Regenerative Braking

Back in 1999, we rented the now famous GM EV1 on a couple of occasions. Long interested in both automobiles and technology, it was only natural for me to be interested in what that project attempted and accomplished.

One of the common press buzzwords for the EV1 was "regenerative braking." GM engineers would refer to it as "regen." They promise of regen was that the EV1 would attempt to recoup some of the energy wasted during deceleration. This energy would put back into the battery pack, rather than lost as heat, as is the case with traditional internal-combustion vehicles and friction braking systems (which the EV1 also utilized).

After reading much hype about the complex engineering and motor/charging control system programming involved in the EV1's regenerative braking system design, I was disappointed when I finally saw empirical data of the increased range. I think that GM claimed something like three or four additional miles of range, and though the EV1 publicized a maximum range of over 100 miles, real-world range was more like 70. So the benefits were single-digit percentage of range improvement, at best.

To be fair, if they really got 5 or 6 per cent improvement, that's pretty impressive, especially given that those first-generation EV1s used lead-acid batteries. Lead-acid batteries take on charge at a significantly lower rate than the lithium-ion and lithium-polymer packs that power today's EVs and hybrids.

Most if not all of today's mass-produced plug-in electric vehicles and hybrid vehicles employ some sort of regenerative braking system in an attempt to increase the range/efficiency of these vehicles. The millions of hybrid vehicles that have been sold utilize regenerative braking systems and small battery packs to improve the energy efficiency of those vehicles.

SO WHAT IS REGENERATIVE BRAKING?

The idea behind regenerative braking is to exploit any opportunity in which the momentum of the vehicle needs to be diminished, and store as much of this energy harvested from either slowing the vehicle or maintaining speed while decreasing altitude on a downhill grade. The friction brakes employed in automobiles (including EVs) convert momentum into heat. When the throttle is lifted on an internal-combustion vehicle, the the vehicle's momentum is also converted into heat at it compresses air pumping through the engine (in a manual-transmission vehicle) or churns the fluid inside the automatic transmission's torque converter. In pursuit of greater efficiency, vehicles with regenerative braking attempt to replace these traditional momentum-transferring mechanisms with systems that store as much of that energy as possible for later use.

Beginning almost a century ago, commuter light rail cars and buses have used mechanical storage mechanisms for this purpose, either spinning up a heavy flywheel or even winding a large spring mechanism as part of the braking system. When these frequent-stopping public transit vehicles were ready to depart for the next stop, the operator released the mechanically-stored energy to assist the vehicle's normal propulsion source in getting the vehicle moving from a standstill. Crude as these systems might seem today, the ideas are still valid (and still in use, in some cases) and provide useful energy-reducing benefits by salvaging some of the energy normally lost as heat.

Today, in addition to the more well-publicized electrical battery regen systems employed in vehicles, there are ultra-high pressure compressed gas batteries used in urban public transit and delivery vehicles in much the same way as those flywheel and spring systems, to store as much energy as possible from any given stop to offset the enormous task of overcoming the loaded vehicle's inertia when stopped. Though none have been put into practical use in vehicles, many experiments have utilized high-speed electrically-powered flywheels as batteries for storing and returning energy from braking.

ELECTRIC REGEN

Electric motors and electric generators are very similar. Indeed, many electric motor designs function very well as generators.

Thought Experiment: Two identical, high-efficiency, permanent magnet electric motors are mounted on a tabletop. On each of these motors driveshafts is mounted a hand-crank. Between the two motors are connected two wires, so that the two motors and wires complete a circuit. If the hand crank of one motor is spun with sufficient speed and force, the other motor will begin to turn. If instead, the hand crank is turned on the second motor, the first motor will turn from the electrical energy passing through the circuit. If while turning the "generator" crank someone else places a load on the "motor" - perhaps by dragging their hand on the motor's spinning shaft/crank, the person cranking the generator will feel the effort increase. Likewise, if a low wattage light bulb powered by the generator is replaced by a higher-wattage bulb, the generator operator will feel the additional effort. Note that when these experiments are performed, both motors, the light bulbs and the wires are likely to become warm to the touch. Some of the heat is from mechanical friction from the moving parts of the motors, but most is from electrical resistance. This is evidence of energy leaving the system in the form of heat, and thus a loss of efficiency. This loss is in practice unavoidable. 

Electrically-based regen systems use an electric generator - typically the very same electric motor used for propulsion - and an electronic control system to reverse the flow of electrons from the battery to the motor whenever the system detects an opportunity to do so. As in the thought experiment above, applying regen creates a torque load in the opposite direction of travel to wheels connected to the motor, so regen systems must be designed not to upset the stability of the vehicle through excessive application of this braking torque (i.e., locking up the driving wheels in slippery conditions because the braking torque is too high). But it should be as aggressive as possible to reap the maximum efficiency.

Regenerative braking systems will attempt to "harvest" the momentum of the vehicle under several conditions:

• when the driver applies the brake pedal
• the regen system attempts to achieve maximum generator braking torque, but if this is inadequate to the request signaled by the driver's brake pedal pressure (i.e., an emergency), then the friction brakes must work in concert, and have priority
• if the brake pedal pressure is below a certain threshold, then the system has the opportunity to modulate braking torque entirely through regen, with no friction braking
• below a certain road speed, motor regen no longer generates effective braking torque, and so a transition from regen to friction braking must take place during a full stop
• A common malady of vehicles employing regenerative braking is that the transition to friction braking is typically non-linear. Most often (at least in my regen driving experiences of about a dozen different vehicles), there is a sudden increase in braking effect as the friction brakes take over. I think this is chosen as a more desirable transition than having the braking effect suddenly diminish, but it causes drivers unfamiliar with those cars to nose-dive during this "grabby" increase in braking effect. With some practice, one learns to feather off the brake pedal just at the transition. 
• when the throttle position is insufficient to maintain current speed for the current conditions
• when the vehicle encounters a downhill grade or tailwind
• the regen system will attempt to convert excess momentum to battery charge
• depending upon how much the throttle is lifted, and which regenerative braking mode is selected, the system attempts to slow the vehicle with motor braking/regen

Different manufacturers have different philosophies and strategies about how aggressively their regen systems attempt to harvest. Some vehicles (including our Ford Focus Electric) provide the driver with feedback for driving behavior that maximizes regenerative braking gains. These vehicles coach the driver with visual display aids to generally brake over longer distances at modest deceleration rates, which allow the regen system to perform nearly all the speed abatement, while using the friction brakes as little as possible to avoid needless energy loss as heat.

It's important to grasp that it's impossible to recoup all of the energy from a moving vehicle's momentum with a regenerative braking system. Some energy will inevitably be lost as heat from friction and other mechanical inefficiencies, and there will be loss in electrical and electronic control systems. But systems have become efficient enough for vehicle manufacturers to profit by manufacturing and selling them, and for vehicle owners benefit from measurable energy-use reductions. If you drive up a 400 foot incline and the regen system harvests during the 400 foot descent, you still use significantly more energy lost as heat than if the road were level.

IS DRIVING A CAR WITH REGENERATIVE BRAKING DIFFERENT?

I would say that there is a place and time for a car's regen to be completely undetectable, but for certain users - particularly the current crop of early-adopters of EV and regen technology - having noticeable differences in operation due to regenerative braking is a desirable trait. Certainly those vehicle owners who wish to take a more active role in exploiting the energy-saving benefits of EV technology - the same population who coined the term "hypermiling" to describe challenging oneself to drive their vehicle while using as little fuel as possible - are not only willing to accept additional consequences of the technology, but embrace them.

REGEN-BRAKE TRANSITION

As much as manufacturers would like to make cars with regenerative braking seem absolutely no different to the user than any other vehicle they've operated, such has (in my opinion) not yet been achieved. I haven't driven every vehicle equipped with regen, but I've driven several examples from many different manufacturers, most of whom have developed their own regen technology. And they all suffer from a similar issue which I'll call poor regen-brake transition.

A difficult task engineers face with regen is that for maximum recovery of energy during deceleration and hill descent, friction-braking should take as little part in the process as possible. But for reasons of safety, cost-efficiency and common sense, friction-brakes must be part of the vehicle's braking system. This is partly because the electrical braking torque available in a given vehicle regen system is never adequate for maximum braking, and because electrical generators no longer function effectively below a certain rotational speed. So during a given traffic stop, the system controller will somewhat suddenly hand over braking duties from regen (or a mix of regen/brake, during heavier braking) to friction-brake only. Because electrical braking torque is dependent upon battery load in a regen system, there are conditions under which regen is typically unavailable.

During a typical traffic stop, the EV's control system will attempt to harvest as much energy as possible during the moments that the driver indicates that they wish to lose velocity by pressing on the brake pedal. If the brake pedal is pressed moderately over a long, gentle stop (which several EVs encourage through dashboard brake-coaching displays), the motor controller will keep the wheels engaged to the motor - now functioning as a generator - and route the generated power into the battery pack. If the brake pedal pressure indicates a need for braking which exceeds regen, then traditional friction brakes continue to work as in a conventional vehicle.

ALTERNATE REGEN MODES

Many vehicles with regenerative braking offer two distinctively different regen modes, both of which actually affect the behavior of the vehicle with respect to throttle (and perhaps should actually be called "throttle modes"). Neither of these modes causes the vehicle to behave as a typical internal-combustion vehicle with an automatic transmission. The two modes have varying names, but the concepts are the same:

• Normal, "coasting" mode - When the throttle is lifted, the vehicle provides NO additional braking force except mechanical friction from moving parts and aerodynamic drag. Because most EVs and hybrids deliberately use low-drag bodywork and even tires, very little speed loss results from decreasing the throttle at medium and low speeds (where aero drag has less effect). In the most extreme case of differences between Internal Combustion (IC) cars and low-resistance EVs, lifting the throttle may give the driver the impression that the throttle is still applied, because the deceleration is nearly imperceptible.
• IC cars with automatic transmissions provide significant "engine braking": the engine is partially coupled to the road wheels through the torque converter, and when engine speed is reduced, a braking torque is applied to the wheels. So we're all accustomed to a certain deceleration rate when we lift off the throttle. EVs in "coast" mode barely slow down in an attempt to preserve momentum.
• "Low gear," "Braking," or "Regen" mode - When this mode is selected (often using the vehicle's "shifter," even though this is actually an electrical or software change), the regen system responds to any reduction in throttle position immediately, aggressively slowing the vehicle through electrical braking torque, and sending any harvested electrical energy to the battery pack for storage. Manufacturers have a difficult time explaining this mode, and why the user might employ it. Most manufacturers present the feature in much the same way as manually selecting a lower gear (2nd or 3rd gear) in an IC car with an automatic transmission to provide engine braking on long downhill descents. But then, most people never use that feature of IC cars, and most people with EVs won't do a lot of long downhills. My thoughts about Braking/Low Mode:
• To get the most out of regenerative braking, I drive in this mode most of the time, except during high-speed highway driving. However, it takes a bit of practice to drive smoothly.
• This is far more demanding of the driver. Driving in this mode requires disciplined control of your throttle foot. In most EVs, I liken the effect to driving a 5-speed manual vehicle in a gear about halfway between 2nd and 3rd. Lifting abruptly off the throttle in this mode at 60 mph causes enough deceleration that it could alarm passengers, and in traffic, creates the potential hazard of slowing you significantly while not activating brake lights. With throttle practice and experience, it need feel no different than any other car.
• There is NO DIFFERENCE between holding the throttle in a position in this mode that slowly loses speed and lifting off the throttle in "coast" mode.
• It would certainly be possible to use more energy through unnecessary slowing in this mode.
• I recommend against using this mode while using cruise control, since canceling cruise then results in somewhat more abrupt slowing than conventional cars.

NOTE: I presented the tabletop generator/motor experiment to illustrate that the braking torque utilized in regenerative braking systems depends upon an electrical load. In the case of regen, that load is a partially-discharged battery. In the case of our Ford Focus Electric, if I leave home with a fully-charged battery pack and put the car in "Low" mode, I get no braking torque effect for the first mile or so of operation, because there is no discharged battery to provide a resistive load. In fact, occasionally if I happen to be in Low and braking for a traffic stop during that first few minutes of operation, the Focus might abruptly slow as it suddenly adds regen braking torque when the battery pack falls below full charge. Toyota Prius hybrids apparently maintain their batteries at around 40 to 60 per cent of their full capacity, so that they can always have "headroom for regenerative braking." 

SO WHICH REGEN MODE SHOULD I USE?

If this all sounds like a lot of trouble, don't worry about it. Just select the default drive "coast" mode and have a good life. You may initially feel as though your car isn't slowing down as much as it should when you lift off the throttle, but that's by design. 

There is lively discussion in online forums about which of these modes is "best," or most efficient. Personally, I prefer the idea that if I see an opportunity for maximum regen harvest (a traffic light turns yellow ahead), that it's easier to fully lift off the throttle than to apply only enough brake pressure to trigger regen, but not so far that I waste precious momentum in friction braking. So thus far, I've tended to stay in "Low" mode in our Focus Electric as much as possible during city driving. I operate in Drive mode on the highway to avoid subjecting cars behind me to unexpected slowing without any brake lights, but if I slowing traffic or am approaching an impending exit ramp, I'll throw the vehicle into Low mode for maximum regen. I'm an "involved" driver in any kind of vehicle, so this isn't an imposition for me, but for most drivers, I think this would be too much to do. (I intend to experimentally drive in normal Drive mode for an extended period to compare efficiency results.) Initially, it may be tricky to gently transition off-throttle, but as with most things, one becomes accustomed to it with practice.

HOW WELL DOES REGEN WORK?

Our Focus Electric reports than in 2,885 miles of operation, 627 miles were from regenerative braking. Since I have no way to disable its regenerative braking, I can't provide a comparative figure, and I have to take their word for the reported figure. But if it's accurate, then regen has saved us almost 28% of our energy cost.

(I just noticed for the first time that our Focus Electric has occasionally logged my wife's wireless key fob as the current driver, even though I've almost exclusively driven the car. And the dashboard display only shows statistics from the currently logged key fob. So I just updated the figures above to reflect the 159 miles previously excluded from calculation. That makes for an even more impressive effect than the 19% energy savings I previously cited.)

While the big picture of ecological impact of the manufacture, servicing and recycling of battery electric hybrids is still in question, manufacturers and government organizations have been convinced enough of regenerative braking's validity that increasing numbers of automobile models are adopting the strategy to achieve energy and emissions goals. 

As energy storage technologies continue to mature, regenerative braking will play a incrementally larger role in our energy and transportation future.