Electric vehicle battery packs aren’t as simple as a nickel-metal hydride [NiMH] rechargeable “AA” battery, but then, is even the rechargeable lithium-ion [Li-ion] battery that powers my laptop?
Electric vehicles are significantly cleaner than their conventional counterparts, but can’t be driven exactly the same, that is, recharging on practically any corner after a few hundred miles of driving. In order to keep from ruining an electric vehicle battery pack, the manufacturer limits cycling parameters and available capacity, but not for the reasons you think.
The problem with electric vehicle battery capacity and cycling rates isn’t in the battery cells themselves, but with our understanding of the chemical reactions taking place inside individual cells. Because of this lack of understanding, and our inability to see what exactly is going on inside each cell, battery manufacturers apply something like the Pareto Rule to charging and discharging. In order to maximize the life of the battery, roughly 20% of the battery’s capacity remains unused.
We’ve always known that discharging a battery to less than about 10% of its capacity, or overcharging it, can irreparably damage the cells, so manufacturers have built in controls to prevent that from happening to highly expensive electric vehicle battery packs. What if, however, we didn’t need to have that 20% buffer or, as Ilan Gur, Advanced Research Projects Agency for Energy [ARPA-E] Project Manager asked, “What if we remove the blinders? How much more could you get out of a battery system if you know what was happening inside the cells?” In order to make it possible, we’d have to be able to see, in real time, the changes occurring in each cell, temperature, stress, voltage, and chemical composition.
One particular project under ARPA-E, being conducted at the Palo Alto Research Center [PARC], could make it possible to see what’s going on inside individual electric vehicle battery pack cells. The new sensor is made of non-conductive and non-reactive fiber optic material, so it can be embedded directly in the cell without impacting performance. It is estimated that each embedded sensor might add 5% to the cost of the cell, but would effectively pay for itself by a 25% reduction in the number of cells required in an electric vehicle battery pack with no reduction in range or performance.
Image © PARCk3