Ultracapacitors are super high capacity versions of electric capacitor components that store energy in an electric field. These are devices that have raised the “capacity” of a capacitor so high that ultracapacitors or “supercapacitors” are now thought of as energy storage devices that can replace batteries in some applications. They are available in various cylindrical, square, and flat shapes and sizes. With increasing production volumes and economies of scale, their prices can be competitive with batteries.

Ultracapacitor Characteristics

Ultracapacitors, sometime called Ucaps, are high-power/low-energy devices while batteries tend to be high-energy/low-power devices. The lower equivalent series resistance of an ultracapacitor gives it higher roundtrip (in and out) energy storage efficiency, especially with lower losses at high currents. Ultracapacitor packs are ideal for storing high power braking regeneration energy and supplying quick acceleration energy. The original Honda FCX hybrid fuel cell car used ultracapacitors for the energy storage, as do some of the new hybrid electric transit buses.

Being a capacitor, the state of charge (SOC) energy is precisely determined by the square of the voltage (E=½CV²), where C is the capacitance. The SOC discharge profile is significantly different from a battery, but offers some unique characteristics and advantages. Low temperature performance down to a -40°C (-40°F) shows little degradation. Above 89°C (192°F) internal temperature the electrolyte will vaporize and cause a flash explosion. In general the cycle life is limited by the temperature history of the electrolyte, which slowly decomposes. Over a million complete 100% charge/discharge cycles can be expected.

Ultracapacitor Construction

Capacitors are electric components that store energy in an electric field between two electrically conducting plates or “electrodes”. Ultracapacitors or “supercapacitors” have electrodes are made from a high porosity carbon much like the carbon in air and water filters. This high porosity carbon (in some new devices carbon nanotubes) is somewhat like a microscopic sponge that has extremely high surface areas that increase the “capacity” of a capacitor to such a high density that ultracapacitors are now thought of as energy storage devices that can replace some battery applications.

Ultracapacitors are known as double layer carbon devices because the actual construction is like a layer cake with carbon electrodes on the top and bottom. In between the electrodes is an electrolyte to facilitate the movement of charged microparticles. A special separator sheet lies in the middle of the electrolyte to isolate the voltage between the top and bottom electrodes. Ultracapacitors are rolled up like a “jelly roll” to make cylindrical devices and stacked in flat sheets to make prismatic devices. The cylindrical devices dissipate most of the charging/discharging heat through the end electrode connections while the prismatic devices dissipate most of the heat through the flat sides.

Unlike some modern batteries of similar construction no chemical reaction takes place to store or release energy. One of the electrodes in a battery is made from a different material and different electrolytes are chosen to act as catalysts for the chemical reaction.

Ultracapacitor and Batteries

The combination of ultracapacitors with batteries offers potential advantages in power, energy, temperature range, and life. However, the different SOC voltage profiles make such combinations challenging especially in high power vehicle applications. There have been over 50 technical papers published on combining batteries and ultracapacitors.

I maintain that, for many applications, a simple parallel ultracapacitor pack combination with a battery pack could double the life of the battery pack and pay for the extra complexity.

At least one manufacturer is developing a type of hybrid battery/ultracapacitor cell for use in vehicles. An actual vehicle test using an energy storage pack built with hybrid battery/ultracapacitor cells more than doubled the pack life. The AFS Trinity hybrid SUV offered a combination of battery and ultracapacitor packs. Small scale testing in a camera circuit demonstrated the combination advantages and I suspect that there are some small cordless tool applications that have already implemented this design.

A Short Primer on Ultracapacitors is a post from: CleanCarTalk

The history of battery packs for electric vehicles is filled with stories of early failures and fires. Optimum and safe pack performance requires balancing and equalization management of the pack state of charge (SOC). A battery pack SOC is the equivalent of a fuel gauge which tells you how much usable charge remains in the pack.

Battery pack balancing and equalization management is the regulation or protection of individual battery cells from hitting the maximum or minimum limits when fully charging or discharging the whole pack.

Here are several methods used by battery pack designers to keep a battery pack in balance in order to get top performance out of the pack.

1. Ignore it – For only few cells in series and a limited number of charge/discharge cycles (like a 12-volt starting battery with six cells) that don’t deplete the battery, balancing doesn’t add much to the battery life. For large numbers of individual batteries in a full parallel and series matrix connection the distribution of cells may be good enough to average out variations and provide some fault tolerance without balancing, however, it can create a huge connection and cooling problem. For example, the Tesla Roadster sports car is thought to have a matrix connected type of battery pack with over 8,000 small AA batteries and connections. It almost always turns out that the cells in the middle of the pack are the hottest and hardest to cool. I’ve suggested that a good university student project would be to model such a design and determine sensitivities to manufacturing tolerances, aging, and failures of individual cells.
    
2. Charge each cell or sub pack separately – For example, if a number of 12-volt packs are connected together for a higher voltage pack, each 12-volt battery is charged to a full voltage SOC with a separate 12-volt charger rather than charging the whole pack with a single high voltage charger. If this is done periodically, like once a day, the assumption is that the charge/discharge cycles during the day won’t result in enough unbalance to be a problem. Multiple chargers and connection points can make this a bit cumbersome and time consuming.
    
3. Use a shunt clamp - During charging, as each battery in the pack reaches a predetermined clamp voltage, shunt (switch) the current around the battery to the next battery in line. This process is better known as equalization because every battery in the pack is “equalized” to the same maximum beginning voltage. This requires extra switching circuits that can be tricky for high voltage and high current applications.
    
4. Shuffle the charge - During operation constantly move some charge (current) from batteries with a high SOC to batteries with a low SOC. This requires even more complex circuits and can be limited by how much current can be moved at any time. It works well if the batteries don’t go through a lot of rapid deep cycling because it could take a while to complete all the shuffling. A failing battery in the pack could put a heavy demand on the shuffling system.
    
5. Monitor the pack - Continuously monitor the individual battery voltages in the pack and create a maintenance flag to replace the failing battery. This can cause disruptive and costly down time if not well planned, and typically requires many sensors, extra wiring connections, data acquisition network, some form of processing and reporting (display). This approach is commonly used in addition to balancing and equalization.
    
6. Use a passive network - Connect a resistor in parallel with each cell in the pack. This is a cheap and easy equalizing voltage divider that may work well depending on the resistor value and the length of time available for voltage equalization. However, it can create inefficiencies, generate extra undesired heat, and shorten the battery pack life because of extra deep discharges.
    
7. Others - Dozens of issued patents and published patent applications offer a source reference for finding other schemes to solve this problem.

By managing the battery pack SOC and limiting the operation to stay between a full charge and full charge depletion, battery pack life can be significantly extended.

Why is it bad to fully deplete the pack? Some battery chemistries change the composition of the electrode surfaces at the extreme ends of the SOC (fully charged or full depletion). For example, lead acid batteries at a depleted SOC deposits a sulfate coating on the plates thereby reducing the available electrode surface area. The electrode surface area is where the electric action takes place and the amount available determines the battery performance. Other chemistries such as Li Ion seem to be much less susceptible to electrode damage. However, all chemistries are susceptible to individual cell damage due to imbalances at the extreme ends of the SOC. Most battery pack manufacturers will recommend operating in the middle 30% to 50% SOC for optimum battery pack life.

Take the Toyota Prius as an example. Toyota does extremely well in the battery pack design and operation in the Prius hybrid car. The Prius Nickel Metal Hydride (NiMH) battery pack has 800 to 2000 full charge cycles because of the nature of the battery chemistry. However, by allowing use of only 7% of available battery capacity (usually in the middle of the SOC, about 50-60%), Prius’ battery packs are achieving 8 to 10 years and 100,000 to over 300,000 mile life of the pack. When Toyota says you can drive the Prius on electric alone, this really means the system allows you to drive on electricity alone using 7% of available SOC before the engine comes on for recharging. Because pack life performance has been much better than expected, Toyota has increased the operating SOC range to 15% to increase driving efficiency by capturing more of the braking energy.

The only downside of all of this complex battery pack SOC management is the fact that you are carrying around all the extra battery weight that it appears does not get used. You charge the battery pack only to 50-60% SOC, then use only 7% of that before recharging again. You may be tempted to somehow shave off battery weight to lighten the load the car has to lug around. In reality, it’s actually cheaper to extend the life of the battery pack (through maintaining ideal SOC range, hence heavy battery pack weight) than to save fuel by carrying around less weight.

Similar considerations apply to plug-in hybrids and all electric vehicles that we are going to look at in future posts. You probably don’t need to know all this to drive your hybrid or electric car, but you can give yourself extra battery life by taking advantage of every convenient charging opportunity and not waiting until the battery is fully depleted. We’ll talk more about leakage depletion and how to avoid shelf life damage.

Design Considerations for Keeping Your Battery Pack in Top Performance: Balancing and Equalization vs. Pack Monitoring is a post from: CleanCarTalk