Pairing ultracapacitors and batteries aids reliability

Pairing ultracapacitors and batteries aids reliability

In applications like hybrid cars, ultracapacitors improve system lifetime and performance

BY CHAD HALL
Ioxus
Oneonta, NY
http://www.ioxus.com

Ultracapacitors provide rechargeable energy to prolong the lifespan of other energy sources, such as batteries, in transportation and other green energy applications. Battery packs paired with ultracapacitor packs and control electronics can increase the life of a battery, such as those used in forklifts (lead acid), or plug-in hybrid electric vehicles (nickel-metal hydride or lithium ion). The battery is then used as an energy source and the ultracapacitor pack is the peak power component. This combination can extend the battery cycle life by up to 400%, reducing the warranty costs for the manufacturer and battery replacement costs and inconvenience for the vehicle owner.

Batteries, fuel cells, and ultracapacitors

Ultracapacitors or electrochemical double layer capacitors (EDLC) have been commercially available in a wide range of sizes up to 5,000 farads for approximately 10 years, and smaller sizes, on the order of from one to a few Farads, have been around since the late seventies. When first introduced, they were used in low-voltage applications in computers and other electronic appliances as a power source for internal clocks or volatile memory. All of the early applications involved low current draws and essentially used the ultracapacitor device as a back-up power source.

The voltage limitation of ultracapacitors is a result of the underlying chemical nature of these devices and is limited by the chemical potential of the electrolyte. In order to use them in higher voltage applications, the capacitors can be configured in a series string of identical value cells with the voltage divided by the number of cells in the string to stay within cell voltage ratings. However, the cells in series strings may have slightly different capacitances due to variations in manufacturing, presenting a potential problem, as it is possible to exceed the nominal voltage rating of one of the elements in the string, resulting in degradation of the ultracapacitor and premature failure of the string.

High-voltage applications have lagged because of the necessity to cell balance, either using a passive resistance element across each ultracapacitor or a more expensive active dynamic balancing system, and the cost of the devices themselves. With improved manufacturing techniques and falling prices, ultracapacitors are becoming more attractive for a variety of applications including their use with batteries and fuel cells.

Energy and power density

The terms energy and power density are expressed in a variety of ways, for example the closely related kW/kg or J/kg. The energy density is simply a measure of the total energy that a power source is capable of giving, and power density is a measure of the rate at which the system can deliver that energy. Batteries and fuel cells both produce electrical energy by virtue of a chemical reaction, and the rate of energy delivery is limited by the reaction rate, which is influenced by a variety of factors such as concentration of reactants and the internal resistance. Typically batteries and fuel cells are capable of delivering enormous amounts of stored energy, but they cannot always deliver that energy at a sufficient rate under heavy load conditions. An ultracapacitor, on the other hand, stores energy in an electrostatic field and when energy is required the field will collapse almost instantaneously with the rate of delivery limited only by the internal resistance.

For example, consider two power sources with differing internal resistances but the same voltage connected to a load by a switch. At the instant a load is impressed across the supply, current flows from each power source to the load. In the circuit on the left in the figure , the internal resistance of one source is r and the other is kr, where k is a variable parameter that determines the ratio of the internal resistance of the two sources. The currents flowing in each branch are easily calculated and are found to be inversely proportional to the internal resistances of the power sources. In particular, if current i1 flows out of source V1 then ki1 flows out of V2 provided the V1 and V2 are equal. FAJH_Energy_1_Nov2009

The two circuits above are equivalent at the time the load is initially impressed across the power source.

Apply the math

In the circuit on the right in Figure 1, one power source could be a battery or fuel cell and the other a fully charged ultracapacitor device. This simple analysis shows that the current delivered to the load is dependent on the internal resistance of each device. A more in-depth analysis that takes into account the fact that the voltage across the capacitor changes with time yields the same result at time t = 0. The mathematics involves solving the relevant differential equations that describe the system which gives the essentially the same result as the simple example discussed above, but also includes the time dependence of the system as it settles into its steady state.

In the design of an actual supply the effectiveness of the ultracapacitor will depend directly on the ratio of the internal resistance of the ultracapacitor with respect to the energy source. If the internal resistances are roughly equal, the ultracapacitor will help only a small amount. However, if the ESR of the ultracapacitor is an order of magnitude smaller than that of the battery or fuel cell, the effect will be dramatic. The ultracapacitor does the brunt of the work when the load is initially switched on and allows the battery or fuel cell to pick up load gradually, preventing high current draw from the primary energy source. By allowing the battery or fuel cell to gradually take on load, it is insulated from high current drain, which is beneficial for the energy source since high reaction rates produce thermal, chemical, and mechanical stresses which result in faster wear out.

Ultracapacitors from Ioxus, as an example, feature a lifetime of 10 years. After 500,000 cycles between rated voltage and half rated voltage, under constant current at 25°C, capacitance has a less then 30% decrease and ESR less than 200% increase. Operating temperature range is –40° to +65°C and the ESR is just 4.5 mohms for the 400-farad RSC2R7407SR device, which is rated at 2.7 V and costs about $18 each at quantity 1,000.

Optimization

Generally speaking, the more capacitance the better. However, practical limitations are a factor as the voltage of the system rises. The maximum voltage rating of a typical ultracapacitor is 2.5 to 2.7 V. The sizing of a system needs to be looked at on a case-by-case basis, and in many cases it is desirable to build a more elaborate parallel supply by incorporating power electronic components such as boost-buck converters so that an ultracapacitor bank of a lower voltage can be used in a high voltage application. The idea of a parallel combination is clear; the engineering challenge is the design control circuitry that maximizes efficiency. ■