When testing electric vehicles (EVs) for range in laboratories using a dynamometer, engineers often do not see the same EV’s range on the road. So the first step is isolating each of the variables and studying how they impact EV range individually.
This article explains how regenerative braking, cabin heating and cooling, and driving speed profiles affect EV range, with data and numbers to support the discussion.
How does regenerative braking efficiency change with speed?
Differences in regenerative braking efficiency occur when the electric vehicle operates at low versus high speeds. At low speeds (usually 20km/hr or less), the regenerative braking efficiency is very low. This phenomenon occurs because, at low speeds, regenerative braking is usually disabled due to reduced kinetic energy.
As speed increases beyond 20 km/hr, the regenerative mechanism captures a useful share of the kinetic energy, which continues to increase with speed. When we look at Figure 1, it is clear that the entire graph can be split at the 20 km/h mark. The dataset covers five BEVs logging braking regeneration efficiency across a full speed range.
Four of the five vehicles show braking regeneration efficiency between 90 and 100% at speeds above 30 km/h. The fifth trails the group at 80 to 88%. The steep low-speed ramp matters most in city driving, where braking events are pronounced at below 20 km/h.
How much range does HVAC load cost in cold weather?
Ambient temperature testing on a chassis dynamometer usually happens inside a thermal chamber. That setup lets engineers isolate temperature as a single variable to understand its effect.
The isolation matters because there are at least two different mechanisms that drive the range losses during cold weather. On the one hand, cold temperatures slow lithium-ion reactions and raise a cell’s internal resistance. This in turn reduces the usable battery energy (UBE) available at a given state of charge.
On the other hand, the cabin needs more than the usual energy from the battery during winter to keep itself at the vehicle’s comfortable interior temperature.
A recent study tested a Chevrolet Bolt and a Nissan Leaf across four temperature conditions on the Urban Dynamometer Driving Schedule (UDDS) and the Highway Fuel Economy Test (HWFET) cycles. Each test isolated the range contribution of UBE, HVAC load, and motor losses separately at every temperature.
As shown in Figure 2, HVAC load is the dominant contributor to range loss for both vehicles at -18° and -7° C. It exceeds usable battery energy loss and motor loss combined at those temperatures.
At -18° C, the Bolt loses roughly 350 km of its 647 km UDDS baseline to HVAC alone. The Leaf shows a smaller version of the same pattern, losing roughly 260 of its 523 km baseline to HVAC at the same temperature.
However, the pattern flips at 35° C. Motor losses actually add back to the vehicle range, while HVAC still subtracts range to run cabin cooling. Cabin heating, in other words, costs considerably more range than cooling does across the conditions tested.
How accurate are dynamometer-based EV range estimates?
Aerodynamic force rises with the square of speed, so the power demand at highway speed grows much faster than speed itself. Meanwhile, drivetrain efficiency typically peaks somewhere in the middle of a motor’s operating range rather than at the lowest speed tested. Those two effects combine to produce a consumption curve that is not simply lowest at low speed and highest at high speed.
Table 1 presents a dynamometer test on a Volkswagen e-Up that quantified the combined effect of five velocity profiles by measuring distance and energy consumption for each profile.
As shown in Table 1, energy consumption ranges from 8.35 kWh per 100 km at a 55 km/h profile, the most efficient point tested, up to 23.94 kWh per 100 km at 130 km/h. That is nearly a threefold swing from driving speed alone.
The 35 km/h profile measured 11.69 kWh per 100 km, higher than the 55 km/h result. This means that the lowest speed profile is not necessarily the most efficient.
Summary
If you are validating an EV range model, do not depend on specific conditions and variables of the dynamometer. The variables that we discussed above are a few among others, but they have a pronounced effect on EV range.
Pay special attention to correcting the HVAC load, as it is often overlooked. Correct for the actual driving profile, since speed alone can swing consumption by nearly three times.
References
- Effects of Ambient Temperature on Electric Vehicle Range Considering Battery Performance, Powertrain Efficiency, and HVAC Load, U.S. Department of Energy
- Data-driven evaluation of electric vehicle energy consumption for generalizing standard testing to real-world driving, Patterns, Cell Press Journal
- Battery Electric Vehicle Efficiency Test for Various Velocities, Vehicles, MDPI
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