Regenerative braking question

[Ciprico1]

So going down a reasonably steep incline, if I take my foot off the "gas" the car slows to eventual halt.

So I need to keep the pedal slightly depressed to maintained equal speed - I assume this sends some current to the motor to decrease its resistance and regen capability

So it feels like I have to use some juice to drive down a hill....

If I knock the car into neutral the car accerates under gravity.

So my question is, if I'm going down a reasonably steep hill, but have to keep the pedal depressed to maintain speed, am I actually charging or discharging the battery....?

Also - out of curiousity, is going down a short very steep hill equivalent in terms of regen to going down a longer less steep hill - all other things being equal, ie does the battery deal with a short sharp burst of energy the
same as a longer less intense exposure to energy.

I apologise in advance for my lack of knowledge of electric motors and EV's...

Maybe I am just over thinking !!

 

[eNate]

You're not overthinking it, but consider the accelerator pedal to be an input device to a computer, not a direct connection to the motor.

The software running this system is designed to allow the driver to have one-pedal control over the speed, even while the car is pointed down a steep hill. So there's an engineering-made (i.e. human made) decision here, not some some random result of vehicle weight and slope vs. regenerative-caused resistance.

Watch the power meter under the speedometer to get a sense of how much electricity is flowing back towards the battery in this scenario.

 

[SSi3]

Use the BC button to toggle to the realtime power usage info and watch it to see what its doing in comparison to your pedal behaviour. And as eNate points out, also watch the swinging power meter, the centre is "neutral", the car essentially free-wheels there, it takes a soft foot to free-wheel down a steep hill, but once you get the hang of it it's easy and fun!

 

[TheAirman]

The owners manual says that if you keep the power meter pointer right in the center, you are actually coasting.

Not really coasting in a sense in my mind, but since the manual is a poor translation from German, maybe what they mean is it's not using energy, nor recovering it.

 

[alohart]

The owners manual says that if you keep the power meter pointer right in the center, you are actually coasting.

Not really coasting in a sense in my mind, but since the manual is a poor translation from German, maybe what they mean is it's not using energy, nor recovering it.

Because the motor is neither consuming nor producing energy when the energy gauge is centered, the motor is spinning without much resistance other than bearing friction, so that would be coasting to me. That's the same as when the shifter is in the 'N' position.

 

[TheAirman]

Agreed. But maybe "coasting" in an energy use sense. But the effects on the car are different than if you put it neutral and actually coasted.

Going down a hill (keeping the meter in the center) the car seems to maintain the speed. Put it neutral, and the car gains speed.

 

[JoeCool88]

Agreed. But maybe "coasting" in an energy use sense. But the effects on the car are different than if you put it neutral and actually coasted.

Going down a hill (keeping the meter in the center) the car seems to maintain the speed. Put it neutral, and the car gains speed.

It will depend on the glide angle if the car maintains speed or even accelerates when you keep the meter centered. Due to the charging losses during recuperation, the most efficient way to drive downhill is keeping the meter centered until reaching your desired speed or a given speed limit. And then using the remaining slope downforce (if any) to charge your battery by recuperation in keeping this speed (by lifting the pedal). By the way the German expression for coasting is "segeln", which means sailing.

 

[Clanduncan]

In the UK it's not unusual for me to gain back 2-3% on downward hills, all while keeping slight pressure on the pedal to maintain a safe speed. Balance that with the thought if you reverse your journey you're still using power.

 

[Badgersbits]

So going down a reasonably steep incline, if I take my foot off the "gas" the car slows to eventual halt.

So I need to keep the pedal slightly depressed to maintained equal speed - I assume this sends some current to the motor to decrease its resistance and regen capability

So it feels like I have to use some juice to drive down a hill....

If I knock the car into neutral the car accerates under gravity.

So my question is, if I'm going down a reasonably steep hill, but have to keep the pedal depressed to maintain speed, am I actually charging or discharging the battery....?

Also - out of curiousity, is going down a short very steep hill equivalent in terms of regen to going down a longer less steep hill - all other things being equal, ie does the battery deal with a short sharp burst of energy the
same as a longer less intense exposure to energy.

I apologise in advance for my lack of knowledge of electric motors and EV's...

Maybe I am just over thinking !!

The regen process is only around 60% efficient, due to losses in the motor (generator during regen) and the power electronics. So the most efficient way to slow down is to keep the power meter centralised, as others have said. No power is being consumed in this state. So from the same height hill, a gentle slope will usually consume less power, because you can effectively coast. A steeper gradient will cause you to use regen and possibly the brakes, wasting some of the potential energy you had at the top of the hill.

 

[Mike B]

If you are going down a "reasonably" steep hill, and maintaining the speed you want, the chances are you will get some regen charging. All you have to do is look at the screen in front of you to see what the car says is happening. I assume you are driving a BMW i3.

 

[eNate]

...the most efficient way to drive downhill is keeping the meter centered until reaching your desired speed or a given speed limit. And then using the remaining slope downforce (if any) to charge your battery by recuperation in keeping this speed (by lifting the pedal).

This seems like an overly complex alternative explanation to "use the accelerator pedal to drive at the desired speed and the car will coast/regen/apply drive power as necessary."

 

[TheAirman]

This seems like an overly complex alternative explanation to "use the accelerator pedal to drive at the desired speed and the car will coast/regen/apply drive power as necessary."

Just the way the Germans like it in their cars. "Overly complex."

 

[McDuff]

The owners manual says that if you keep the power meter pointer right in the center, you are actually coasting.

Not really coasting in a sense in my mind, but since the manual is a poor translation from German, maybe what they mean is it's not using energy, nor recovering it.

the part of the manual that I read a few days ago mentioned "coasting" with reference to having selected Neutral, so I don't think that the translation is that far out. And if you have the power meter centred you are putting nothing in but getting nothing out, so a downhill (or following wind, perhaps ) stretch would give you coasting.

 

[Fordychap]

So going down a reasonably steep incline, if I take my foot off the "gas" the car slows to eventual halt.

So I need to keep the pedal slightly depressed to maintained equal speed - I assume this sends some current to the motor to decrease its resistance and regen capability

So it feels like I have to use some juice to drive down a hill....

If I knock the car into neutral the car accerates under gravity.

So my question is, if I'm going down a reasonably steep hill, but have to keep the pedal depressed to maintain speed, am I actually charging or discharging the battery....?

Also - out of curiousity, is going down a short very steep hill equivalent in terms of regen to going down a longer less steep hill - all other things being equal, ie does the battery deal with a short sharp burst of energy the
same as a longer less intense exposure to energy.

I apologise in advance for my lack of knowledge of electric motors and EV's...

Maybe I am just over thinking !!

A tip (or more of a guide), that I wish I had been given at the beginning of our five years of i3 experience (and we have two battery only i3’s) is that the “ideal” when driving an i3 is to use the “accelerator” 99% of the time to control your speed.
Only use the brakes if you need to, and just back off the accelerator gently to slow down - if you can safely.
The white blob indicator “the swingometer” is key, and the more “centred” you can keep it, the better.
Slower acceleration, and gentle deceleration are key - and your range benefits markedly if you learn to keep the white blob in the centre lower 1/5 of the dial where the bars are thickest, plus the drive is smooth and relaxed..
Practice that, keeping it mostly centred using your right foot, and it will become second nature.
Of course there are always times when an i3 is a hoot to drive at the greater extremes - and the low centre of gravity certainly makes it entertaining...

 

[Ciprico1]

...my question really was if you find a hill that the car would freewheel down in neutral without gaining or losing speed....

And then if you drive down the same hill "in gear" you will need to depress the accelerator.

In theory both these scenarios should be taking no power from the battery.

...but as you have to press the accelerator pedal in the second case it would seem there is some draw from the battery....?


The swingometer shows us what BMW want us to see. They may not want us to see slight current draw whilst driving down slight inclines.

Compare to a petrol car if driving down a hill in gear you may need to depress the accelerator to remove the braking effect of the engine to maintain speed.

The EV may need to draw some power (much less than ice)
To negate the effect of turning the motor, before real regen kicks in?

Ok this doesn't really matter, and using the brakes is clearly best avoided, and free wheeling in neutral also probably not such a great idea...

 

[eNate]

as you have to press the accelerator pedal in the second case it would seem there is some draw from the battery

I don't think so. If you coast down a really steep hill, the car absolutely creeps without any throttle input. There's clearly something holding it back -- probably regen, but could even be electrical resistance braking.

Gently goosing the throttle in that scenario isn't supplying power to the motor, it's acting as a "reverse throttle" of sorts and releasing or decreasing whichever force is holding the car back.

This gets back to the throttle pedal not being connected to a rheostat in the sense of a simple motor controller, but an input device to the computer that's communicating "I want to go faster than this" (or slower...). The computer takes it from there and figures out what parameters need to be changed to accomplish this.

 

[i3mart]

There isn't an actual Neutral gear in the i3 anyway, you can't disconnect the drive wheels from the electric machine rotor.

 

[eecummings]

Too much overthinking. If the regen is holding the car at a constant speed down a steep incline, then it's charging the battery.

 

[Fordychap]

...my question really was if you find a hill that the car would freewheel down in neutral without gaining or losing speed...

And then if you drive down the same hill "in gear" you will need to depress the accelerator.

In theory both these scenarios should be taking no power from the battery.

...but as you have to press the accelerator pedal in the second case it would seem there is some draw from the battery....?


The swingometer shows us what BMW want us to see. They may not want us to see slight current draw whilst driving down slight inclines.

Compare to a petrol car if driving down a hill in gear you may need to depress the accelerator to remove the braking effect of the engine to maintain speed.

The EV may need to draw some power (much less than ice)
To negate the effect of turning the motor, before real regen kicks in?

Ok this doesn't really matter, and using the brakes is clearly best avoided, and free wheeling in neutral also probably not such a great idea...

Actually, I think, having used an ODBC-connected battery monitor and battery monitoring software, the main motor (power) battery is neither charging nor discharging when the swingometer is in the middle.

Don’t forget all the ancillary circuits are powered by the 12V battery.

The only way I find this indicator misleading is that the regeneration side runs to a lower maximum - since the regen is affected by the poor performance of an electric motor used for traction being used as a generator.

The power consumption side however runs as you would expect - 0-100% of the selected power range limits - (Eco Pro or Comfort for example) - and the throttle mapping is different too.

Personally, I believe that if you keep your right foot managing the swingometer - you won’t go far wrong.
I often “coast” down hills, just keeping the swingometer in the central part, but often find it needs slight regen to keep the best speed.

In my experience that modest regen and modest acceleration are the key to good range since that is where everything is at optimum efficiency.

 

[Fordychap]

I hope that helped!