How to kill an ultracapacitor

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By eeNews Europe

When working within their specified operating parameters, ultracapacitors will provide a stubbornly long and trouble-free existence. Ultracapacitors will outlast most other electronic components in any given application, which is why they are the perfect power delivery device in the most demanding environments.

However, an ultracapacitor can meet an early demise if you really want it to — though the list of things you can do to kill it is pretty limited. Paying attention to the following circumstances will ensure the maximum lifetime of your system’s ultracapacitors.

Operating temperature: An ultracapacitor doesn’t rely on electrochemical reactions to generate or deliver power or energy. It operates (simplistically speaking) electrostatically. So it stands to reason that electrochemical reactions in an ultracapacitor are not only absent by design, but also unwanted. Increasing temperature catalyzes chemical reaction rates. This generally applies to ultracapacitors, as they are chemical systems. Therefore, if you want to kill an ultracapacitor, operate it outside of its specified operating temperature, and you will be sure to degrade its capacitance excessively, increase its resistance at unacceptable rates, and decrease its efficiency dramatically. The magnitude of the effects is proportionate to the temperature beyond specification and the time spent at that temperature.

Operating voltage: Electrochemical systems generally exhibit a stable operating voltage range beyond which the system becomes unstable, electrochemically speaking. Similar to over-temperature, the effects of overvoltage include increasing decay rate in capacitance and increasing rate of resistance rise. Overvoltage will also increase the gas generation rate inside the cell. If the overvoltage is severe enough, the gas will increase to a level that will result in over-pressuring the cell from the inside out and venting, followed by cessation of function. Ultracapacitors, unlike some other electrochemical storage technologies, do not have an under-voltage limit. In fact, reverse voltage, while not desirable, will generally not kill an ultracapacitor or create a dangerous situation, as long as the reverse voltage is not over the absolute value of the voltage specification when normally operated. As with temperature, the impacts of overvoltage are dependent on the overvoltage magnitude and the time spent at that condition.

High voltage and high temperature: The worst of all worlds is an overvoltage condition coupled with an over-temperature condition. In this case, electrochemical reactions and gas generation are both accelerated, resulting in high rates of capacitance and resistance decay, rapid gas pressure buildup in the cell, and irreversible degradation of the active materials in the cell, including the electrode active materials matrix and the electrolyte. Exposing the cell to this lethal combination is the easiest way to kill an ultracapacitor.

Mechanical abuse: Exceeding mechanical specifications for ultracapacitors, such as vibration specifications or shock events, will kill all but the heartiest of devices. Sometimes death will be instantaneous. Maxwell ultracapacitors have a distinct advantage over competitors in this area due to a robust and unique dry electrode and specially engineered cell construction. However, all manufacturers have their limits, and exceeding any manufacturer’s specifications can result in the rapid demise of the device.

Ultracapacitors are not susceptible to dying young unless they are exposed to out-of-specifications conditions. It is important to understand why manufacturers specify their devices as they do and what evidence and data is available to substantiate that the particular device is up to the task.

Specifications are defined by the manufacturer and, as such, are sometimes subject to interpretation. My advice is to ask questions, demand data, and operate the device inside the specifications. This way you won’t be disappointed by your decision to use ultracapacitors in your most demanding applications.

Mike Everett joined Maxwell Technologies in 2002 and was appointed chief technical officer in 2005. Over a 25-year engineering career, Mike has been responsible for all levels of new product development, primarily focused on systems engineering and now the research and development of new technology.

This article first appeared on EE Times’ Planet Analog website.

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