by Imagine a battery. You’re probably picturing a standard AA or AAA cell, the kind you buy to power various small electrical devices like your TV remote control or a smoke alarm.
Now imagine the battery of an electric vehicle. The image you conjured up probably looks more like a large rectangle than a small cylinder.
Although your mind perceives these two types of batteries as very different power storage devices, both the typical store-bought battery for your various electronic devices and the battery pack in an electric vehicle operate on the same general principles. However, the battery in a hybrid or electric vehicle is a bit more complicated than the lipstick-like cells you’re used to.
The battery in an HEV, PHEV, or BEV (ie, hybrid electric vehicle, plug-in hybrid electric vehicle, or battery electric vehicle, respectively) can be made from a variety of materials, each with different performance characteristics. The individual cells stored in these large battery packs also come in many different shapes and sizes.
How does an EV battery work?
The cells in an electric vehicle battery pack each have an anode (the negative electrode) and a cathode (the positive electrode), both of which are separated by a plastic-like material. When the positive and negative terminals are connected (think turning on a flashlight), ions travel between the two electrodes through a liquid electrolyte in the cell. Meanwhile, the electrons emitted by these electrodes pass through the wire outside the cell.
When the battery supplies current (e.g. the light bulb in the flashlight mentioned above) – a process known as discharging – ions flow through the separator from the anode to the cathode, while electrons flow down the wire from the negative (anode) to migrate the positive (cathode) terminal to power an external load. Over time, the cell’s energy becomes depleted as it powers whatever powers it.
However, when the cell is charged, electrons flow in the other direction (from positive to negative) from an external energy source and the process is reversed: Electrons flow back from the cathode to the anode, increasing the energy of the cell again.
Construction of EV batteries
When you think of the AA or AAA batteries above, picture a single battery cell. But the batteries in electric vehicles are not a giant version of this single cell. Instead, they are made up of hundreds, if not thousands, of individual cells, usually grouped together into modules. Up to several dozen modules can be in a battery pack that makes up the complete EV battery.
EV cells can be small cylindrical cells like an AA or AAA cell with various standardized dimensions. This is the approach taken by Tesla, Rivian, Lucid, and a few other automakers, who wire thousands of these tiny cells together. The advantage, these companies claim, is that small cells are much cheaper to produce in large quantities. Still, Tesla plans to switch to a smaller number of larger cylindrical cells to reduce the number of connections in their cars’ battery packs.
But EV cells come in two other formats: prismatic (rigid and rectangular) or pouch (also rectangular but in a soft aluminum case that allows for some expansion of the cell walls in extreme heat). There are few standardized prism or pouch cell dimensions, and most automakers – such as General Motors and Ford – specify their own in cooperation with the cell manufacturer, such as CATL in China, Panasonic in Japan, or LG Chem in Korea.
Types of EV Batteries
The chemistry of an electric vehicle battery — or the materials used in its cathode — varies by cell type. Today, there are two main types of battery chemistries, both under the lithium-ion umbrella, meaning their cathodes use lithium along with other metals.
The two types of lithium ion batteries
The first, which is most common in North America and Europe, uses a mixture of either nickel, manganese, and cobalt (NMC) or nickel, manganese, cobalt, and aluminum (NMCA).
These batteries have a higher energy density (energy per weight or energy per volume) but also a higher propensity to oxidize (catch fire) in the event of a drastic short circuit or severe impact. Cell manufacturers and battery engineers spend a lot of time monitoring cells and modules, both during manufacture and during use throughout the life of the car, to limit the chance of oxidation.
The second type, which is far more commonly used in China, is known as lithium iron phosphate, or LFP. (This is despite the fact that Fe is the symbol for iron on the periodic table, while F is actually fluorine.) Iron phosphate cells have a significantly lower energy density, requiring larger batteries to provide the same amount of energy (and therefore driving range) than NMC based batteries.
However, this is countered by the fact that LFP cells are less likely to oxidize in the event of a short circuit. LFP cells also do not use rare and expensive metals. Both iron and phosphate are used in a variety of industrial applications today, and neither are remotely considered rare or resource-constrained. For these reasons, LFP cells are cheaper per kilowatt hour.
The lower costs prompted Tesla (and last Ford) to use LFP cells in its base-model electric vehicles, saving the more expensive and high-energy chemicals for more expensive models in the lineup.
The other cell electrode, the anode, is mostly made of graphite today.
Battery software for electric vehicles
Unlike your simple AA or AAA cell, an EV battery requires a lot of software to keep track of things. You can expect an AA or AAA cell to last a few years at most. However, automakers often warrant the battery components of their electric vehicles for around a decade or up to 150,000 miles.
All EV batteries lose some charge capacity over time. Given the limited data available, it is difficult to examine the details of these losses. Generally, range loss after 100,000 miles is on the order of 10 to 20 percent. In other words, an EV that was originally capable of delivering a range of 300 miles would still have a range of between 240 and 270 miles at this point in its lifecycle.
To ensure this, the battery modules and the pack itself have a number of sensors that monitor the power delivered by each component – ideally identical across all cells and modules – and the heat of the pack. A set of software known as a battery management system (BMS) monitors this information.
Like humans, batteries are susceptible to temperature changes and work best at around 70 degrees Fahrenheit. When an EV’s battery pack shows signs of getting too hot, the BMS in most modern HEV, PHEV, and BEV batteries circulates coolant through the pack to dissipate heat and bring the temperature closer to 70 degrees. Batteries deliver less power in extreme cold. If an EV owner preconditions their vehicle, then their control software and BMS can use grid power (if connected) or maybe some battery power to warm up the battery. Preconditioning allows an EV battery to deliver a specific level of performance as soon as the driver drives off.
New battery technology for electric cars
Battery technology is constantly evolving. Although today’s EVs predominantly use lithium-ion packs, many of tomorrow’s battery-powered cars will likely use packs with different chemistry. For example, solid state batteries, which use solid electrolyte cells, are a promising alternative that many manufacturers are investing in. In fact, Toyota plans to launch a vehicle with a solid-state battery by the middle of the decade.
Solid-state batteries are said to offer higher energy density, which should allow better range compared to a similar lithium-ion battery. However, this groundbreaking technology still has work to do as engineers work to reduce the cost of materials used to manufacture solid-state cells. Likewise, the lifetime of these cells needs to be drastically improved to handle the thousands of full discharge cycles of an HEV, PHEV or BEV.
Regardless, the future for battery-powered vehicles is bright. Look for new technologies to improve electric car efficiency and range, and be aware that the cost of lithium-ion battery packs will drop significantly in the years to come.
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