Imagine a battery. You’re probably picturing a standard-sized AA or AAA cell, the kind you buy to power various small electrical devices, like your TV remote control or a smoke detector.
Now, imagine the battery of an electric vehicle. The image you conjured probably looks more like a large rectangle than a small cylinder.
Although your mind may perceive these two types of batteries as very different electricity storage devices, both the typical store-bought battery for your various electronic devices and the battery in an electric vehicle operate on the same general principles. That said, a hybrid or electric vehicle battery is a bit more complicated than those lipstick-like cells you’re used to handling.
The battery of an HEV, PHEV, or BEV (i.e., hybrid electric vehicle, plug-in hybrid electric vehicle, and battery electric vehicle, respectively) can be made from a variety of materials, each of which has different performance characteristics. . The individual cells stored inside these large battery packs also come in many different shapes and sizes.
How does an EV battery work?
The cells inside an electric vehicle battery 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 of turning on a flashlight), ions travel between the two electrodes through a liquid electrolyte inside the cell. Meanwhile, the electrons given off by these electrodes pass through the wire outside the cell.
If the battery provides power (eg the aforementioned flashlight bulb), an action known as discharge, ions flow through the separator from the anode to the cathode, while electrons travel down the wire from the negative (anode) to the cathode. the positive terminal (cathode) to provide power to an external load. Over time, the cell’s energy is depleted as it drives everything it’s feeding.
When the cell is charged, however, electrons flow from an external energy source in the other direction (positive to negative) and the process is reversed: electrons flow from the cathode back to the anode, increasing the cell’s energy again.
Construction of EV batteries
When you think of the AA or AAA batteries mentioned above, you imagine a single battery. But electric vehicle batteries are not a huge version of this single cell. Instead, they are made up of hundreds, if not thousands, of individual cells, usually grouped into modules. Up to several dozen modules can reside within a battery pack, which is the complete EV battery.
EV cells can be small cylindrical cells, like an AA or AAA cell, of various standardized dimensions. That’s the approach taken by Tesla, Rivian, Lucid and some other automakers, connecting thousands of these tiny cells. The advantage, these companies claim, is that small cells are much cheaper to produce in volume. Still, Tesla plans to move to a lower number of larger cylindrical cells to reduce the number of connections inside its cars’ battery packs.
Three different cylindrical battery cells that are used by the hundreds or thousands to make up a vehicle battery.
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 in the cell walls under extreme heat). There are few standardized dimensions of prismatic or pouch cells, and most car manufacturers (General Motors and Ford, for example) specify their own in cooperation with the cell manufacturer, such as CATL from China, Panasonic from Japan or LG Chem from Korea.
Types of EV batteries
The chemistry of an electric vehicle battery, or the materials used in its cathode, varies according to the different types of cells. Today, there are essentially two types of battery chemistries, both of which fall under the lithium-ion umbrella, meaning their cathodes use lithium along with other metals.
This is a battery pack from GM’s Ultium family, which uses cells with a nickel-manganese-cobalt-aluminum (NMCA) blend.
The two types of lithium-ion batteries
The first, most common in North America and Europe, uses a mixture of nickel, manganese and cobalt (NMC) or nickel, manganese, cobalt and aluminum (NMCA).
These batteries have higher energy densities (energy per weight, or energy per volume) but also a greater propensity to oxidize (fire) during a drastic short circuit or a severe impact. Cell manufacturers and battery engineers spend a great deal of time monitoring cells and modules, both during manufacture and while in use over the life of the car, to limit the possibility of oxidation.
The second type, much 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 in the periodic table, while F is actually fluorine.) Iron-phosphate cells have considerably lower energy density, so larger batteries are needed to provide the same amount of power (and therefore drive). range) as NMC-based batteries.
Offsetting this, however, is that LFP cells are less likely to oxidize if shorted. 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 is considered remotely rare or resource limited. For these reasons, LFP cells are less expensive per kilowatt-hour.
The lower cost led to Tesla (i most recently Ford) to use LFP cells in their entry-level electric vehicles, saving the more expensive and higher-energy chemistries for the more expensive models in the lineup.
As for the other cell electrode, the anode, today most are made of graphite.
EV Battery Software
Unlike your basic AA or AAA cell, an EV battery requires a lot of software to control things. You can expect an AA or AAA cell to last a couple of years at most. Automakers, however, guarantee the battery components of their electric vehicles, often for a decade or up to 150,000 miles of use.
All electric vehicle batteries lose some capacity over time. With limited data available, it is difficult to delve into the details of these losses. In general, the loss of range after 100,000 miles can be on the order of 10 to 20 percent. In other words, an electric vehicle originally capable of 300 miles of range would still have 240 to 270 miles of range at this point in its life cycle.
To ensure this happens, the battery modules and the pack itself have a large number of sensors to monitor the power supplied by each component, ideally identical to all cells and modules, and the heat of the pack. A suite of software known as a battery management system (BMS) controls this information.
Like humans, batteries are susceptible to temperature changes and work best at about 70 degrees Fahrenheit. If an EV’s battery pack shows signs of overheating, the BMS in most modern HEV, PHEV and BEV batteries will circulate coolant through the pack to remove the heat and bring the temperature closer to 70 degrees. Batteries offer less power in extreme cold. If an EV owner conditions their vehicle, their control software and BMS can use mains power (if connected) or perhaps some battery power to heat the battery. Preconditioning allows an EV battery to provide a specific level of power as soon as the driver pulls off.
New battery technology for electric cars
Battery technology is always evolving. While today’s electric vehicles overwhelmingly use lithium-ion packs, many of tomorrow’s battery-powered cars will likely use packs with different chemistries. For example, solid-state batteries that use cells with a solid electrolyte are a promising alternative that many manufacturers are investing in. In fact, Toyota plans to introduce a vehicle with a solid-state battery by the middle of the decade.
Solid-state batteries should offer a higher energy density that should allow for better autonomy relative to a similar lithium-ion battery. However, this innovative technology still has a way to go as engineers work to reduce the material costs of producing solid-state cells. Similarly, the lifetime of these cells will need to improve dramatically in order to accommodate the thousands of full discharge cycles of an HEV, PHEV or BEV.
Regardless, the future of battery-powered vehicles is promising. Look for new technologies to improve the efficiency and range of electric cars, and for lithium-ion battery costs to drop significantly in the coming years.
Collaborating editor
John Voelcker edited Green car reports for nine years, publishing more than 12,000 articles on hybrids, electric cars, and other low- and zero-emission vehicles and the energy ecosystem that surrounds them. He now covers advanced automotive technologies and energy policy as a reporter and analyst. His work has appeared in print, online and radio media including Wiring, Popular Science, Technology Review, IEEE Spectrumand NPR’s “All Things Considered.” He divides his time between the Catskill Mountains and New York City, and still has hopes of one day becoming an international man of mystery.
