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Getting to Know the Battery

 

Getting to Know the Battery

The battery dictates the speed with which mobility advances. So important is this portable energy source that any incremental improvement opens new doors for many products. The better the battery, the greater our liberty will become.

Besides packing more energy into the battery, engineers have also made strides in reducing power consumption of portable equipment. These advancements go hand-in-hand with longer runtimes but are often counteracted by the demand for additional features and more power.
The end result is similar runtimes but enhanced performance.

The battery has not advanced at the same speed as microelectronics, and the industry has only gained 8 to 10 percent in capacity per year during the last two decades. This is a far cry from Moore’s Law* that specifies a doubling of the number of transistors in an integrated circuit every two years. Instead of two years, the capacity of lithium-ion took 10 years to double.

In parallel with achieving capacity gain, battery makers must also focus on improving manufacturing methods to ensure better safety. The recent recall of millions of lithium-cobalt packs caused by thermal runaway is a reminder of the inherent risk in condensing too much energy into a small package. Better manufacturing practices should make such recalls a thing of the past. A generation of Li-ion batteries is emerging that are built for longevity. These batteries have a lower specific energy (capacity) than those for portable electronics and are increasingly being considered for the electric powertrain of vehicles.

People want an inexhaustible pool of energy in a package that is small, cheap, safe and clean, and the battery industry can only fulfill this desire partially. As long as the battery is an electrochemical process, there will be limitations on capacity and life span. Only a revolutionary new storage system could satisfy the unquenchable thirst for mobile power, and it’s anyone’s guess whether this will be lithium-air, the fuel cell, or some other ground-breaking new power generator, such as atomic fusion. For most of us, the big break might not come in our lifetime.

Meeting Expectations

Many battery novices argue, wrongly, that all advanced battery systems offer high energy densities, deliver thousands of charge/discharge cycles and come in a small size. While some of these attributes are possible, this is not attainable in one and the same battery in a given chemistry.

A battery may be designed for high specific energy and small size, but the cycle life is short. Another battery may be built for high load capabilities and durability, and the cells are bulky and heavy. A third pack may have high capacity and long service life, but the manufacturing cost is out of reach for the average consumer. Battery manufacturers are well aware of customer needs and respond by offering products that best suit the application intended. The mobile phone industry is an example of this clever adaptation. The emphasis is on small size, high energy density and low price. Longevity is less important here.

The terms nickel-metal-hydride (NiMH) and lithium-ion (Li-ion) do not automatically mean high specific energy. For example, NiMH for the electric powertrain in vehicles has a specific energy of only 45Wh/kg, a value that is not much higher than lead acid. The consumer NiMH, in comparison, has about 90Wh/kg. The Li-ion battery for hybrid and electric vehicles can have a specific energy as low as 60Wh/kg, a value that is comparable with nickel-cadmium. Li-ion for cell phones and laptops, on the other hand, has two to three times this specific energy.

The Cadex-sponsored website www.BatteryUniversity.com generates many interesting questions. Those that stand out are, “What’s the best battery for a remote-controlled car, a portable solar station, an electric bicycle or electric car?” There is no universal battery that fits all needs and each application is unique. Although lithium-ion would in most instances be the preferred choice, high price and the need for an approved protection circuit exclude this system from use by many hobbyists and small manufacturers. Removing Li-ion leads back to the nickel- and lead-based options. Consumer products may have benefited the most from battery advancements. High volume made Li-ion relatively inexpensive.

Will the battery replace the internal combustion engine of cars? It may come as a surprise to many that we don’t yet have an economical battery that allows long-distance driving and lasts as long as the car. Batteries work reasonably well for portable applications such as cell phones, laptops and digital cameras. Low power enables an economical price; the relative short battery life is acceptable in consumer products; and we can live with a decreasing runtime. While the fading capacity can be annoying, it does not endanger safety.

As we examine the characteristics of battery systems and compare alternative power sources, such as the fuel cell and the internal combustion (IC) engine, we realize that the battery is best suited for portable and stationary systems. For motive applications such as trains, ocean going ships and aircraft, the battery lacks capacity, endurance and reliability. The dividing line, in my opinion, lies with the electric vehicle.                                                           

*   In 1965, Gordon Moore said that the number of transistors in an integrated circuit would double every two years. The prediction became true and is being carried into the 21st century. Applied to a battery, Moore’s Law would shrink a starter battery in a car to the size of a coin.