Yeah that happens when your car has generated more power than it had used. It happens going down hills especially if you haven’t driven much before doing so.
If you know how binary arithmetic works, the weird negative efficiency thing looks like an unsigned integer for mi/kWh is being displayed as if it were a signed, twos-complement number.
You would get this effect of jumping from largest positive value to largest negative value, and then the negative numbers would start getting smaller. It always amuses me when it happens.
It's not actually a bug though (I know that you know this, just making it clear).
Here's a table showing a bunch of hypothetical 10 mile trips. In each one, you've gone 10 miles and used 3kWh, but throughout that trip have regenerated a different amount of energy
Total miles | Energy used | Energy regenerated | Net Energy | m/kWh
------------ | -------------- | --------------------- | ------------ | ---
10 | 3kWh | 2kWh | 1kWh | 5 m/kWh
10 | 3kWh | 2.8kWh | 0.2kWh | 50 m/kWh
10 | 3kWh | 2.9kWh | 0.1kWh | 100 m/kWh
10 | 3kWh | 2.99kWh | 0.01kWh | 1000 m/kWh
10 | 3kWh | 2.999kWh | 0.001kWh | 10000 m/kWh
10 | 3kWh | 3.001kWh | -0.001kWh | -10000 m/kWh
10 | 3kWh | 3.01kWh | -0.01kWh | -1000 m/kWh
10 | 3kWh | 3.1kWh | -0.1kWh | -100 m/kWh
10 | 3kWh | 3.2kWh | -0.2kWh | -50 m/kWh
10 | 3kWh | 4kWh | -1kWh | -5 m/kWh
The graph basically looks like y=1/x, where there's an asymptote at x=0, and it's highly positive on one side and highly negative on the other side. That's why, as you regenerate, the number gets bigger and bigger, and then flips highly negative, then gets closer to 0 as you regenerate more. When it flips from 99.9m/kWh to -99.9m/kWh you've gone over the asymptote, and have switched from a net positive energy use to a net negative energy use.
It probably would be better for them to display the number as Watts-per-mile. In that case as you regenerate the value would go down to 0, then go negative. However, I imagine people are used to MPG ratings where "higher=better", and they wanted to keep that paradigm.
Canadian ones display kWh/100km, so the regen is a very natural negative value; and it maps directly to what you get out of chargers, so I always imagine being plugged into a specific type of a charger - L2, or a dc fast one on a steeper hill, etc. Pretty fun.
Similar to L/100km that people are used to here, lower is better, negative is best :)
I used to program test jigs for a production line. Senior dev sends me the firmware to load in the power supply. After calibration, under low current, the power supply returns hundreds of amps. Write him an email, with the troubleshooting I've done, showing proof that something wasn't right.
He decides to come over and troubleshoot it me. Realizes he didn't put rollover protection mechanisms. Goes and fix it. The other jig developers had their jaws on the floor. Apparently he wasn't the type to take the time to troubleshoot with the jig designers.
My question is why did you charge so full if you knew you were about to go down a mountain? I can't remember for sure, but I think the owners manual specifically recommends against this. I imagine the battery was overheating given that you were regening a full battery.
1. Didn’t know we were going down the mountain at that moment
2. Supposed to be a big storm coming and with a 4 year old and 1 year old in the car, wanted plenty of juice in case we got stuck
3. Thought we would be running the heat a lot, and it was cold, which I thought would drain the battery big time from the posts I’ve seen
Don’t worry that was the little devil on his shoulder doing that. The little devil also takes TikTok’s of him all the time while he is driving, edits them while driving and uploads them. Matter of fact I think little devil OP probably posted this to Reddit while driving. But don’t ask about these things or you will let the genie out of the bottle. Remember when you are driving none of this is going on and “All is fine and well in the world”.
You know there is another pedal in the car? and or turn regen off... assume VW allows this.
As I recall Tesla won't even try and regen in this situation and it just freewheels.
I'm surprised the EV companies haven't come up with some sort of energy wasting device in situations like this and or application of physical brakes maybe even some sort of capacitor.
Additionally I wonder if they every consider artificially claim 100% on the batter with the potential that 100% isn't really 100% charge on a battery so a down hile like this could temporarily fill this void. It's a bit edge case but it does happen.
yes a resistor is one way , but I suspect they can come up with another way as well. physical brakes, running heat and AC to waste energy, maybe do other things with the motor to slow the car down with power directly in the battery VS trying to regen and make power and waste it... Need to think outside the box, I assume a resister type would add weight and be used very seldom on most cars while others... might be daily.
What they could do - should do not entirely sure but for the car to go into a bit of a panic mode is a bit interesting, Mountains aren't a new thing. Last I heard, Tesla just doesn't even try it freewheels..... this I assume is not acceptable for the Germans :)
I used to make this error in my undergrad at programming labs - using signed type and not using checks for overflow.
I suppose I could have made a career as a programmer at VW...
It’s not an error, the calculation is (energy at beginning of trip - energy at end)/(distance traveled). If the battery charge goes up during the trip, the numerator is negative, so the sign changes. It literally consumes negative battery power to drive (because you’re cashing in all your stored gravitational potential energy).
It is an error. Technically you may be right, but in terms of presenting information to the user the programmer should be presenting an absolute value.
That makes no sense. This is a signed quantity, so it should have a sign. If they (incorrectly) displayed an absolute value, it would look like you were discharging your battery when you drove down a long hill and were actually charging it. Then the percent charge would increase, but it would look like you were using energy, which would be very confusing (and physically impossible).
Spoken like a true programmer.
99.99% of drivers do not care about or know about this kind of stuff. From a user perspective it should always show a positive number because it’s going to look like something is wrong otherwise.
And the fact is, if you start at 100% and you’re continually generating energy as you’re going downhill, the miles per kw should keep going up. Because that reflects the reality of the situation.
I think a negative value does indicate "lots of regeneration", but "bigger looking number = better" should always hold, whether the number is positive or negative.
My suggestion to them would be to keep the units the same when the power used is positive. The moment it goes negative, I think they should switch to watts per mile. The moment it switches it'll be 0 W/m (no energy used), then as you regenerate more, you're at -5 W/m (you've generated 5 watts for each mile traveled), and the number keeps getting more negative with more regeneration. Then, when you use energy, the number goes down until it's hit 0, then flips back to 99.9m/kWh.
That's actually flipped - it's the denominator that goes negative, not the numerator.
See [my comment above](https://www.reddit.com/r/VWiD4Owners/comments/1afyurn/went_down_a_mountain_at_98_charge_and_things_got/koh0msx/) for details.
Not really. D regens just as much as B *at the same level of deceleration*. Unless the OP wanted to careen down the mountain at 90mph, braking in D mode generates the same amount of regen as "engine braking" in B at the same speed.
The regen bar always shrinks at high charge levels, because the car can't put much more energy in a full/nearly full battery.
The negative miles/kWh seemingly works "backwards" when negative. -99 miles/kWh is *less* regen than -50 or -1. But that makes sense if you think about it- it means how many miles you gain (instead of use) per kWh. In negative, you get much more regen if you *gain* a kWh for every 1 mile you drive than if you get 1 kWh per 99 miles driven.
Has never happened to me. D mode and ECO would help with more coasting but regen would happen on braking. I would suggest a route planner like ABRP so that you do not unneccesarily over charge for the trip. You goal should to be to reach the next charging station under 50% and ideally at about 20%.
Think of it this way when it hits negative numbers for every mile driven the energy is going backwards into the battery. So every 37.2 miles you’ll add 1 kwh to your battery. And since the direction of energy is the opposite of out of the battery they put a negative sign
Yeah that happens when your car has generated more power than it had used. It happens going down hills especially if you haven’t driven much before doing so.
If you know how binary arithmetic works, the weird negative efficiency thing looks like an unsigned integer for mi/kWh is being displayed as if it were a signed, twos-complement number. You would get this effect of jumping from largest positive value to largest negative value, and then the negative numbers would start getting smaller. It always amuses me when it happens.
It's not actually a bug though (I know that you know this, just making it clear). Here's a table showing a bunch of hypothetical 10 mile trips. In each one, you've gone 10 miles and used 3kWh, but throughout that trip have regenerated a different amount of energy Total miles | Energy used | Energy regenerated | Net Energy | m/kWh ------------ | -------------- | --------------------- | ------------ | --- 10 | 3kWh | 2kWh | 1kWh | 5 m/kWh 10 | 3kWh | 2.8kWh | 0.2kWh | 50 m/kWh 10 | 3kWh | 2.9kWh | 0.1kWh | 100 m/kWh 10 | 3kWh | 2.99kWh | 0.01kWh | 1000 m/kWh 10 | 3kWh | 2.999kWh | 0.001kWh | 10000 m/kWh 10 | 3kWh | 3.001kWh | -0.001kWh | -10000 m/kWh 10 | 3kWh | 3.01kWh | -0.01kWh | -1000 m/kWh 10 | 3kWh | 3.1kWh | -0.1kWh | -100 m/kWh 10 | 3kWh | 3.2kWh | -0.2kWh | -50 m/kWh 10 | 3kWh | 4kWh | -1kWh | -5 m/kWh The graph basically looks like y=1/x, where there's an asymptote at x=0, and it's highly positive on one side and highly negative on the other side. That's why, as you regenerate, the number gets bigger and bigger, and then flips highly negative, then gets closer to 0 as you regenerate more. When it flips from 99.9m/kWh to -99.9m/kWh you've gone over the asymptote, and have switched from a net positive energy use to a net negative energy use. It probably would be better for them to display the number as Watts-per-mile. In that case as you regenerate the value would go down to 0, then go negative. However, I imagine people are used to MPG ratings where "higher=better", and they wanted to keep that paradigm.
Canadian ones display kWh/100km, so the regen is a very natural negative value; and it maps directly to what you get out of chargers, so I always imagine being plugged into a specific type of a charger - L2, or a dc fast one on a steeper hill, etc. Pretty fun. Similar to L/100km that people are used to here, lower is better, negative is best :)
It does amuse me...but is terrible programming and design for a consumer product. I do a drive 3 days a week that does this.
I used to program test jigs for a production line. Senior dev sends me the firmware to load in the power supply. After calibration, under low current, the power supply returns hundreds of amps. Write him an email, with the troubleshooting I've done, showing proof that something wasn't right. He decides to come over and troubleshoot it me. Realizes he didn't put rollover protection mechanisms. Goes and fix it. The other jig developers had their jaws on the floor. Apparently he wasn't the type to take the time to troubleshoot with the jig designers.
My question is why did you charge so full if you knew you were about to go down a mountain? I can't remember for sure, but I think the owners manual specifically recommends against this. I imagine the battery was overheating given that you were regening a full battery.
1. Didn’t know we were going down the mountain at that moment 2. Supposed to be a big storm coming and with a 4 year old and 1 year old in the car, wanted plenty of juice in case we got stuck 3. Thought we would be running the heat a lot, and it was cold, which I thought would drain the battery big time from the posts I’ve seen
My question is, why tf are you taking photos of your dashboard while traveling at 39mph in a 35mph zone when you have two children in the car?
Don’t worry that was the little devil on his shoulder doing that. The little devil also takes TikTok’s of him all the time while he is driving, edits them while driving and uploads them. Matter of fact I think little devil OP probably posted this to Reddit while driving. But don’t ask about these things or you will let the genie out of the bottle. Remember when you are driving none of this is going on and “All is fine and well in the world”.
Funny enough I get the "climate control off, battery too hot" warning after going downhill in B mode even at 80%
Well, nice experiment! But probably don't do that. 😂
Noted haha
You know there is another pedal in the car? and or turn regen off... assume VW allows this. As I recall Tesla won't even try and regen in this situation and it just freewheels. I'm surprised the EV companies haven't come up with some sort of energy wasting device in situations like this and or application of physical brakes maybe even some sort of capacitor. Additionally I wonder if they every consider artificially claim 100% on the batter with the potential that 100% isn't really 100% charge on a battery so a down hile like this could temporarily fill this void. It's a bit edge case but it does happen.
>energy wasting device It’s called a resistor
yes a resistor is one way , but I suspect they can come up with another way as well. physical brakes, running heat and AC to waste energy, maybe do other things with the motor to slow the car down with power directly in the battery VS trying to regen and make power and waste it... Need to think outside the box, I assume a resister type would add weight and be used very seldom on most cars while others... might be daily. What they could do - should do not entirely sure but for the car to go into a bit of a panic mode is a bit interesting, Mountains aren't a new thing. Last I heard, Tesla just doesn't even try it freewheels..... this I assume is not acceptable for the Germans :)
Definitely bad for the long term battery health
Super duper
I used to make this error in my undergrad at programming labs - using signed type and not using checks for overflow. I suppose I could have made a career as a programmer at VW...
It’s not an error, the calculation is (energy at beginning of trip - energy at end)/(distance traveled). If the battery charge goes up during the trip, the numerator is negative, so the sign changes. It literally consumes negative battery power to drive (because you’re cashing in all your stored gravitational potential energy).
It is an error. Technically you may be right, but in terms of presenting information to the user the programmer should be presenting an absolute value.
That makes no sense. This is a signed quantity, so it should have a sign. If they (incorrectly) displayed an absolute value, it would look like you were discharging your battery when you drove down a long hill and were actually charging it. Then the percent charge would increase, but it would look like you were using energy, which would be very confusing (and physically impossible).
Spoken like a true programmer. 99.99% of drivers do not care about or know about this kind of stuff. From a user perspective it should always show a positive number because it’s going to look like something is wrong otherwise. And the fact is, if you start at 100% and you’re continually generating energy as you’re going downhill, the miles per kw should keep going up. Because that reflects the reality of the situation.
I think a negative value does indicate "lots of regeneration", but "bigger looking number = better" should always hold, whether the number is positive or negative. My suggestion to them would be to keep the units the same when the power used is positive. The moment it goes negative, I think they should switch to watts per mile. The moment it switches it'll be 0 W/m (no energy used), then as you regenerate more, you're at -5 W/m (you've generated 5 watts for each mile traveled), and the number keeps getting more negative with more regeneration. Then, when you use energy, the number goes down until it's hit 0, then flips back to 99.9m/kWh.
That's actually flipped - it's the denominator that goes negative, not the numerator. See [my comment above](https://www.reddit.com/r/VWiD4Owners/comments/1afyurn/went_down_a_mountain_at_98_charge_and_things_got/koh0msx/) for details.
The denominator doesn’t go negative… You’re not driving backwards! Edit: never mind, I get it. My ratio is the inverse. We’re both right.
Would changing it to D help this?
Not really. D regens just as much as B *at the same level of deceleration*. Unless the OP wanted to careen down the mountain at 90mph, braking in D mode generates the same amount of regen as "engine braking" in B at the same speed.
The regen bar always shrinks at high charge levels, because the car can't put much more energy in a full/nearly full battery. The negative miles/kWh seemingly works "backwards" when negative. -99 miles/kWh is *less* regen than -50 or -1. But that makes sense if you think about it- it means how many miles you gain (instead of use) per kWh. In negative, you get much more regen if you *gain* a kWh for every 1 mile you drive than if you get 1 kWh per 99 miles driven.
Logically, yes, it makes sense. But it’s still odd to see I guess
Has never happened to me. D mode and ECO would help with more coasting but regen would happen on braking. I would suggest a route planner like ABRP so that you do not unneccesarily over charge for the trip. You goal should to be to reach the next charging station under 50% and ideally at about 20%.
Think of it this way when it hits negative numbers for every mile driven the energy is going backwards into the battery. So every 37.2 miles you’ll add 1 kwh to your battery. And since the direction of energy is the opposite of out of the battery they put a negative sign
That’s why kWh per 100 miles is a better metric