So… How Far Does it Go on One Charge?

Range Screen

A very common question EV owners have to field is range – how far can the car go.  As with most questions, the answer is that it depends on a variety of factors.  To be fair, this is true of gasoline-powered vehicles as well, but EVs are particularly variable in this regard.  While my experiences are based on my year with the Bolt, they should generally apply to other electric vehicles as well.

Chevy (and the EPA) both claim that the Bolt is good for 238 miles, in normal driving, on a full charge.  Given a battery capacity of 60kWh, this equates to roughly 4 miles/kWh.  I think this is a decent first-order approximation (I’ve had no trouble achieving it) and most EVs are somewhere between 3.5 and 4.5 miles/kWh, so the Bolt, despite it’s decidedly unaerodynamic shape, is squarely in the middle.

One neat thing about the Bolt is that it displays the amount of power being drawn at any given time.  You can easily estimate your efficiency simply by dividing your current speed by that power, to obtain distance/energy.  For example, 60 mph at 12 kW yields 60 / 12 =  5 kWh.

When looking at EV range, the biggest variables are typically speed, terrain, acceleration, battery temperature and heating.  Let’s take them one at a time.

Speed

The relationship between engine power output and speed is not linear.  In particular, aerodynamic drag is roughly proportional to the square of speed and at higher speeds makes for a large part of a vehicle’s power consumption.  In my experience, on a well-paved flat road, cruising at 50mph requires roughly 8kW from the Bolt.  Speed up to 60mph and this increases to ~13kW.  By 70mph, the motor needs to provide at least 18kW.  Or in more concrete terms, speeding up by 40% required a 125% increase in power.

So for maximizing range, speed is your enemy, especially above 50mph or so.  Some crazy people managed to coax 466 miles out of a Bolt mainly by limiting speed to around 20mph.   That’s going a bit far, but if you can stay under 55mph you will generally achieve better range than Chevy’s estimates.  Assuming reasonable pavement quality, I find taking secondary roads (vs. the interstate) is a nice way to enjoy the scenery during road-trips, as well as decrease the number of times I need charge.

Terrain.

It’s no surprise that going uphill requires more energy than staying on level ground.  How much more is the question, and this in turn depends on speed and angle of incline (grade).  Unfortunately these tend to vary a lot, but even a gentle incline usually takes twice the power as the same speed on level ground, and a steep incline can drop efficiency to as low as 1 mile/kW.  If that sounds like a lot, it is.  On one trip in the mountains, a relatively gentle climb of 4,000 feet of cumulative elevation over 28 miles at 45mph required 10.5kWh (2.7 miles/kW).

The saving grace is that going downhill, the Bolt actually gains energy.  Like most EVs and hybrids, the Bolt essentially runs the motor as a generator by reversing the current in a process termed ‘regenerative braking’.  While the efficiency of this process is typically only 50-60%, the net result is that you can recoup a fair amount of energy ‘lost’ from the ascent.  On the same 28 mile drive above, returning (descending 4,000 feet) consumed only 3.5kW over 28 miles (10.9 miles/kWh)

This process appears to work best if the climb is short and steep and the descent is long and gentle.  On another trip that involved 55 miles beginning and ending at the same elevation, but climbing and descending 6,000 feet in between, the Bolt required ~17kWh traveling eastbound, but only 12kWh returning.  The difference was that on the return, most of the climbing was done in a 10 mile stretch, vs. the majority of the descent being spread over 45 miles, whereas the reverse occurred on the way out.

All in all, I’ve found efficiency is best on flat roads, and worst on those with steep inclines.

Acceleration

One of the fun parts of having an EV is that they’re very quick ‘off-the-line’.  Having full torque available from 0 RPM means that it’s easy to keep up with even high-end gasoline-powered vehicles up to 30mph or so.  The Bolt has a 150kW motor, but it is rare that I’ve ever used more than 30kW even accelerating from a stop.  Still, compared to cruising, accelerating does use a lot more power, and jack-rabbit starts use the most.  Perhaps surprisingly, the Bolt does better in slow stop-and-go traffic than faster, steady traffic.  The reason is that the losses due to acceleration (go) are mitigated somewhat by regenerative braking (stop), and the overall lower speeds makes the bigger difference.

Battery Temperature

The chemistry of the Bolt’s lithium-ion battery is that it performs best when warm but not too hot.  The exact numbers aren’t available, but I suspect that similar to charging, 77-104F is probably the optimal temperature.  While the Bolt does have a radiator system with a heater to regulate the battery temperature, it tends to do so fairly minimally (which also impacts charging times) only coming on at very low or very high temperatures.  But the basic point remains – when the air and battery temperature is cold, it requires more power to do the same drive, even if the battery heater and cabin heater are not operating.  A 15-20% decrease in efficiency in the wintertime is not uncommon, even in California where temperatures rarely drop below freezing.

Heating

The Bolt has a resistive heater to warm the cabin.  It works adequately, but is quite power-hungry, and can consume up to 7.5kW when on full blast.  Fortunately once it has been on for a bit, it typically drops down to 2-2.5kW but even that is a substantial power draw (consider that the vehicle at 50mph may only be using 8kW).  The air-conditioning by contrast uses very little power, and the fan almost none at all.  A few EVs use heat pumps for cabin heating which is much more efficient at normal temperatures.  Note that the defog setting will enable the Bolt’s heater, and thus requires a decent amount of power.  Make sure to disable ‘Auto Defog’ in the settings, or even the fan alon may occasionally turn on the heater.  I do regret not getting the seat warmer option as they are a much more efficient way of keeping warm without using a lot of power.

Compared

Few of the factors mentioned above are uniques to EVs.  ICEVs (Internal Combustion Engine Vehicles) also must deal with drag, roads that change elevation, accelerating in traffic, and differences in cold temperature as well as other things.  The main distinction is that EVs are very efficient (85-90%) and that efficiency doesn’t vary with engine RPM, so if the motor requires twice the power to move the car, that translates directly to drawing twice the power from the batter.  ICEVs on the other hand are at best around 40% efficient and efficiency varies a lot with engine RPM, so generating twice the power may only require 20-30% more fuel overall, as so much already is going into waste heat and unused motion.  (Although that waste heat is used to warm the cabin so it isn’t always completely wasted).

Working in the favor of EVs, regenerative braking and high efficiencies at low RPMs means that in traffic they are very efficient – more so than cruising at high speeds.  ICEVs waste a huge amount of energy running at low loads, idling, or when slowing down (in that case, kinetic energy).  MPGe, the measure used by the EPA, is based on the amount of fuel/energy needed to produce 115,000 BTUs.  By this measure, the Bolt is more than twice as efficient as the best hybrid – 120 MPGe vs. < 60 MPG.

Conclusions

In general, 240 miles on a full charge is a reasonable estimate for the Bolt.  With careful driving in mild weather, I’ve managed almost 320 miles.  On the flip side, in cold weather with hilly terrain and the heater on, I’ve seen as low as 150 miles.  It is always good to be a little conservative.  This is somewhat more variable than in traditional vehicles, which is largely due to the higher efficiency of the Bolt.  My average, after 23,000 miles is about 4.5 miles/kWh, a solid 13.5% above the official rating.

Comments are closed.