The world’s most recognisable electric vehicles (EVs), such as the Tesla Model S and Nissan Leaf, run on hundreds, or even thousands, of small battery cells.
BMW's new i3 electric runabout spreads 96 battery cells across eight modules in its pack. The Leaf uses almost 200 thin laminated film cells that are packaged into 48 modules, and the Model S has more than 6,800 small lithium-ion battery cylinders.
A visit to Quora.com, the online question and answer community, yields a nuanced view of why so much variation exists around how many batteries an EV uses, and why the industry is not quite ready for a mega-battery.
Follow the money
Leigh Christie, an EV engineer, says manufacturers’ embrace of smaller batteries boils down to cost. "The capital cost for manufacturing equipment for 18650-size cells is as about as low as it gets,” he wrote. “This cell has been manufactured longer than pretty much any other lithium-ion cell."
The numbered cell refers to the small type used in Tesla's battery packs, which are also manufactured for use in laptops. To keep up with demand, Tesla officials have publicly considered building their own manufacturing plant to supply all the battery cells their vehicles need.
A matter of managing heat
However critical a factor cost may be, heat from EV battery cells is something all manufacturers must learn to manage. "The gaps between the cells allow for cooling and minimize the possibility of thermal runaway," Christie added. That's why Nissan's flat laminated cells are designed with a large surface area that quickly disperses the batteries’ heat. Because of this, the Leaf does not require a separate battery-cooling unit, such as those in the i3 and Model S.
Michael Graham, an engineer, argues that larger batteries may ultimately be a better way to go for EVs, given their greater energy storage capacity. However, that day has not yet arrived. In addition to temperature control benefits, he said smaller batteries were "easier to stack in unique ways to distribute weight and make use of small spaces in a vehicle chassis." That's exactly what Nissan does with its flat cells, which can be stacked both vertically and horizontally within the Leaf’s pack.
But Quora user Frank Schmitt argues that it's not just the ability to transfer heat efficiently or stack cells that make small batteries the optimal choice. Manufacturing defects in larger batteries can be difficult to identify and treat, which can shorten a battery's lifespan. Tesla agrees. "The small cell size enables efficient heat transfer, allows for precise charge management, improves reliability, and extends battery pack life," the automaker concisely states on its website.
Less is more?
EV makers clearly utilise a variety of battery technologies and varying numbers of cells. While their approaches to delivering electric propulsion may differ, there’s broad consensus around the goal of reducing the number of cells. The trick, as Quora user Graham said, is doing this while increasing each cell’s attendant storage capacity. That ultimately will strengthen EVs’ desirability in the long term.