So what is an ultracapacitor? It can simply be thought of in the context of a "burst." As in a sudden burst of energy. In a battery, there is a chemical reaction going on inside; in an ultracapacitor, the energy is stored in an electric field. There is a positive charge on one side and a negative charge on the other, and the two are kept apart by a separator until needed — and then the opposites attract, pronto.
Another benefit of storing the charge in a field is that the ultracapacitor can be recharged many more times than a battery.
An example of this need for a burst of power is in automotive start-stop applications, where ultracaps are being used, with Maxwell, for example, supplying General Motors and PSA.
Think of a battery as something that takes a comparatively long time to discharge and then recharge (which is the case for your phone as it is for a car). This is not the case with ultracaps. So while a battery is appropriate for powering a car over the long run, an ultracapacitor provides more of a transient push.
One way that ultracaps can be recharged in a vehicle is through regenerative braking, wherein the kinetic energy of braking is captured and used to generate electricity that is then stored in the ultracap. This is something that Lamborghini is doing with Maxwell technology.
Another use of an ultracapacitor in a vehicle is to take care of peak transient load requirements. When the battery system alone just can't get it done, then the ultracap can kick in to handle the electrical demand.
Last year, for example, Zhejiang Geely Holding Group announced a technology partnership with Maxwell. (Geely owns Volvo, which, as you'll remember, has announced that it is putting an electric motor into every new vehicle it launches starting this year and plans to have 50 percent of its sales fully electric by 2025.)
Qingfeng Feng, vice president and CTO of Geely, said, "The Maxwell ultracapacitor subsystem has proven to provide significant support for our hybrid cars and will be able to economically handle the challenging peak power demands in a way that batteries alone cannot."
In this context, it is worth noting that one of Maxwell's markets is for power smoothing on utility grids, and Tesla has a business of providing energy storage for everything from municipalities to homes, so this ultracap tech is relevant there, too.
As for Tesla's vehicles? Perhaps the ultracapacitors will give rise to some sort of Ultra-Ludicrous mode.