We’re car guys and gals here. We know about compression ratios, bore and stroke, horsepower versus torque, and what overdrive means. But can we explain the difference between the battery used in a pure electric car versus a battery used in a plug-in hybrid? Probably not, but odds are our kids can. Fear not, ye intrepid Gas2 faithful. When you are done reading this, you will be able to talk batteries like a pro.
When talking about batteries for an electric car, the discussion has to begin with the difference between power and energy. Here we turn to our friends for guidance. “Energy is the capacity to do work,” author Rob Lewis writes at CleanTechinca. “In physics, work is the act of exerting a force over a distance.” Leaning on a couch may mean you have the capacity to do work, but until you have actually caused it to move, you have performed no work.
“Pushing…..things around is work, and it takes energy to do it,” Lewis continues. “If we know the strength of the force we need in order to move an object, and the distance we’re going to move it, we can calculate the amount of energy we’ll need.” So what is power? Simply put, power adds time to the equation. “While energy measures the total quantity of work done, it doesn’t say how fast you can get the work done,” is how Lewis puts it. Moving that couch from one side of the room to the other before your wife gets home from the store takes energy. Moving it there before she gets from her car to the front door requires power.
The power to energy ratio is to electric car engineers what the bore to stroke ratio is to internal combustion engine designers. “It’s kind of like designing an engine,” says 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.” In general, hybrid cars need batteries with more power. Electric cars need batteries with more energy. Plug-in hybrid cars fall somewhere in between.
Battery engineers can control the energy to power ratio by specifying the thicknesses of both the current collectors and their chemical coatings. Current collectors are aluminum or copper plates. They are the pathways through which electrons exit and enter the battery. In a hybrid car, brief spurts of peak power mean higher electrical current. That’s why hybrids use thicker collectors than battery electric cars do. When it comes to the chemical coatings applied to the collectors, though, the opposite is true. Thin coatings allow the electrons to flow through the battery quicker for higher power delivery in hybrids.
A battery electric car typically has more individual cells. It can discharge each cell more slowly through thicker coatings that help increase energy capacity. In general, cell chemistry is not critical to making an individual cell more power or energy dense. The thickness of the collector plates and coatings are how battery engineers control that ratio.
The way battery packs are assembled also affects the energy to power relationship. When batteries are connected in series, the voltage of each cell is cumulatively added. Cells wired in parallel increase the capacity of the battery pack. Each pack is wired to achieve the voltage and capacity characteristics needed for a particular application.
In the Chevrolet Bolt battery pack, GM welds the tabs of three cells in parallel, then wires 96 of those triplets in series. A Volt plug-in hybrid battery wires two cells together in parallel then connects them to other diads in series. The battery pack for the Malibu Hybrid has all of its cells wired in series.
If you every have a job interview with Elon Musk, he will ask you to explain the difference between energy and power. Now that you’re an expert on the subject, you stand a decent chance of getting the job.
Source: Car and Driver