Technology

The energy of battery cells, the power of supercaps in a safe package

Carbon based Power Capacitors

This technology is a new type of hybrid supercapacitor with a much higher energy density. It bridges  a gap between traditional (often Lithium based) supercapacitors and lithium-ion battery cells, but using activated carbon as its active ingredient. Contrary to batteries there is no chemical reaction involved when charging or discharging. Hence, they pose no safety risk, can work at low temperatures and have long lifetime. The combination of a power and energy density in a safe energy storage device is what the market really needs.

 

Power and Energy Density

The Carbon based Power Capacitors are a new technology for storing electric charges as the following Ragone chart shows. They offer an energy density that is similar to Lithium-ion battery cells (from 80 to 230 Wk/kg) but with a Power density as found in supercapacitors (a few 100 to 1000’s W/kg). Practically speaking, systems equipped with these Power Capacitors will be able to store as much energy but can deliver it up to 20 times faster, which implies that they can be charged faster than traditional lithium based batteries. Technically, the power capacitors are classified as assymetric, i.e. hybrid supercapacitors.

 

What makes Power Capacitors safer and more reliable than ever?

Safety of a system comes in layers. If a safety issue exist, then it has to be addressed at multiple levels. The first level is prevention, i.e. make sure by design or by technology choice that the safety risk doesn’t exist at all. This is the level where power capacitors offers intrinsic safety and reliability. Nevertheless, power capacitor battery packs can store and release enormous amounts of electric energy. Just like with any such system, general principles of safety  with power electricity still apply. Let’s now focus on the inherent safety of the technology:

  • Electrolyte: while the power capacitors contain a small amount of electrolyte, its purpose is mainly to act as a filler and as a pathway for the charges to move. The electrolyte is soaked up by the nano-carbon material, so that even when the cell’s skin is breached, very little or none will leak out.
  • Use of Nickel and Cobalt. These metals offer a trade-off between energy and power density. More Nickel means more energy, more cobalt means more stability. The 18650 cells contain no Nickel but provide the highest power density. The metal are only present on one electrode and bounded in specific compounds so that no free metal ions can create dangerous dendrites.
  • The power capacitors come in different variants that reflect the dominant lithium battery technologies. This provides a trade-off between price, safety, reliability and performance but as it still functions as a capacitor, the electric energy is stored as electric charges with no active chemical reactions involved.
  • The nano-carbon materials increase the surface of the electrodes and hence increase the capacitance, allowing to store as much energy as traditional lithium battery cells.
  • Fault tolerant, resilient power pack batteries. The powerpack is created by connecting the capacitors is a small grid mesh. If for some reason a cell would become defective (e.g. because it is damaged), then the electrolyte that doesn’t get soaked up by the carbon material might leak out and will evaporate. As no dendrites can form to create a short circuit between the electrodes, the above failure mode becomes an open circuit and while the power pack will loose some capacitance, it will continue to function as a battery.
  • No dramatic warming up: as each cell has a low internal resistance and in combination with the meshing connections, a power capacitor pack warms up very little even when high currents are used.
  • Destructive tests have demonstrated the safety of the cells: short circuit, overcharging, forced discharging, fire, drop test, nail puncture (internal short circuit), even shooting on it with a gun showed no fire and no explosion.
  • Our stress and abuse tests have demonstrated extreme robustness. Even when subjected to load conditions way beyond the permissible values, the cell keeps functioning.  See the results at https://kurt.energy/unparalleled-robustness-of-blue-cell-power-capacitors/

Besides the safety and reliability benefits using power capacitors also translates into a simpler system design. Firstly, no BMS (Battery Management System) is needed to actively balance the cells when charging or discharging. A failing BMS is often a root cause for a battery fire.  Secondly, as the cells operate from very cold freezing temperatures to very warm desert like temperatures, most of the no active cooling is needed as the cells barely warm up. Hence, most of the space is occupied by the cells and not by supporting subsystems.

The interested reader can find a very detailed overview of battery safety issues in the excellent special article of Best Magazine UK (url: https://www.bestmag.co.uk/emags/BESTMAG65-Summer2019/?page=39

A relevant extract can be downloaded here: BestMagAug2019 safety extract

Benefits of both technologies

  • High energy density
  • High Power density
  • No complex Battery Management System
  • No thermal management needed
  • No thermal runaway risk
  • Operates and charges from -40°C till up to +80 °C
  • Batteries are resilient in case cells fail
  • Much longer lifetime

Benefits at system level

Power Capacitors enable applications that are often not optimal when using lithium based batteries or lithium based supercapacitors.

  • High peak demands can be met with much smaller battery packs
  • Fast charging drastically reduces unproductive time
  • Lower life cycle costs
  • More energy can be captured from external sources
  • Reduced system complexity
  • Safe and reliable at all temperatures

Using clean electric energy has always been hampered by the limitations of how to save and store it in batteries, how to use the energy from it and especially on how to charge these batteries. With Carbon based Power Capacitors many applications become practical again en they enable to reach clean energy sustainability as never before.