ISE is one of the few hybrid system suppliers offering energy storage systems based on ultracapacitors - energy storage devices that can store and release energy more efficiently than chemical batteries. In heavy-duty vehicle applications, ultracapacitors can perform the most important energy storage functions - capturing regenerative braking energy and providing surge power for acceleration - with systems smaller and lighter than most battery technologies. While ultracapacitors are a relatively new technology, laboratory testing indicates they have much higher cycle lives than batteries. A pack of ultracapacitors is projected to have an operating life of at least five years in a heavy-duty hybrid application, and test data suggest that operating lives of ten years or more may be achievable. Another advantage of ultracapacitors is that they do not have to be externally recharged to maintain the health of the energy storage system, as is the case with many battery technologies.

Integrated ultracapacitor pack

Zebra battery pack

Cobasys NiMHax battery pack

ISE has teamed up with Maxwell Technologies, the leading U.S. manufacturer of ultracapacitors, to develop and market integrated ultracapacitor packs (pictured here). Two such packs, weighing a total of approximately 200 kg., provide peak power of 200 kW and 0.6 kilowatt-hours of total energy storage. This is adequate energy storage capacity to capture most available regenerative braking energy, and to supply large amounts of supplementary power for brief periods (e.g., 200 kW for up to ten seconds). The main disadvantage of ultracapacitors is that they do not store sufficient energy to operate vehicles without engine power for more than a few hundred yards. Achievement of significant all-electric operating range therefore requires use of batteries in lieu of, or in addition to ultracapacitors.

ISE offers an advanced battery energy storage subsystem based on an advanced technology battery using nickel sodium chloride technology. This "Zebra" battery, marketed by a Swiss company, MES-DEA, is more expensive than most lead-acid batteries, but has several desirable characteristics. Most notably, the Zebra batteries have a specific energy of 94 watt-hours per kilogram (Wh/kg), about three times that  of a typical lead-acid battery. This means the batteries can store substantially more energy per unit weight. In addition, the Zebra batteries are available in pre-integrated packs (shown here) with integral battery monitoring, thermal control, and charging capabilities.

Depending on specific vehicle requirements, anywhere from one to three Zebra packs can be used. Each Zebra battery pack (Figure 5) stores 17.8 to 20 kWhr of energy, can supply or absorb up to 33 kW of power, and weighs about 450 lb. For 30-foot and smaller transit buses, and for 40-foot buses operating on low-speed urban routes, one Zebra pack is usually adequate. For heavier buses and trucks, or vehicles with unusually high power requirements such as refuse trucks with large accessories, use of two or three packs may be required for optimal vehicle performance. Adding a second or third battery pack can also improve fuel efficiency by maximizing regenerative braking recapture, and provides greater all-electric operating range (operation of the bus with the engine completely turned off). Use of multiple packs also can reduce the depth of discharging of the packs, prolonging their life. ISE can assist vehicle fleet operators in conducting the economic and performance analyses required to determine the ideal number of battery packs.

The battery packs are designed to function in ambient temperatures ranging from -40 to +50 degrees Celsius, so battery performance is not adversely affected by the outside temperature. Another attractive feature of the Zebra battery is that it can provide high power down to an 80% depth of discharge level, which means that the ISE three-pack configuration can supply 50-90 kW of power until the batteries are depleted to their minimum safe level of 20% state of voltage.

Yet another attractive feature of the Zebra battery product is the fact that the cell monitoring and equalization systems are fully integrated into an enclosed pack, which greatly simplifies battery subsystem integration and maintenance. Achieving maximum battery life currently requires periodic external charging of the battery packs to maintain optimal cell equalization and balance between the three packs, but ISE and MES-DEA are working together to develop new charging algorithms that will enable the onboard APU to equalize the packs, without the need to ever recharge the batteries via external grid power.

The Zebra battery packs are fully warranted by MES-DEA for one year. Warranty extensions of up to five years are also available at an additional cost. By avoiding deep discharging of the batteries and equalizing the packs on a regular basis, ISE estimates the battery packs will last two to five years in most applications. These are preliminary estimates and are now in the early stages of being validated through testing of ISE demonstration buses using Zebra batteries.

ISE also offers a new energy storage subsystem based on an advanced battery using Nickel Metal-Hydride technology. This NiMHax battery system is offered by Cobasys. Nickel Metal-Hydride technology is used in most hybrid passenger vehicles. The NiMHax system is based on Cobasys, Series 1000 module and proprietary battery management system. This module is a high power design, therefore perfectly suited for hybrid-electric vehicle applications.