Batteries are an essential ingredient for the growth of electronics from small devices used for IoT as well as large batteries for electric cars. Historically, battery energy density improves 5%-8% per year. While this is much slower than the historical improvements from Moore’s Law, it’s still the kind of growth that can result in leaps in efficiency, opening the door for a better experience and more applications.

The performance improvement of different battery chemistries looks relatively flat when compared to the Moore’s Law improvement percentage. Mobile energy requirements have exceeded battery capabilities and continue to push battery performance to a higher level. In the past 10 years, the latest two lithium battery chemistries have started to reach the mobile customer requirements.

Cylindrical cell, lithium-ion cell, and button cells have been popular options for mobile devices and IoT applications that run on low power. A stable power source is an issue that continues to challenge the use of batteries for continuous or long term power needs in mobile devices. Energy harvesting offers a power source to work with or without batteries. Energy harvesting has been around for decades but is recently getting more attention as the explosion of the trillion-sensor IoT market is threatened by the prospects of energy shortages. In addition, worldwide regulations associated with the reduction of climate change impacts is forcing the adoption of alternative power sources. Semico will be releasing an updated report on energy harvesting in September 2019.

Another option that is also seen as an alternative to batteries in some applications is the supercapacitor. The supercapacitor, also known as ultracapacitor or double layer capacitor, differs from a regular capacitor in that it has very high capacitance. A capacitor stores energy by means of a static charge as opposed to an electrochemical reaction. Applying a voltage differential on the positive and negative plates charges the capacitor. This is similar to the buildup of electrical charge when walking on a carpet. Touching an object releases the energy through the finger. Manufacturers of supercapacitors include AUX, Eaton, KEMET, Murata, NIC Components, Panasonic, Samsung, Surge Components, Taiyo Yuden and Vishay.

There are three types of capacitors, and the most basic is the electrostatic capacitor with a dry separator. This classic capacitor has very low capacitance and is mainly used to tune radio frequencies and filtering. The size ranges from a few pico-farads (pf) to low microfarad (μF).

The electrolytic capacitor provides higher capacitance than the electrostatic capacitor and is rated in microfarads (μF), which is a million times larger than a picofarad. These capacitors deploy a moist separator and are used for filtering, buffering and signal coupling. Similar to a battery, the electrostatic capacity has a positive and negative that must be observed.

The third type is the supercapacitor, rated in farads, which is thousands of times higher than the electrolytic capacitor. The supercapacitor is used for energy storage, undergoing frequent charge and discharge cycles at high current and short duration.

The charge time of a supercapacitor is 1 to 10 seconds. The charge characteristic is similar to an electrochemical battery, and the charge current is, to a large extent, limited by the charger’s current handling capability. The initial charge can be made very fast, and the topping charge will take extra time. Provisions must be made to limit the inrush current when charging an empty supercapacitor as it will suck up all it can. The supercapacitor is not subject to overcharge and does not require full-charge detection; the current simply stops flowing when full.

The following table compares the supercapacitor with a typical Li-ion solution.



Applications

The supercapacitor is often misunderstood; it is not a battery replacement to store long-term energy. If, for example, the charge and discharge times are more than 60 seconds, a battery is still recommended. If the charge time and discharge time are 60 seconds or shorter, then the supercapacitor becomes economical.

Supercapacitors are ideal when a quick charge is needed to fill a short-term power need, whereas batteries are chosen to provide long-term energy. Combining the two into a hybrid battery satisfies both needs and reduces battery stress, which results in a longer service life.

Japan employs large supercapacitors. The 4MW systems are installed in commercial buildings to reduce grid consumption at peak demand times and ease loading. Other applications include start-up backup generators during power outages and providing power until the switch-over is stabilized.

Supercapacitors have also made critical inroads into electric powertrains. The virtue of ultra-rapid charging during regenerative braking and delivery of high current on acceleration makes the supercapacitor ideal as a peak-load enhancer for hybrid vehicles as well as for fuel cell applications. Its broad temperature range and long life offers an advantage over the battery.

Supercapacitors have low specific energy and are expensive in terms of cost per watt. Some design engineers argue that the money for the supercapacitor would be spent better on a larger battery.

In terms of large battery applications the big performance competitor to batteries in the automotive arena is the fuel or gasoline engine. The big question is how long will it take for battery energy density to equal gasoline?

We would have to wait 48 years at an 8% compounded annual battery energy improvement before the gravimetric energy density (the energy capacity divided by weight) would exceed that of gasoline! But gravimetric energy density is not the only way to look at the performance and improvement benchmarks. Weight and efficiency have to be incorporated into the calculations.

We expect the storage per unit mass and volume of batteries will probably plateau within 10 to 20 years. At the same time, the market penetration of lithium batteries is doubling every 4 to 5 years. Direct comparisons between fuels and batteries must include the machinery needed to use that fuel and its efficiency.

The Tesla’s range is 200+ miles even though the battery pack is only around 150WH per kg, versus gasoline’s 12,000WH per kg. However, the gas engine has only about 25% efficiency versus 90%+ for the electric battery. Therefore, a Tesla has 3000 WH per kg when efficiency is factored in. An internal combustion engine weighs 840 pounds for a typical 340HP car. The Tesla motor weighs only 70 pounds for a similar horsepower rating.

A comparison of the energy density when incorporating the engine weight and energy efficiency results in the following formula:

Energy weight * Efficiency + Engine weight = Total Gravimetric Energy

The following table compares the specifications discussed above for an internal combustion engine and a Tesla electric engine at 340HP. Surprisingly, electric cars and gasoline cars have about the same energy density.

In conclusion, batteries are critical to portable electronics that make our lives more enjoyable and productive. Our desire for better battery life continues to push the envelope for better performance, increased efficiency and more sustainable solutions. When it comes to electric vehicles, their efficiency, weight and performance is now about equivalent to the energy density of internal combustion engines without the emissions. In the future, Semico has high expectations for supercapacitors, energy harvesting and other energy solutions not even on the table yet.