Freedom from the power cord for mobile products typically requires batteries along with their associated added weight. New technologies such as fuel cells and ultracapacitors offer an attractive alternative.
The key for maximizing system performance is sizing the components for the most efficient use. The fuel cell is a good source of continuous average power, while the ultracapacitor is better suited for peak power requirements. Ultracapacitors such as Maxwell's BOOSTCAP products can deliver up to 10 times the power with 10 times the expected life of a battery and offer lower weight. In addition, ultracapacitors also operate more reliably at high- and low-temperature extremes.
A mobile industrial servo system provides an example of an ultracapacitor and fuel cell replacing a battery and off-board power supply. By sizing the fuel cell for continuous instead of the peak power requirements, a smaller, less expensive fuel cell can be used. In addition, the ultracapacitor responds to the peak requirements better than a larger fuel cell. In one specific application, a 3.5A, 150V dc fuel cell supports the 2A continuous power for the system and provides 1.5A to charge the ultracapacitor. The ultracapacitor has to deliver 30A for 500 msec with a voltage drop of less than 25V to meet the specification. To determine the number of ultracapacitors needed, the application voltage (150V) is divided by the voltage per cell (2.5V), and this shows that 60 cells are required for the application. Using a 350F cell in the pack leads to a calculated pack capacitance of 350F/60cells=5.8F. This pack will then experience a reactive voltage drop of 500 mS * 30A / 5.8F = 2.6V. Knowing the ESR of the cell allows for a calculation of the resistive voltage drop as follows: 30A * 0.0032 (Ω per cell) * 60cells = 5.76V resistive voltage drop. Summing the two drops, at 30A for 500 mS the total voltage will drop 8.36V. That is well below the 25V requirement of the specification and provides plenty of operating margin. Optimization could lead to selection of a cell with lower capacitance as a way to result in less operating margin and reduce costs.
The total weight of the 60 ultracapacitors in series is 3.6 kg. To handle the peak loads in this application, the fuel cells alone would have been too large and expensive to be practical. The combination of ultracapacitors and the fuel cell operates longer and more reliably than the previous battery solution and the ultracapacitor pack has less weight and volume than the battery pack.
Power Combo: Fuel cells and ultracapacitors can provide power for portable applications based on the unique capabilities of both technologies. The continuous power comes from the fuel cells and the peak power from the ultracapacitors.
CONTACT: John Dispennette, Maxwell TechnologiesTel: 858-503-3300; e-mail: firstname.lastname@example.org://rbi.ims.ca/4394-509