What is a supercapacitor, and where are they used in electric vehicles?

What is a supercapacitor, and where are they used in electric vehicles?

Earlier this year, Tesla announced that it was to acquire supercapacitor company, Maxwell, in a deal valued at around $235m. At over 50 years old, Maxwell is no small business, and at the time of the acquisition, Maxwell employed around 380 employees and had annual revenues of about $100M, despite racking up some hefty losses. A $200m+ valuation might seem pretty low in comparison to some other deals happening recently, so why did Tesla’s stakeholders go for this deal?

Firstly, just in case people were wondering; let us talk about what precisely a Supercapacitor is.

A capacitor is an energy storage device based on electrostatic rather than electrochemical principles (as used in batteries), meaning it is more robust and can handle more power than batteries, as there are no electrochemical reactions to worry about. However, there is a catch, in that while they are great for power, their energy density is poor, and they lose charge over time. Therefore, they are not suitable for many applications. A supercapacitor is essentially a double-layered capacitor with the potential for much higher power capacity, and therefore, many more application opportunities.

Maxwell’s supercaps have already seen use in rail applications, along with BEV and FCEV buses. In these instances, the supercapacitors were used to support more conservative, old-tech battery systems that could not deliver the required power for the application. However, this market is likely to fade away as current improved batteries take out all of these needs for supercaps with higher power and energy ratings and longer cycle lives.

Furthermore, Maxwell’s products are used in automotive stop-start systems where the supercapacitor is used to support the battery, providing more starting power for repeated start/stop operations. In the USA this includes heavy-duty trucks which typically have a 12V electrical system as well, rather than the 24V found in Europe. This has been a good market but is likely to decline sharply as 48V hybrids, and strong/full hybrids take over.

So, with most of Maxwell’s applications featuring in declining markets, why exactly were Tesla so interested in buying?

One potential reason for this is the dry battery electrode technology that Maxwell has developed. This is a manufacturing process technology that can potentially deliver significant energy density and cost reduction benefits to Lithium batteries. The process eliminates volatile solvents from the electrode coating process, and instead utilises a thick carbon and polymer binder film to coat the battery electrode in a mechanical rolling process. This results in an improved coating structure on the battery electrodes, and pretty decent benefits to performance and energy density. The battery would still require a liquid electrolyte so would not be “solid-state”. However, the gains made, combined with the understood performance, production readiness, durability and flexibility of a liquid electrolyte system all look very worthwhile indeed.

The second possible reason is the potential of maxwell’s supercaps. One of the historical applications of supercapacitors has been to provide high power density energy storage. This is often supporting high-energy-low-power battery systems. Current batteries have improved from a power and energy point of view, but the generally accepted view is that as energy density improves further, power density will fall. Especially with the possible advent of solid-state battery technology. As the materials are thinned out to give more active surface area inside the battery, it becomes difficult to pass large currents through the pack. In these battery systems, there could be some real advantages in deploying supercaps to support a high energy-density battery that cannot deliver the power required in the application by itself.

There is one other application that springs to mind where Maxwell’s skills might be useful to Tesla. Typically, power electronic devices, such as traction inverters, DCDC converters and battery chargers all require a DC link capacitor. These are perhaps the most expensive components in these power devices. Traditionally, DC link capacitors are large film capacitors; they are heavy, costly and also can be a weak point in the power device design.

One can imagine that improving the performance of a DC link capacitor might be something of interest. Particularly when a company is designing a lot of their own power-electronics devices, as is the case with Tesla. Maxwell, with their wealth of capacitor knowledge, will undoubtedly be of use to Tesla in designing and optimising DC-link capacitors for their cars. While this is most likely not Tesla’s main reason for acquiring Maxwell, it is nonetheless a pleasant bonus for them.

To listen to more on this topic we discussed it in length on episode 33 of the AVID Learning: EV Technology podcast.

AVID Learning Electric Vehicle Technology Podcast Cover Art

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About the Author 

Ryan Maughan

Ryan Maughan

The Managing Director of the AVID Technology Group Ltd. AVID is based in the North East of England and is a leader in the design and manufacture of electrified powertrain systems for heavy-duty and high-performance vehicles.