From the November 2016 issue
The battery pack of an electrified vehicle isn’t just a fuel tank for its motors. It’s also a metaphorical fuel pump dictating the peak power that can be delivered to a motor. While electric vehicles require both range and power from the battery pack, hybrids require similar power with far less energy. Accordingly, the individual cells are optimized to deliver brief bursts of power in hybrids or long-lasting energy to maximize range for EVs. A plug-in hybrid’s battery pack straddles both priorities and lands somewhere in the middle. A battery’s output-to-storage-capacity ratio (what the industry calls its power-to-energy ratio, or watts per watt-hour) characterizes these differences.
“It’s kind of like designing an engine,” said Pablo Valencia, senior manager in global battery engineering at GM. “Am I trying to get high-speed power or am I trying to get fuel economy? Your piston-diameter-to-stroke ratio is one of the first fundamentals that you do in an engine design.”
Battery engineers establish the power-to-energy ratio early in the design process by defining the thicknesses of both the current collectors and their chemical coatings. A hybrid’s brief spurts of peak power mean higher electrical current, and higher current requires larger wires. The current collectors—aluminum or copper plates through which electrons exit and enter the battery—are a battery’s internal analog to wires. Hybrids use thicker collectors than EVs to carry more current.
The opposite is true of the chemical coatings applied to the collectors. Thin coatings allow the electrons to flow through the battery quicker for higher power delivery in hybrids. EVs, with more individual cells, can discharge each cell more slowly through thicker coatings that help increase their energy capacity. These coatings are the secret sauces (such as lithium-manganese-oxide) that define the electricity-generating chemical reactions, but the chemistry is not critical to determining if the cell is more power- or energy-dense.
When batteries are connected in series, the voltage of each cell is cumulatively added; cells wired in parallel increase the capacity of a pack, adding the ampere-hours of each cell together. The pack is wired to achieve the necessary operating voltage and capacity, with more cells in parallel for plug-in vehicles. In the Chevrolet Bolt EV’s pack, GM welds the tabs of three cells in parallel, then wires 96 of those triplets in series. A Volt plug-in hybrid uses pairs of cells wired in parallel, while all 80 cells in the Malibu hybrid’s pack are wired in series.
| Malibu Hybrid |
Volt |
Bolt EV |
||||
| Energy capacity | 1.5 kWh | 18.4 kWh | 60.0 kWh | |||
| Discharge power | 52 kW | 120 kW | 140 kW | |||
| Power-to-energy Ratio | 34.7 W/Wh | 6.5 W/Wh | 2.3 W/Wh | |||
| Pack weight | 95 lb | 403 lb | 948 lb | |||
| Pack volume | 1.4 cu ft | 5.4 cu ft | 10.1 cu ft | |||
| Cell count | 80 | 192 | 288 | |||
| Cooling | Air | Liquid | Liquid | |||
| Cell supplier | Hitachi | LG Chem | LG Chem |
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