Regenerative braking and charging limit

[Ohji]

I have tried to find the answer to this question all over, and I can't quite figure it out, so I apologize if this has been answered before...

I live at the top of a fairly steep hill that is about 1000 feet above sea level. I pretty much lose the entire 1000 feet over the first mile of my commute. If I set a charging limit of 90% and leave that limit constant, will the regnerative braking system exceed that limit as I begin what is essentially an almost entirely downhill trip to work (assuming I start out with a full charge to 90%)? It is only a minimal amount of excess charge, likely.

I know the regenerative system disengages when the battery is fully charged (or does it just dump the excess electricty into a resistor?), so I'm wondering if the same thing would happen if a charge limit is set and the energy supplied by the regenerative braking system would result in a charge above that set limit...

 

[David99]

Interesting question. I don't think the charge limiter affects regen. Even when set to 90% and then going downhill for a while the battery will charge higher than 90%. At least that's what I assume. Give it a try and let us know.

Once the batter is full regen will be almost zero. There are no large resistors to keep regen active. You have to use your brakes. If you know you are going down a large hill I would probably only charge to 80% or even less. Just to make sure you can really capture the full amount possible when going down.

Frankly 1000 feet isn't a huge hill. You might gain 3-5%.

 

[Ohji]

Interesting question. I don't think the charge limiter affects regen. Even when set to 90% and then going downhill for a while the battery will charge higher than 90%. At least that's what I assume. Give it a try and let us know.

Once the batter is full regen will be almost zero. There are no large resistors to keep regen active. You have to use your brakes. If you know you are going down a large hill I would probably only charge to 80% or even less. Just to make sure you can really capture the full amount possible when going down.

Frankly 1000 feet isn't a huge hill. You might gain 3-5%.

Thanks! My Model S won't be delivered until November, so we will have to wait a while for that experiment!

And you are right, 1000 feet isn't very high at all, but the hill is so steep that when I did my test drive, the energy consumption to climb it was quite alarming. I just want to be sure I can at least regain some of that excess energy with the regen on the way down!

 

[WarpedOne]

I did some calculations... 1000 feet climb on 10% grade will add some 1000Wh/mile of additional consumption over what one usual gets as similar speed over level road.

If you get 300Wh/mile at 50mph, you will see over 1300Wh/mile going 50 up that hill. And this is just average number, smoothed out the whole climb. Temporary consumption may get off the scale.

 

[Ingineer]

The charge set point does not affect Regen. The pack will regen until it's at 100% then the regen will go offline and you will have to use brakes. It's important to be sure you will have enough spare room in the pack to regen all the way down or 2 things happen:

1. You throw away the excess energy.

And more importantly:

2. You risk overheating your brakes which could create a dangerous condition.

When you get the car home the first time, note how much energy you use going up the hill. Set the car to display SOC rather than range, and note the SOC at the beginning of the climb. Subtract the SOC you are at when you arrive home and leave almost that much "open" when you charge. It's perfectly safe to regen to 100%, and won't be a problem as long as you use it right away, but if you don't need the capacity, then only charge to 80%, etc.

 

[rfmurphy81]

Check out Maximum Regen - Can Someone Top This? for some data another forum member posted about his 10,000 foot descent from Haleakala in Maui.

 

[tnt1971]

I have put almost 8,000 miles on my car and I have only seen regen restrictions in two conditions. First, when it is cold. Usually this only happens when it gets down to the 40s and below. The other was when I did my first and only range charge, which gave me 275 miles of range (85D). By the time I got down to 260 miles of range the restriction was gone, which is about 95% SOC. 1,000 feet from 90% won't get you to the range restriction.

 

[Polly Wog]

@Ohji, if you are descending about 1,000 feet over the course of about a mile, you could potentially gain up to 5 miles of range. I don't know what model you will have, but in a 90D, that is still less than 2%, so you won't have any problems in that regard. Charge to 90% regularly (assuming you are getting anything except a software limited 60), and you will be fine.

 

[Ohji]

Thank you all for the replies! I appreciate the help!

 

[Big Virgil]

Ohji - That incline is 1 ft of elevation gain for every 5.25 ft of distance. That is approx 25% grade for 1 mile? I ride mtn bikes on Colorado trails and rarely encounter a grade that steep. Are you sure? Any sand or rain, and I'm not sure most cars would hold traction. Likely doesn't matter for Regen, but just curious.

 

[bxr140]

The charge set point does not affect Regen. The pack will regen until it's at 100% then the regen will go offline and you will have to use brakes.

Not exactly. Regen limiting kicks in at higher SOCs (and at cold temps). You need to use brakes long before 100%
SOC.

OP--the commute from my last house dropped 1500 feet in 4 miles. I would frequently start to get regen limiting toward the bottom of the decent, especially during colder months. Dropping initial SOC helped a bit.

 

[WannabeOwner]

the hill is so steep

It might exceed the braking effect of Regen ... so you might have to, also, use the brakes - in which case, sadly, you won't get 100% of the Free Lunch. Nice to leave on 90% charge and gain a bit more before the real journey starts, but a bummer when limping home on low charge ...

... I've been telling my kids that when they buy their first house it should be to the East of their work, so they are not driving into the sun Morning and Evening in Autumn, Winner and Spring; now I need to add to that to live at the bottom of a steep hill for maximum limp-home Regen

 

[wdolson]

Sounds like the West Hills in Portland.

I too live up a hill, but it's about about 250 feet. Leaving home I have to go up about 50 feet before going down the hill. I've been experimenting to get the lowest Wh/Mi by the bottom of the hill. My results are all over the map. I've made it to the bottom of the hill at 80 Wh/Mi, but it's usually around 200 Wh/Mi. I did experiment setting the charge level to 100% after taking the car off the charger but before leaving and it was one of the times I got below 100 Wh/Mi, but I may have just hit the sweet spot that day. I need to do some more experiments though.

My only conclusions thus far are that fairly minor variations in how I drive the hill can have a big effect on the regen.

 

[bxr140]

My only conclusions thus far are that fairly minor variations in how I drive the hill can have a big effect on the regen.

Could be, but my longer hill was pretty consistent. The main variable was ambient temp. I could get close to -200 wh/mi in the summer by the time I hit the bottom...but sometimes as low as -50 in the winter, partly because I'd always get cold regen limiting toward the bottom.

 

[wdolson]

My hill has some plateau stretches and even one slight rise on the way down. If I don't manage my speed right, I end up crawling along at 15 mph on the plateaus.

 

[Boatguy]

I also live on a steep hill and with a the charger set at 90% I get full regen on the way down the hill.