked Carwings and putting in a notification for predetermined charge levels i suspect would only require an automatic polling of Carwings status at a preset interval of time.

I could write a simple application to monitor Carwings and text me as soon as the charge reaches the desired number of bars. I'd still have to unplug the car since Carwings does not seem to provide a "stop charging" function. That would still save me from having to check on the car, provided I could be satisfied with the rather rough granularity of the charge bars. Maybe when I feel like spending a little time on this...

 

[Mottyski82]

I hope my comment is not out of place here, but I feel people may find it useful information...
I took my Leaf up and down the MT Washington Auto Road twice in September and used 46 and 48 percent battery charge respectively going up, and regenerated 25 % both times coming back down. So, the thing I want to point to is that approximately half the energy consumed going up is regenerated back down! That's not bad in my book. If I had started from 100% SOC, the LEAF probably could have climbed MT Washington three times on that one charge.

 

[DaveinOlyWA]

I hope my comment is not out of place here, but I feel people may find it useful information...
I took my Leaf up and down the MT Washington Auto Road twice in September and used 46 and 48 percent battery charge respectively going up, and regenerated 25 % both times coming back down. So, the thing I want to point to is that approximately half the energy consumed going up is regenerated back down! That's not bad in my book. If I had started from 100% SOC, the LEAF probably could have climbed MT Washington three times on that one charge.

would have been more than half. there are things that are always present including gravity, friction, accessory overhead, etc.

other than that; I think your 50% claim is amazing