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Discussion Starter · #1 ·
I've been watching a number of things going on with the Volt 'behind the scenes' with my nifty DashDAQ. Jetjock gets the honors for finding it first and he and Rusty get the honors for blazing the trail. There so much to discover that I'm overwhelmed with data but it's been fun.

I live on a hill and I've noticed some very strange behavior with the car when the raw SOC of the battery reaches 88% and I'm going down the hill in low. This doesn't happen every day but if I'm careful I can make it happen. I need to start out with a full charge (85.7 SOC) and if I play my cards right the last few hundred feet of the hill or so I'll be at 88%. At that point the car starts making a high pitched gear noise and loses some resistance that you'd normally have in Low which causes the it to speed up at precisely the point it hits 88% even though the hill is a steady slope. If I'm coming down the hill not at 88% then it will decelerate this slope consistently.

I captured this on the DashDAQ today and thought some would find it interesting. See attached image or link.

Here is the setup. I'm descending a pretty steep hill and the car is in low and I have no need to brake. It normally holds it's speed steady on this stretch unless I hit 88% SOC when it starts to speed up.

You will see the following:

At the point where SOC goes to 88%, Motor B goes from GENERATING 40A to DRAWING 140A. At the same time Motor A comes online from doing nothing to GENERATING 150A and the Transmission Ratio goes from what the car displays as 0 (I'm assuming thats 1:1) to 3.984.

You can see when all this is happening the light blue line at the bottom - thats me hitting the brakes out of instinct and too many years around electricity when I saw one motor go to -150A and the other go to +140A. ;-)

So, Rusty, help me out here - are they giving up on charging the battery and dumping regen from one motor to the other to dissipate the energy then getting the deceleration for low from the transmission? Thats what it looks like to me but I'm a computer geek and hotrod guy not an automotive engineer.

Uncompressed image is here.

Does anyone know how to tell the site to NOT reduce the size of the images I attach? These graphs really need to be not reduced in size to be visible.
 

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At the point where SOC goes to 88%, Motor B goes from GENERATING 40A to DRAWING 140A. At the same time Motor A comes online from doing nothing to GENERATING 150A and the Transmission Ratio goes from what the car displays as 0 (I'm assuming thats 1:1) to 3.984.
That's just absolutely.... uh.... well... fascinating! Your guess appears to be as good or mine, or better. That's certainly a, well... yeah, that could do it. I think.

And the trouble/challenge with the DashDAQ (and the Volt) is that my next question set would be: What are the Motor A and Motor B volt (and hence with volts and amps - watts), RPM, and torque signals generating when this happens? What are the HV battery Volt and Amps signals (and also hence watt) generating? What are the Transmission torque and RPM signals generating? Since I don't see any of it, I think I can guess what the brake regen signal is generating (nothing), so you're showing the brake torque command here (which is turning into all friction), right?

I haven't figured out what to do with the transmission ratio yet either. I'm fairly sure you have it right that at 0 it represents the ring gear locked and the direct drive transmission ratio (which I've yet to confirm). I haven't done the math yet for motor A/B RPM => transmission ratio => transmission RPM => speed mappings yet. I'm sure that's another whole (unrelated to this question) issue worthy of detailed study.

Fascinating. Just... fascinating...

P.S.

Does anyone know how to tell the site to NOT reduce the size of the images I attach?
Maximum size for a bmp, doc, jpe, png, psd, or txt file is currently 20KB. Maximum size for a gif, jpg, jpeg, or zip file is 100KB. And the maximum size for a pdf is 2M. YPMV.
 

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Discussion Starter · #3 ·
Yes, that is my problem. One question generates more questions. I don't log EVERYTHING by default so when I discover something I have to go back and log more signals to answer the NEW questions. I'm going to set it up for this and capture the other signals. One thing is I haven't found a regen torque signal that shows actual regen, but only regen requested by the brake pedal. I just watch Motor B current usually to look for regen.

You are correct that the brake torque is generating nothing in my graph until I flinched and tapped the brakes as the current shot to crazy levels. Unfortunately as that was near the bottom of the hill it cancelled the 'event' and you can see a few feet later I came to a stop at a stop sign.

When I get more signals captured I'll let you know. I try to squeak this in with the busy schedule but sometimes it doesn't work out. The other day I had this all set up and my wife says "can you run the kids up to the bus stop on the way out?" which means I have to go UP the hill a bit from my house instead of straight down so I never hit 88% SOC on the way down. It's too bad because that day would have rocked - I discovered that even though the battery settles in at 85.7% as "full" if you unplug the charger near the end of a charge at just the right time, right before it's "full' it's actually OVER 85.7. I think it was at about 86.2 the other day. If I wait until the green light flashes it's always 85.7. I'm not sure whats up but it LOOKS like in the last few minutes of charging it takes the level up just a little over what it thinks is 'full' then lets it drift back down.
 

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When I get more signals captured I'll let you know. I try to squeak this in with the busy schedule but sometimes it doesn't work out. The other day I had this all set up and my wife says "can you run the kids up to the bus stop on the way out?" which means I have to go UP the hill a bit from my house instead of straight down so I never hit 88% SOC on the way down. It's too bad because that day would have rocked - I discovered that even though the battery settles in at 85.7% as "full" if you unplug the charger near the end of a charge at just the right time, right before it's "full' it's actually OVER 85.7. I think it was at about 86.2 the other day. If I wait until the green light flashes it's always 85.7. I'm not sure whats up but it LOOKS like in the last few minutes of charging it takes the level up just a little over what it thinks is 'full' then lets it drift back down.
No disrespect intended, but given your schedule and priorities (including, rightly, your kids), I have to question what you are really intending to learn, and what you are going to do with it if/when you do. It should be no surprise that the battery is protected from overcharging, always with a buffer at the high end, likely with some hysteresis. If you were like most Volt drivers, you would just put the vehicle in D, use the brakes, and the friction brakes would seamlessly apply as needed. But there I go, spending my time even reading and replying to this (mea culpa).
 

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Overall it is an interesting piece of the operational puzzle, but in most cases most users would have used enough battery, so as to not trigger this action by the Volts control systems.

Pat
 

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The Volt is probably doing the same thing the Prius does when it is at max SOC and you have the car in L (or in the Prius case B mode).. The Prius once it hits max SOC will use MG1 to spin the engine (with no fuel) to dissipate the excess energy that the battery can no longer brake. However unlike your case the Prius will maintain maximum regen, and not speed up when it does this. The first time it happened it scared the crap out of me cause I thought our Prius was going to blow up (it is able to spin the motor @ greater than maximum RPM depending on however much energy it needs to burn up). So I would imagine this is why you see that generator in the Volt start drawing 140 amps is it is spinning something. If you did not hear anything I doubt it is spinning the engine like the Prius (it is suprisingly loud), but it is spinning something to dissipate that extra energy since it cannot go in to the battery pack. I guess we just need to see what.. Did you by chance happen to see the engine RPM at this time or? That is where I would start.
 

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When braking in the Leaf at high/max SOC, doesn't it shut down Regen and go all physical braking ?

I wonder what happens in the Fisker - if they used such a bypass for their max SOC as well?
 

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Thanks.. interesting stuff. I had wondered what they would do with the excess generation power. I too live on a hill (though maybe not as steep). I've found its more fun if i play in sport mode for the first 2miles (getting to the hill) then make most it up on regen. (Just gotta remember to be more relaxed when I get off the highway).

I had though earlier postings
http://gm-volt.com/forum/showthread.php?8720-MM-vs.-Normal-D-vs.-L/page3,
had said the regen would be using the traction motor (which I thought ws A) but this suggests its B (Generator) doing regen, at least until you top-off the battery. Am I mixing up the motors? If it is the generator doing regen, any insights why?
 

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This is very interesting. Great you guys with DashDaq are posting your findings. Cool.

Bonaire, with a fixed gear ratio and one motor, the Leaf doesn't have any options other than just using the mechanical brakes when the battery is at maximum SOC.
 

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Same in solectria

I have been driving a Solectria for the last 7 years (Until I got my volt yesterday -more on that at another time ) If you went down a steep hill in the Solectria with the regeneration on and the battery full the car would "buck" and vibrate fiercely I would just turn off the regn for a mile or so. Phil in Vancouver
 

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Well that will be the benefit of the future plug in Prius, and the Volt.. In that you have a mechanical air pump essentially to spin if you need to burn excess power off which is exactly what the current Prius already does. The Leaf obviously does not have that option, and therefore has no way to dissipate energy so it switches from regen to friction brakes. On the Leaf forums you will find that people who live near steep grades will only charge the car to 80 or 90% in so they can have regen braking available to them. In fact I think they mentioned it is even in the owners manual to do that althought I could be wrong..

I would imagine any vehicles that has no way to deal with excess capacity would react once the battery hits its max capacity (or even over capacity).. As then you will be just generating a tremendous amount of heat in the motor and wires because the electrons have nowhere to escape to. I personally have never experienced this (although I wish I had the problem of having to much power!) and doubt I ever will. Is mostly a curiosity thing for me to see how the car works. Can never know to much about it as it may help me troubleshoot in case I have any problems later.
 

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I would imagine any vehicles that has no way to deal with excess capacity would react once the battery hits its max capacity (or even over capacity).. As then you will be just generating a tremendous amount of heat in the motor and wires because the electrons have nowhere to escape to. I personally have never experienced this (although I wish I had the problem of having to much power!) and doubt I ever will. Is mostly a curiosity thing for me to see how the car works. Can never know to much about it as it may help me troubleshoot in case I have any problems later.
I don't think it works that way. When a Lithium battery reaches the full point the manufacturer choose for longevity, it doesn't magically switch out of circuit. The battery would continue to absorb power, raising the system voltage and damaging the battery (eventually overheating the battery.) The electrons never "have nowhere to go" so it should not change the ability of the motor and inverter to generate power or cause them to overheat. Instead, the computer reduces or eliminates the power flow to protect the battery from overvoltage.
 

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I live in a similar situation, but this hasn't happened to me yet. However, I get to the bottom of the "big hill" with more miles left showing than when I left the driveway, even if I started with a full charge - that much happens every time. Could be I just don't have a big enough hill (but it's two miles of pretty steep down), or that the mile or so I go on the flat before starting down is enough to make the car happy (or my battery is really new and this will start happening later).

But I'd expect this and was looking for it to happen myself. The car system would be junk if it didn't protect the battery from an overcharge in regen mode, though perhaps it could do it more gracefully than described. I don't have data aq on it, I'm just going by what the car normally tells me here.
 

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I live in a similar situation, but this hasn't happened to me yet. However, I get to the bottom of the "big hill" with more miles left showing than when I left the driveway, even if I started with a full charge - that much happens every time. Could be I just don't have a big enough hill (but it's two miles of pretty steep down), or that the mile or so I go on the flat before starting down is enough to make the car happy (or my battery is really new and this will start happening later).

But I'd expect this and was looking for it to happen myself. The car system would be junk if it didn't protect the battery from an overcharge in regen mode, though perhaps it could do it more gracefully than described. I don't have data aq on it, I'm just going by what the car normally tells me here.
Well your Estimated Electric Range (EER) could be increasing even if the battery is full.. The EER its saying that if you keep driving like you have been,this is how far you will go. It seems to be a multi-scale predictive filter. Its continually adapting, depending on the last 150 miles, but depending even more strongly on recent miles. If you start at 40, drive 10 miles and still have a near 100% soc (well 88internal), then it would still think you can go 40miles but will probably say even more as you were very efficient in the last 10 miles. (Don't think you can get it above 50, I've tried).
 

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I live in a similar situation, but this hasn't happened to me yet. However, I get to the bottom of the "big hill" with more miles left showing than when I left the driveway, even if I started with a full charge - that much happens every time. Could be I just don't have a big enough hill (but it's two miles of pretty steep down), or that the mile or so I go on the flat before starting down is enough to make the car happy (or my battery is really new and this will start happening later).

But I'd expect this and was looking for it to happen myself. The car system would be junk if it didn't protect the battery from an overcharge in regen mode, though perhaps it could do it more gracefully than described. I don't have data aq on it, I'm just going by what the car normally tells me here.
Well, according to the data in the OP, the car holds 2.3% (~400 Wh) usable reserve above the top of "full". That's enough to stop the car from something over 60 mph, or to go about 1.5 miles in typical conditions. My guess would be that this reserve plus the ~250Wh from the mile you drive first is enough to absorb your hill, since you aren't seeing this behavior from your car.
 

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I live on a hill and I've noticed some very strange behavior with the car when the raw SOC of the battery reaches 88% and I'm going down the hill in low. This doesn't happen every day but if I'm careful I can make it happen. I need to start out with a full charge (85.7 SOC) and if I play my cards right the last few hundred feet of the hill or so I'll be at 88%. At that point the car starts making a high pitched gear noise and loses some resistance that you'd normally have in Low which causes the it to speed up at precisely the point it hits 88% even though the hill is a steady slope. If I'm coming down the hill not at 88% then it will decelerate this slope consistently.
After thinking more about your post, I finally got the chance to go back and read it again. What you are describing sounds like the classic generator load dump situation. That you are getting a high-pitched gear noise is very unusual and can't be good for the vehicle. The energy surge released by the load dump has to go somewhere other than the battery, and you seem to have found evidence of a failsafe (foolproof??) mode designed to use the inductance of the motors to protect the vehicle systems. You are also generating heat, dissipating the load dump, in this situation, into the transmission motors and liquid-cooled power electronics.

If you had simply left your fully charged Volt in "D" going down the hill, you would have dissipated this excess energy (you can't continue to charge a battery at the full charge limit) through the friction brakes - much more conventional and conservative - the brakes can easily take this - I wouldn't want to put that extra stress on my Volt drive / electronic system / battery, which is much more expensive than a little wear on brake pads.
 

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Discussion Starter · #17 ·
If you had simply left your fully charged Volt in "D" going down the hill, you would have dissipated this excess energy (you can't continue to charge a battery at the full charge limit) through the friction brakes - much more conventional and conservative - the brakes can easily take this - I wouldn't want to put that extra stress on my Volt drive / electronic system / battery, which is much more expensive than a little wear on brake pads.
I sometimes do that these days but I discovered it doing this long before I got a DashDAQ. There isn't any indication to the driver this is a problem and I doubt they'd design the car so it presents a problem and doesnt' warn the driver. I'm well aware the energy has to go somewhere which is why I got curious.

Out of habit we just put our cars in 2nd after getting only 15-18k out of each set of brake pads after we moved here. I finally had Les Schwab start doing all my brakes because they have a 30,000 mile warranty and I'd get 2 sets in for the price of one. On the car I just sold I took it in for it's last brake job and to my surprise the told me I had squeaked in a third when I went to pick it up.

Even with this hill it won't always get to that SOC even if I try. I was out today and had the DD all set up to log it better and only hit about 87.5 by the time I hit the stop sign. It really is just the last couple hundred feet of the trip IF I it gets there. It's fun to learn about whats going on just for the sake of knowledge.
 
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@ pauldon,

You have come across a feature of the vehicle which protects the battery from overcharging. Under certain circumstances, the electric motors will resist one another to provide braking in addition to the friction brakes on the vehicle. In order to meet emission requirements, the Volt does not spin the engine, but uses clutch 2 in the drive unit to link both motors. When the vehicle is at low speeds, clutch 2 requires that the resultant planetary gearset speeds increase to compensate. You will hear the electric motors at higher speeds, which is certainly a change from their normally silent operation.

The Volt was validated using the steepest, longest descent in the nation, Pike's Peak. With a full battery, the volt can descend Pike's Peak without issue with a combination of friction brakes and the electric motors. This is part of the Voltec propulsion system which has many more delighting features waiting for you to discover.
 

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^ This is why we bought a Volt. You can't get this type of service with any other vehicle that the vast majority of us could afford.
 
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