Ultracapacitors Give Lift to Material Handling

by Chad Hall, Ioxus, Oneonta, NY

One of the most important benefits of ultracapacitors is their ability to increase the power density of an energy source. Any time a battery is used to supply power to a variable load, an ultracapacitor can be used to supplement the primary energy source by increasing its power density.

Batteries provide electrical energy by virtue of a chemical reaction, and the rate of reaction limits the rate at which energy can be delivered. When a battery is subjected to variable loads, the current draw can become enormous, causing internal heating that will shorten the longevity of the battery. However, when an ultracapacitor is introduced into the equation, this negative effect can be significantly mitigated.

Lead-acid batteries are an ideal candidate for combination with an ultracapacitor bank. A parallel combination of a battery and an ultracapacitor offers the best of both worlds: high energy density and high power density. The material handling industry was a natural avenue for this product, since forklifts are typically powered either by propane or lead-acid storage battery packs. (For some background on ultracapacitors and their applications, see “Ultracaps and Their Uses.”)

The battery packs used in forklifts are doubly useful. They not only provide electrical power, they also act as ballast to the forklift, which is part of the reason they are a popular power source. But in many large warehouse operations, the batteries have some severe limitations. It is not uncommon for a forklift operator to deplete a battery to a level that is inadequate to power the forklift, requiring a battery swap mid-shift. This situation is exacerbated in refrigerated warehouse environments—most commonly found in food distribution centers—because the performance of a lead-acid storage battery degrades with decreasing temperature.

The use of a battery pack in parallel with an ultracapacitor bank provides a power source that will typically last an entire shift. This battery plus ultracapacitor combination has the added advantage of increasing the useful life of the lead-acid battery pack. This latter point is particularly important since prolonging the useful life of the battery pack represents a significant economic advantage to the warehouse operator.

The reasons underlying the improved performance are two fold. First, more useful energy can be extracted from a battery if it is allowed to operate within its comfort range. In other words, batteries operate most efficiently when the current draw is kept moderate. Second, EDLC capacitor packs have the ability to operate efficiently and without degradation in low-temperature environments.

In a typical forklift application, the equipment can draw up to 700 A from the battery under heavy load conditions. But when the battery is operating with a parallel EDLC pack, this battery current can be reduced anywhere from 30 to 45 percent depending on the effective series resistance (ESR) of the EDLC pack and its interconnecting bus. The lower the ESR of the EDLC bank and its connecting bus, the greater the reduction of current drawn from the battery pack.

Due to the ultracapacitor’s ability to accept fast charges, such as those in regenerative braking, the additional benefits of energy harvesting can be used to realize better performance from the energy storage system. A battery, by contrast, has difficulty with this due to the chemical reaction rate, which is too slow to absorb the energy produced in a regenerative braking system.

The ability of a bank of ultracaps to lessen demands on the battery pack can be demonstrated with current measurements taken in an actual forklift application. However, before examining these results, it’s worth noting the role of control circuitry in the application as this circuitry plays a role in determining the effectiveness of the ultracapacitors.

Control Electronics

The use of ultracapacitors often requires sophisticated control circuits and power electronic circuits for an efficient and economical application. In an electric forklift application, the ultracapacitor bank and its associated electronics must include features necessary for the safe operation of the forklift, as well as features that maximize the length between battery charges or changes, and increase the overall battery life. Specifically, the controller employed in this application must perform the following tasks:

• Disconnect the ultracapacitor bank from the power bus in the event the voltage rises above a preset limit

• Disconnect the ultracapacitor pack when the battery pack is removed for maintenance

• Monitor current on the power bus and

• Engage the ultracapacitor pack when power is required.

The controller requirements are designed to protect the ultracapacitor pack from an overvoltage, which could damage the components; to incorporate safety LEDs that illuminate to show the ultracapacitor bank’s charge status; and to ensure the forklift will not operate from energy stored within the ultracapacitor bank when the battery pack is not present. This latter requirement is to ensure the safety of personnel, assuming that when the battery pack is removed, the forklift will be inoperable. Monitoring the current on the power bus allows the controller to engage the ultracapacitor bank under heavy load conditions, which prevents excessive current draw from the battery pack.

Performance Data

Measurements of battery current in an actual forklift application illustrate the effectiveness of ultracapacitors in reducing current draw from the battery under load (see the figure). The red trace in the figure represents the current from the battery without the ultracapacitor bank engaged. The green trace represents the current draw with the ultracapacitor bank engaged. As can be seen from the graph, the peak current of the battery pack alone is about 750 A and with the ultracapacitor bank engaged, the peak current is about 550 A or a reduction of about 25 percent.

Figure. Current drawn from battery in forklift application with and without ultracapacitors (EDLCs).

If a detailed analysis is carried out, one finds that the reduction in current drawn from the battery pack depends on the ratio of the battery’s internal resistance to the effective ESR of the ultracapacitor bank. The effective ESR includes the ESR of the EDLC and any additional resistance introduced by the bus and switching components of the controller that increase the overall series resistance of the ultracapacitor bank. Therefore, it is important to minimize the effective ESR of the ultracapacitor bank and control circuitry.

The other benefit of reducing the current draw of the battery pack is that it reduces the damaging effect of deep cycling the lead-acid storage battery, which is related to the number of charge-discharge cycles such a battery pack can withstand. Deep cycling a lead-acid storage battery reduces its effective life. While it is not clear how long the life is extended (based on the limited data available at this time), it is clear there is significant extension of the overall life of the battery pack. Anecdotal data suggests that battery pack life can be extended by a factor of two to three times, which is considered a conservative estimate.

About the author

Currently serving as chief operating officer of Ioxus, Chad Hall has used his extensive mechanical engineering and business experience to take Ioxus from funding to factory to launch.  Chad is responsible for manufacturing and testing of standard products, as well as further establishing the company and building the team, increasing sales, locating funding, and spurring continued product development. Chad is also responsible for educating the market on Ioxus products and training sales personnel on the technical aspects of the expanding product line.

Chad spent 14 years with Ioxus’ parent company, Custom Electronics, Inc. (CEI). In addition, he has been involved in several high-voltage, high-reliability capacitor projects at national labs. Chad attended State University of New York, Delhi and received a 3-M certificate in electro-mechanical design and drafting. Chad is a member of the American Society of Mechanical Engineers (ASME).