Range estimates are important for electric vehicle drivers, especially when traveling long distances along routes with sparse fast charge infrastructure. Even though EVs do provide range information, drivers do not have much useful real-time information on how environmental conditions of the drive, or elevation changes of the route, are affecting the vehicle's energy use.
For example, short term miles per kilowatt-hour (miles/kWh) or watt-hour per mile (Wh/mile) information is strongly affected by the slope of the road and recent speed changes. However, this is very difficult to use for range estimation, especially on an unfamiliar route, and does not provide much useful feedback to the driver so they can adapt to current conditions.
The fundamental concept of the Total Energy Meter system here is that there are actually three important energy storage mechanisms, which may be intuitively thought of as ‘batteries.’ The first is the potential energy a battery stores from elevation changes. Second is the kinetic energy a battery stores from the vehicle’s speed and mass. Third is the chemical battery that stores the electrical energy.
The Total Energy in these three batteries accurately represents the energy available to the vehicle. Altitude and speed changes merely transfer energy between the three batteries, so the Total Energy consumption represents energy actually being dissipated to the environment by aerodynamic drag, friction, electrical losses, and climate control.
With real-time Total Energy Wh/mile information, a driver can easily adjust their vehicle’s speed and climate settings to stay within an energy budget and achieve a desired range, even in difficult environmental conditions such as hilly terrain, high winds, rain or snow, and extreme temperatures.
How the Total Energy Meter Works
It may be useful to consider the following energy equivalents for a ‘typical’ 2000kg, 260Wh/mi (@65mph) EV: The EV traveling at 65mph has 234Wh of kinetic energy, which represents 0.9 miles of range; On a road with 3.0% down slope the EV will coast at 65mph with no power; The potential energy of a 1000m elevation change is 5.45kWh, which represents 21 miles of range.
In order to provide accurate range prediction in varying driving conditions, it important to determine the energy that is truly being lost to the environment in the form of friction, aerodynamic drag, electrical losses, and auxiliary loads, and not to contaminate this with energy that is merely being transferred between the vehicle’s ‘batteries.’
Vehicle instrumentation that calculates the true energy use (Wh/mi) using the total of the 3 ‘batteries’ can be used to extrapolate accurate range estimations from the most recent few miles of driving. It can also provide the driver with meaningful real-time feedback on their driving choices (such as speed, climate control, cargo racks, and tire pressure) that can be easily interpreted to ensure that a desired range is attained.
An EV with total energy metering will indicate an energy use (Wh/mi, to be preferred over the mi/kWh shown by some) that remains relatively constant whether the vehicle is on a level road, climbing a grade, or descending. The Wh/mi number will accurately reflect the effect of driving speed, headwinds, temperature, rain, and A/C load on the vehicle’s actual power dissipation even over hilly terrain. As the effects of speed changes (kinetic energy) are properly accounted for, the short term energy use, averaged over only a fraction of a mile, is quite a smooth function during city driving.
When offered more usable energy feedback, there is the potential that a driver may learn to optimize their driving efficiency and enjoy enhanced vehicle utility with reduced energy consumption, battery degradation, and range anxiety.
The Elevation Measurement Problem
Most EVs already have GPS, and this provides altitude information. The short term error of the GPS altitude can be several meters, especially in urban or mountain environments. For 4 percent accuracy of the total energy Wh/mi over a specific distance, for example quarter mile, the altitude must have less than 0.5m error. Another measurement method is required for short term accuracy.
A practical solution has been to use a sensitive longitudinal accelerometer to measure the slope that the vehicle is driving on. For the same accuracy as above, the slope needs a precision of 0.12 percent, or a few mm over the wheelbase. As the sensor must be mounted to the chassis (not the road surface!), the variations of the suspension loads and tire deflections introduce errors greater than desired.
The complete solution has been to use the GPS altitude data (which has excellent long term precision), averaged over several miles, to adjust the accelerometer null used in the total energy calculation. It is interesting to note that the time integral of (accel * mass * speed) is the sum of the potential and kinetic energies, exactly what is needed for the total energy meter system.
There is another detail that needs to be considered when deciding where to mount the accelerometer in the chassis. The location should minimize the cross coupling between the lateral g generated in turns to the desired measurement of longitudinal acceleration. Fortunately most EV s only steer the front wheels, so a location above the rear axle ensures that the lateral g forces are orthogonal to the longitudinal axis of the vehicle. Lower in the chassis is also preferable, as pitch oscillations have less effect.
The EV total energy meter is not just a theoretical discussion. A prototype was developed during the last year and has been implemented in a Hyundai Kona EV. The system has been tested in a wide variety of driving conditions.
The prototype uses a Windows tablet PC with a Bluetooth link to the vehicle’s OBDII port to get battery state of charge (SOC), volts, amps, and motor rpm. This is combined with accelerometer data and GPS altitude to calculate and display energy use information. As the software in the tablet is not linked to the NAV system, the user manually enters the destination altitude for the range calculation.
For an OEM implementation, the only additional hardware requirement over what is currently in most EV s is the accelerometer, which can use a $2 sensor chip and needs to be connected to the vehicle CAN bus. The vehicle dashboard computer could handle the data processing and display.
The prototype system display is for engineering test and evaluation, but much of the basic functionality could be applied to a consumer oriented implementation. This view of the touchscreen shows the range display tab.
The calculated remaining range can either be based on the “E.use” Wh/mi evaluated over the last x miles (“eval dist” user select), or on a target Wh/mile number entered by the user. The target Wh/mi mode has proven valuable when it is important to ensure a desired range is attained; as long as measured energy use is kept below the target, the range requirement will be met.
Note how the blue Wh/mi trace is not affected by altitude changes (red), but does reflect the effect of different driving speeds (white), from 65 mph freeway driving to 35 mph on a twisty mountain road. Both the trip average of 170Wh/mi “trip E” and the 157Wh/mi “E use” were well below the target 210Wh/mi “set targ” at this point in the drive, so the remaining range number would be indicating an increasing margin to the destination. The range calculation is based on the 280m “dest alt” that is set manually. During the drive, shown cruise control was used extensively to maximize efficiency and to generate smoother data records.
An Open Source Invitation
This presentation of the EV Total Energy Meter is an invitation for this concept to be used by OEMs and anyone else as an open source technology to enhance EV products and promote more efficient transportation. The same concept could also be applied to fueled vehicles, substituting gal/mi or $/mi for Wh/mi.