Putting on the brakes: Regenerative braking in accident reconstruction
There can be a split-second delay before the brake lights on an electric car illuminate, indicating the driver is slowing down or stopping.
Regenerative braking is an integral part of an electric vehicle (EV) powertrain. Regenerative braking has become more common with the increased production of EVs. For many years, accident reconstructionists have discussed various deceleration factors including, but not limited to, full braking capacity and rolling resistance. As the number of electric vehicles on North American roads increases, it is important to consider how regenerative braking works.
It’s more than stop & go
Most electric vehicles are powered by various types of AC motors, depending on the manufacturer; however, the working principle of regenerative braking remains the same. The primary function of regenerative braking is to slow down the vehicle, but contrary to traditional braking systems, regenerative braking converts the kinetic energy of the vehicle into electric potential. This electric potential is used to recharge the battery, thereby increasing the range of the electric vehicle.
The rotational motion of the wheels is faced with a resistance inside the motor, which acts as a generator during regenerative braking, slowing the vehicle down. Regenerative braking has multiple advantages over traditional brakes found in an internal combustion engine (ICE) powered vehicles. For example, not all kinetic energy is lost as heat, and the rate of wear and tear to the brake components is reduced as well. It should be noted that despite all EVs being equipped with regenerative braking, they still have the traditional brake system in place for emergency braking.
Typically, in an EV, regenerative braking is activated as soon as the accelerator pedal is released. That is why some manufacturers boast about the ‘one-pedal driving’ system. Tesla provides its customers with the option to toggle between two modes of regenerative braking: standard and low. In the standard mode, the braking/deceleration experienced by the vehicle is higher than in the low mode. The low mode mimics the deceleration experienced by an ICE-powered vehicle during coast down when neither the accelerator pedal nor the brake pedal is engaged.
Regenerative braking and accident reconstruction
A key part of the testimony of a driver involved in a traffic collision is the application of the brakes. Accident reconstructionists take into account the braking behavior of the involved vehicle, in conjunction with physical evidence, to reconstruct the collision. In conventional ICE vehicles, brakes can be applied to different degrees. One could expect drivers to testify that they either did not apply brakes at all, slightly tapped the brake pedal, moderately braked or slammed the brake pedal.
Deceleration is quantified in terms of gravitational force equivalent (g) and 1g is the acceleration we feel due to the force of gravity. Typically, full braking (100% brake application) results in 0.7 g to 0.8 g’s of deceleration. It is rather a straightforward process to assign a range of g’s for each of the aforementioned levels of braking. But what happens when a driver involved in a collision says she was driving an electric vehicle and had just lifted her foot from the accelerator but didn’t touch the brake pedal; in essence, the vehicle was undergoing regenerative braking prior to the collision.
This thought lead us to a study we conducted on seven Tesla vehicles where deceleration due to regenerative braking was measured for each vehicle at six different speeds under both standard and low regenerative braking modes. The vehicles were driven up to a speed of 65 mph, 55 mph, 45 mph, 35 mph, 25 mph, and 15 mph and once the speed was maintained steadily, the test driver would then lift his foot from the accelerator, activating regenerative braking. A VBOX-3i was utilized to measure the deceleration undergone by the vehicle.
Findings
Testing showed that regenerative braking behavior on all three models tested was similar. They all exhibited multi-phase braking. The filtered data was divided into three segments: Phase 1 (ramp-up), Phase 2 (steady-state) and Phase 3 (ramp-down). Phase 2 was the steady-state deceleration, which is discussed henceforth.
The average steady-state deceleration for the two Model 3’s, three Model S’s, and two Model X’s were analyzed at various speeds. In the Standard regen mode, the average steady-state deceleration in the Model 3’s, Model S’s, and Model X’s were measured to be -0.21 g, -0.19 g, and -0.20 g, respectively. In the low mode, the values recorded were -0.11 g, -0.09 g, and -0.11 g, respectively. When compared to an ICE-powered vehicle, the average coast-down deceleration with an automatic transmission is in the range of -0.017 g to -0.024 g at 25 mph.
Brake light activation delay
Another interesting finding of this research was the delay in brake light activation due to regenerative braking. It is common knowledge that regular ICE vehicles are wired such that when the driver applies the brakes, the rear brake lights are illuminated to caution following drivers of deceleration. This can prove to be critical evidence in the realm of accident reconstruction, as dashcam videos from the following vehicle can show whether the leading vehicle applied brakes. Generally, brake light activation and the application of the brake pedal are nearly instantaneous. This, in conjunction with perception-reaction times, can be used to determine the position of both vehicles when the lead driver detected a hazard.
However, in the Tesla Model 3, Model S, and Model X, in the standard mode, it was found that there was a delay in the brake light activation when the driver adhered to regenerative braking as opposed to the application of the brake pedal. This means that once the driver lets go off the accelerator pedal, regenerative braking activates instantaneously, but then there is a delay before the brake lights come on.
This delay can be of significant use to accident reconstructionists and human factors experts. The average light activation delay (upon releasing the throttle) for the Model 3 was 0.35 sec., for the Model S it was 0.55 sec., and for the Model X it was measured to be 0.59 seconds.
In the event of a rear-end accident, the split-second delay before the brake lights activate could affect whether or not a driver following the electric car can react in time to stop and avoid an accident, especially in bumper-to-bumper traffic, if he is following too closely or is distracted.
Omair M. Siddiqui is a forensic engineer with Momentum Engineering Corp, specializing in accident reconstruction for heavy trucks, automobiles, motorcycles, bicycles and pedestrian accidents. Contact him at OMS@momentum-eng.com.
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