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% 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 immerging that is built for longevity. These batteries have a lower energy density 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 power density 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.
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 energy density 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 term nickel-metal-hydride (NiMH) and lithium-ion (Li-ion) does not automatically mean high energy density. For example, NiMH for the electric powertrain in vehicles has an energy density 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 an energy density 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 energy density.
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. We now compare the battery as a power source and begin with the positive traits, and then look at the limitations.
Batteries store energy well and for a considerable length of time. Primary batteries (non-rechargeable) hold more energy than secondary (rechargeable), and the self-discharge is lower. Alkaline cells are good for 10 years with minimal losses. Lead-, nickel- and lithium-based batteries need periodic recharges to compensate for lost power.
The battery holds plenty of energy for portable applications, but this does not apply to large-scale mobile and stationary use. For example, a 100kg (220lb) battery produces about 10kWh of energy (100Wh/kg), an IC engine of the same weight generates 100kW, ten times more.
Batteries have a huge advantage over other power sources in being ready to deliver on short notice — think of the quick action of the camera flash! There is no warm-up, as is the case with the internal combustion (IC) engine; the power from the battery flows within a fraction of a second. In comparison, a jet engine takes several seconds to gain power, a fuel cell requires a few minutes, and the cold steam engine of a locomotive needs hours to build up steam.
Rechargeable batteries have a wide power bandwidth, a quality that is shared with the diesel engine. In comparison, the bandwidth of the fuel cell is narrow and works best within a specific load. Jet engines also have a limited power bandwidth. They have poor low-end torque and operates most efficiently at a defined revolution-per-minute (RPM).
The battery runs clean and stays reasonably cool. Sealed cells have no exhaust, are quiet and do not vibrate. This is in sharp contrast with the IC engine and larger fuel cells that require noisy compressors and cooling fans. The IC engine also needs air and exhausts toxic gases.
The battery is highly efficient. Before reaching 70% charge saturation, the charge efficiency is close to 100%. Discharge losses are only a few percent. In comparison, the energy efficiency of the fuel cell is 20 to 60%, and the efficiency of thermal engines is 25 to 30%. (At optimal air intake speed and temperature, the GE90-115 on the Boeing 777 jetliner is 37% efficient.)
The sealed battery operates in any position and offers good shock and vibration tolerance. This benefit does not transfer to the flooded batteries that must be installed in the upright position. Most IC engines must also be positioned in the upright position and mounted on shock- absorbing dampers to reduce vibration
. Thermal engines also need air and an exhaust.
Lithium- and nickel-based batteries are best suited for portable devices; lead acid batteries are economical for wheeled mobility and stationary applications. Cost and weight makes batteries impractical for electric powertrains in larger vehicles. The price of a 1 kW battery is roughly $1,000 and has a life span of about 2,500 hours. Adding the replacement cost of $0.40/h and an average of $0.10/kWh for charging, the cost per kWh comes to about $0.50. The IC engine costs less to build per watt and lasts for about 4,000 hours. This brings the cost per 1kWh to about $0.34.
With the exception of watering of flooded lead batteries and discharging NiCds to prevent "memory," rechargeable batteries require low maintenance. Service includes cleaning of corrosion buildup on the outside terminals and applying periodic performance checks.
The rechargeable battery has a relatively short service life and ages even if not in use. In consumer products, the 3- to 5-year lifespan is satisfactory. This is not acceptable for larger batteries in industry, and makers of the hybrid and electric vehicles guarantee their batteries for 8 to 10 years. The fuel cell delivers 2,000 to 5,000 hours of service and, depending on temperature, large stationary batteries are good for 5 to 20 years.
Like molasses, cold temperatures slow the electrochemical reaction and batteries do not perform well below freezing. The fuel cell shares the same problem, but the internal combustion engine does well once warmed up. Charging must always be done above freezing. Operating at a high temperature provides a performance boost but this causes rapid aging due to added stress.
Here, the battery has an undisputed disadvantage. Lithium- and nickel-based systems take 1 to 3 hours to charge; lead acid typically takes 14 hours. In comparison, filling up a vehicle only takes a few minutes. Although some electric vehicles can be charged to 80 percent in less than one hour on a high-power outlet, users of electric vehicles will need to make adjustments.
Nickel-cadmium and lead acid batteries contain hazardous material and cannot be disposed of in landfills. Nickel-metal-hydrate and lithium systems are environmentally friendly and can be disposed of with regular household items in small quantities. Authorities recommend that all batteries be recycled.
* 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.
Next month: detailed battery definintions
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc., a company that manufactures battery test and diagnostic equipment, and creator of www.BatteryUniversity.com.