Suggest a battery. You’re probably envisioning a standard format AA or AAA cell, the kind you buy to power various small electrical appliances, such as your television remote control or a smoke alarm.
Now imagine the battery of an electric vehicle. The image you conjured probably looks more like a large rectangle rather than a small cylinder.
Although your mind may think of these two types of batteries as very different electricity storage devices, both the typical store battery for your various electronic devices and the battery pack in an EV work on the same general principles. That said, the battery in a hybrid or electric vehicle is just a little more complicated than those lipstick-like cells you’re used to dealing with.
The battery in an HEV, PHEV or BEV (these are respectively hybrid electric vehicle, plug-in hybrid electric vehicle and battery electric vehicle) can be made from a variety of materials, each of which has 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 inside an electric vehicle’s 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 of turning on a flashlight), ions move between the two electrodes through a liquid electrolyte inside the cell. The electrons that these electrodes give off, meanwhile, pass through the wire outside the cell.
If the battery supplies power (for example, the bulb in the aforementioned flashlight)—an action known as discharge—then ions flow through the separator from the anode to the cathode, while electrons move across the wire from the negative (anode) to the positive. (cathode) terminal to supply power to an external load. Over time, the cell’s energy is depleted as it drives whatever drives it.
However, when the cell is charged, electrons from an external energy source flow in the other direction (from positive to negative) and the process reverses: electrons flow from the cathode back to the anode, increasing the cell’s energy again.
EV Battery Construction
When you think of the above AA or AAA batteries, you imagine a single battery cell. But the batteries in EVs are not a large version of that single cell. Instead, they consist of hundreds, if not thousands, of individual cells, usually grouped together in modules. Up to several dozen modules can be in a battery pack, which is the complete EV battery.
EV cells can be small cylindrical cells, such as 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 together. The advantage, these companies claim, is that small cells are much cheaper in volume to produce. Still, Tesla plans to move to lower numbers of larger cylindrical cells to reduce the number of connections within their cars’ battery packs.
Three different cylindrical battery cells used in the hundreds or thousands to make up a vehicle’s battery pack.
But EV cells come in two other formats: prismatic (rigid and rectangular) or pouch (also rectangular, but in a soft aluminum casing that allows some expansion in the cell walls under extreme heat). There are few standardized prismatic or pocket cell dimensions, and most automakers—General Motors and Ford, for example—specify their own in partnership with diesel manufacturers, such as China’s CATL, Japan’s Panasonic, or Korea’s LG Chem.
Types of EV Batteries
The chemistry of an electric vehicle’s battery – or the material used in its cathode – varies between different cell types. Today, there are essentially two types of battery chemistries, both under the umbrella of lithium-ion, meaning their cathodes use lithium along with other metals.
It’s a battery pack from GM’s Ultium family, which uses cells with a nickel-manganese-cobalt-aluminum (NMCA) mixture.
The two types of lithium-ion batteries
The first, 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 higher energy densities (energy per weight, or energy per volume) but also a greater tendency to oxidize (catch fire) during a drastic short circuit or severe impact. Cell makers and battery engineers spend a lot of time monitoring cells and modules, both during manufacturing and while in use over the life of the car, to limit the chance of oxidation.
The second type, which is used much more in China, is known as lithium iron phosphate, or LFP. (This despite the fact that Fe is the symbol for iron on the periodic table, while F is actually fluorine.) Iron-phosphate cells have significantly lower energy density, so larger batteries are needed to provide the same amount of energy (and therefore drive) series) than NMC based batteries.
However, compensating for this is that LFP cells are less likely to oxidize if they are short-circuited. 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 remotely considered scarce or resource limited. For those reasons, LFP cells are cheaper per kilowatt hour.
The lower costs led to Tesla (and most recently Ford) to use LFP cells in its base-model electric vehicles, saving the more expensive and higher-energy chemistries for more expensive models in the lineup.
As for the other cell electrode, the anode, most of them today are made of graphite.
EV Battery software
Unlike your basic AA or AAA cell, an EV battery requires a lot of software to keep things up to date. You can expect an AA or AAA cell to last a few years at most. However, automakers warranty their EVs’ battery components, often for around a decade or as much as 150,000 miles of use.
All EV batteries lose some charging capacity over time. With limited data available, it is difficult to dig 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 EV originally capable of delivering a range of 300 miles will still have between 240 and 270 miles of range at this point in its life cycle.
To ensure this happens, the battery modules and the pack itself have a slew of sensors to monitor the power delivered by each component – ideally, identically across all cells and modules – and the heat of the pack. A set of software known as the battery management system (BMS) keeps track of this information.
Like people, batteries are susceptible to changes in temperature, and they perform best at around 70 degrees Fahrenheit. If an EV’s battery pack shows signs of overheating, the BMS of most modern HEV, PHEV and BEV batteries will circulate coolant through the pack to shed heat and bring the temperature closer to 70 degrees. Batteries produce less power in extreme cold. If an EV owner prescribes their vehicle, their control software and BMS can use grid energy (if plugged in) or perhaps some battery energy to warm up the battery. Pre-conditioning allows an EV battery to deliver a specific power level once the driver jumps off.
New battery technology for electric cars
Battery technology is always evolving. Although today’s EVs 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 in which many manufacturers are investing. In fact, Toyota plans to introduce a vehicle with a solid-state battery by the middle of the decade.
Solid state batteries should offer greater energy density which should provide better driving range compared to a similar lithium ion battery. However, this breakthrough technology still has some way to go as engineers work to lower the material costs of manufacturing solid-state cells. Likewise, these cells’ lifespans will need to improve dramatically to accommodate the thousands of fully charge cycles of an HEV, PHEV or BEV.
Regardless, the future for battery-powered vehicles is promising. Look for new technologies to improve the efficiency and range of electric cars, and for the cost of lithium-ion batteries to drop significantly in the coming years.
Contributing Editor
Edited by John Voelcker 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 around them. He now covers advanced automotive technologies and energy policy as a reporter and analyst. His work has appeared in print, online and radio outlets that include Wired, Popular Science, Tech Review, IEEE Spectrumand NPR’s “All Things Considered.” Splitting his time between the Catskill Mountains and New York City, he still hopes to one day become an international man of mystery.
