Extending EV Range Through Regen Braking

There are many techniques we can use to make sure we get the best range from our electric vehicles (EVs). In fact, most of the very same techniques can be used for non-EVs to but strangely, very few seem to want to reduce their fuel consumption in spite of petrol and diesel costing well over £6 per gallon. However, the ones I am going to talk about are rather more specific to electric vehicles… coasting and regenerative braking.

Regenerative braking is very specific to EVs but what exactly is it?

Electric motors are rather weird beasts. Put power into them and they can be used to power the car. However, when you lift off the throttle and allow the car to drive the motor it generates power which can be used to charge the battery. This is where the name “regeneration” comes from. The more power it generates the more difficult it is to turn the motor and that slows the car down in a way similar to how the engine does in engine braking with an internal combustion engine (ICE) car. Hence the name “regenerative braking”. This then turns the speed of the car into battery power.

Without regen braking, such as with an ICE car, the only ways to slow the car is by lifting the throttle and using engine braking alone or by pressing the brake pedal and using the brakes. Both of those ways are getting rid of the speed energy (called kinetic energy) into other forms of energy such as heat. That energy, once it is converted to heat, cannot be recovered and is always lost and that is why every time you need to slow the car you are wasting energy and the more quickly you need to slow the more energy is lost. More lost energy means more fuel used and so to get the best fuel consumption from an ICE car you need to use anticipation to allow smooth driving with minimum use of the gas and brake pedals.

In an EV with regen braking the same basic principle applies. Conserve energy whenever possible and so the same technique of good anticipation, minimum use of gas and brake pedal still applies. However the difference is that when you lift off the gas pedal instead of the cars kinetic energy (speed) being converted into heat when turning the engine it turns the electric motor and turns that kinetic energy into first electrical energy and then converts again into chemical energy in the battery and it is then recovered and available to use again later.

This is great isn’t it. Energy is saved and so regenerative braking allows for less energy wastage and so extends the range of an EV. The annoying thing though is a fact of physics that when converting energy from one form to another it is never 100% efficient. There is always some energy lost. So, although regen braking is a good thing for an EV, converting from kinetic energy (speed) to electrical energy to chemical energy (battery), it means that inevitably there will be some energy lost in the process.

However, there is an even better technique EV drivers can use to extend the range even further. Instead of lifting off the throttle and allowing the car’s speed to drive the motor and hence using regen braking to slow the car and charge the battery a bit, you might be able to use even more anticipation and lifting just enough not to drive the car but not lifting enough to regen brake. It is a mid-way point between using power and generating it. In this state the car will still slow down because of rolling resistance, wind resistance etc but as far as the EV electrical system is concerned the car is in a kind of neutral state… it isn’t losing power and it isn’t gaining it. This is in a state of “coasting”. Coasting isn’t simply a result of lifting off the gas pedal. In most EVs that would cause some regen braking. Coasting is where there is no power being gained from regen braking and no power being used either. It often requires some delicate balancing on the throttle or brake pedal to achieve a coasting state.

So why is coasting so important? In a coasting state it is the most efficient way an EV can run. You are not using power but neither are you regen braking and as we have seen, regen braking results in some power losses through conversion of energy from one form (kinetic) to another (electrical and chemical). So, by coasting when slowing down we do not unnecessarily waste energy thereby maximising the range possible.

Now some of you might be saying that you can’t always coast. Slowing down by coasting can often be difficult or just not possible at all. In normal traffic coasting to slow down can annoy the cars behind because by not using the brake it needs you to leave a much bigger gap to the car in front. I am not for a moment suggesting that coasting, or regen braking, should be used when it is not appropriate. Often, particularly in busy traffic, it is difficult to use either. However, when it is appropriate, and when it is safe to do so such as in very light traffic with no cars behind, then doing so will extend your range.

I think it is fair to say that it won’t extend range dramatically but by using coasting and regen braking as a matter of habit when you can it all mounts up and it might make a difference between running out of power or not.

EV drivers then have regen braking and coasting as additional techniques to help extend range but which one should we use when there is a choice? As regen braking wastes some energy the choice should always be coasting first then regen braking if you need to slow a little more quickly then normal brakes when you need to slow even more quickly.

In the Nissan Leaf there are a couple of ways to determine whether the car is coasting or regen braking. Firstly there are the power use dots. Dots light up to the right when the car is using power and to the left when regenerating power. The single dot indicates that it is very close to coasting. This is the easiest way to find the coasting point. If you can drive the car with as few dots alight as possible, ideally just the one, then you will be running at the cars most efficient giving you the best range. To help find the coast point even more accurately there is a page in the information area directly in front of the driver. With this display there is a bar that moves from left to right as more power is used. keeping that bar as close as possible to the left will get you closer to the coast point. Be careful not to go inadvertently into regen braking though. You want to always see a slither of white bar just so you know you are not regen braking. If you do as I do and use this display page in my normal driving it soon then becomes second nature finding the coast point and the displays become less important.

Coasting and regen braking are not the only techniques that we can use to maximise the range but by using them alongside good anticipation and other eco-driving techniques, whenever it is safe and appropriate to do so, we can get the most from our EV.

One thought on “Extending EV Range Through Regen Braking”

  1. Thanks for your comment Scott.

    I don't have the definitive answer and my thoughts, like yours, are more by discovery than anything official.

    However, the way I uderstand it is:

    – if the battery is 100% charged then all braking is mechanical until such time as the regen rings light up and then regen becomes available.

    – if you press the brake pedal and command a little braking that can be satisfied by regen only then it will not use mechanical braking at all.

    – if you command more braking than is available by regen alone (that varied depending on battery state, temperature and other factors) then it will give you max regen plus sufficient mechanical blended) to give you the braking required.

    – when the car slows to such a speed that regen is not possible (about 5 mph) so the mechanical brakes gradually take over as regen reduces so that the requested braking is maintained.

    In my experience the blend from regen to mechanical when almost stopped is not 100% smooth and could be improved but it isn't at all bad.

    That is about all I know and even that is mostly conjecture based on observation.

    I hope that helps.

    Paul

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