Deep dive: The 2017 Chevrolet Bolt's electric powertrain
Many have started comparing the Bolt to Tesla's forthcoming Model 3. But unlike the Tesla, we actually know what's going on under the Chevrolet's skin.
It's been just four months since Chevrolet introduced its all-new electric Bolt at CES. But in that time, it's already become one of the most hotly anticipated EVs in the world, thanks in part to its estimated 200-plus-mile range and sub-$40,000 price tag.
It's not the only game in town, though. Tesla Motors recently took the covers off its Model 3 electric sedan, which should have similar price and range figures to the Bolt. Chevrolet's not often viewed as being at the cutting edge of new vehicle technologies, whereas Tesla is treated like an Apple-like deity. Thus, the onus is on Chevrolet to deliver a vehicle that's going to make waves in its segment.
Chevrolet's primary goals with the Bolt were to eliminate range anxiety, provide superior stats to its previous electric effort, the Spark EV, and keep the battery from infringing on interior space. In order to meet all those targets, Chevrolet had to start from scratch. The motor and battery in the Bolt might resemble the Spark's ever so slightly, but rest assured, they're radically different components. We'll start by taking a look at the largest part, the battery.
Where the juice comes from
The main battery that rests under the Bolt's floor only deals with propulsion -- there's a smaller, 12-volt, lead-acid battery that deals with accessory power. The big mamma jamma (weighing 436 kg vs. the Spark EV's 215 kg) is a lithium-ion unit with high nickel content, which allows for a higher heat tolerance. It runs warmer than the Spark's battery, yet its overall waste heat is low.
That focus on efficiency allows the battery to contain about three times the Spark's energy with only twice the mass. Compared to the Spark, overall capacity is higher (60 kWh vs. 18.4), max power is higher (160 kW versus 120), and nominal voltage is lower (350 volts vs. 360). The Bolt's battery capacity is a shade lower than the 70 kWh figure in the smallest pack Tesla currently sells.
Whereas Tesla uses cylindrical cells in its batteries, the Bolt relies on "landscape" cells, which are long and flat. In each battery there are 288 of these cells, which are bundled into 96 groups of three, connected in parallel to form the battery pack. The cells are double-stacked in the last row, taking advantage of the rear seat's height compared to the floor.
From left to right, you can see two different Volt batteries, the Spark EV's battery, and the Bolt's.
Integration is vastly improved over the Spark, as well. The battery is actually a significant structural component of the Bolt. If the battery were to be removed, the car's torsional rigidity would drop by 28 percent. Cross-members are built into the battery's tub to better transfer forces across it -- in stress tests, only the rear shelf of the battery experiences significant flexing.
Keeping the battery within its ideal operating range is a liquid cooling system using Dex-Cool coolant. All 6.9 liters of the stuff flows through small metal plates underneath each cell module, and the system is tested to handle vibration and g-force well above what's experienced in normal driving situations.
For safety's sake, there are two separate disconnects. For service concerns, there's a manual disconnect hidden underneath the rear seat. In front of the battery is a rather hefty crash-safety system, which cuts power in a collision, making it safer for first responders.
The battery is capable of DC fast charging at 50 kW using an SAE Combo plug. At full clip, it'll add about 90 miles of range in 30 minutes. A full charge on a Level 2 charger takes about 9 hours.
GM claims the cells are as tightly packed as possible. The company even moved several components -- power distribution and charging modules, mostly -- to the engine bay in order to give the battery as much elbow room as possible. Any further improvement to the Bolt's battery must come from cell capacity and efficiency breakthroughs.
As for the warranty, the battery is covered for eight years or 100,000 miles, whichever comes first. It's built to last the life of the vehicle, which GM said is beyond those figures, but declined to get more specific. Despite being such an integral component, I'm told removing it is no more difficult than removing the transmission in a gas-powered car.
In case you were thinking that there had to be room in that battery pack for more cells...you're wrong.
Where the juice goes
The Bolt's drive unit, which includes the engine and transmission, was built to be as compact as possible while delivering the torque and performance that Chevrolet wanted. The goal was to top the Spark's acceleration, max launch grade and EPA range all at the same time, and Chevrolet delivered (with LG's help).
While Chevrolet engineered the motor itself, LG's responsible for assembling it, which is done in Korea. This permanent-magnet electric motor weighs 76 kg, only 8 more than the Spark's motor, but the Bolt's unit packs a smaller outer diameter (204 mm vs. 213), a higher peak power density, higher peak axle torque (2,500 newton meters vs. 1,710) but a lower peak torque (360 Nm vs. 540). It takes significantly less transmission fluid, as well (2.9 liters vs. 4.2).
In this shot, the motor is on the left, and the transmission is on the right. Power reaches the axle by way of a shaft going through the center of the motor.
The Bolt's motor rotates significantly faster than the Spark's (8,810 rpm max speed vs. 4,500), but that's translated to the road by way of a much higher final drive ratio (7.05:1 vs. 3.87:1).
Despite the lower peak torque, the Bolt accelerates faster than the Spark. 60 mph arrives in fewer than 7 seconds (7.5 for the Spark), 30 mph comes up in just 2.9 seconds (3.1 for the Spark), and it can launch from a 30 percent grade (28 percent in the Spark). Range is also up significantly, from the Spark's 82 miles to "over 200 miles" -- the EPA hasn't put out official numbers yet.
All in, the Bolt weighs 3,580 pounds, which is about 700 pounds heavier than the Spark. But it's a larger car -- GM actually calls it a crossover -- with a much bigger battery. GM hasn't yet announced what will become of the Spark EV, which is only sold in states that require zero-emissions vehicles.
Some folks might get all up in arms about the Bolt not having the same kind of flash or panache as Tesla's hot-to-trot Model 3, but after taking a closer look at the parts of the Bolt that most users will never see or (ideally) worry about, it's pretty obvious that Chevrolet isn't half-assing the Bolt one bit.
If you want a closer look at what's going on inside the Bolt's motor, this one's for you.
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