In portable applications where battery life is important, designers may evaluate various battery chemistries to determine the best energy source. For some designs, traditional alkaline batteries will provide acceptable performance and battery life. In other cases, higher performance and thus higher-cost batteries must be considered. However, in designs where alkaline batteries are insufficient, there is a lower cost alternative to high-performance batteries. That alternative is to parallel ultracapacitors with the batteries. This option is particularly useful in portable power tools where peak current demands are high.
Consider the example of a tube-cutting tool designed by Superior Tool Co. (Cleveland, Ohio). This product puts demands on the battery for high peak loads experienced during the onset of cutting. This loading is typical of many portable devices where the peak load is much higher than the average power demand.
For any portable device, the first step will be to specify the required power source. In the tube-cutting example, Superior Tool planned to produce tools capable of using either primary alkaline cells or rechargeable NiMH cells. The initial target specification for the cutting tool necessitated a ½-in. copper pipe to be cut in less than 10 sec, with the ability to make at least 100 cuts using either primary alkaline cells or rechargeable NiMH cells as the energy source. Lab testing determined the work required to cut a piece of ½-in. copper tubing in 10 sec to be on the order of 50 Wsec.
Once energy figures are determined, the next step is to work out the kind of batteries that could be suitable. For the tool cutter, initial calculations determined four series-connected alkaline AA cells could provide the necessary energy. A typical alkaline AA battery is rated at 2500 mAH. Assuming 33% efficiency, four cells can produce roughly 54,000 Wsec yielding 360 cuts. So, the energy storage for the specified 100 cuts is not a problem.
However, the internal impedance of the alkaline cells and the high initial current load demanded by the cutting tool were not considered in these calculations. With the typical internal impedance of a AA alkaline battery at 200 mΩ and a peak load current draw between 4 A and 5 A, it was quickly determined the initial voltage drop rendered the tool inoperable. In the tool-cutting application, it was determined — as a rule of thumb — that peak voltage sags higher than 0.5 V were too demanding and caused undesirable effects on the battery sizes required for this application.
This example shows how it is vital to consider both the overall energy-storage requirements on the batteries and the peak power characteristics of the application. For many applications, the battery has traditionally been sized for the peak power demands, meaning that a larger and heavier battery has been used than is required for the overall energy-storage demands. This is where ultracapacitors, connected in parallel, can help manage the peak power demands.
For the tube-cutting application, the peak power demand is the limiting factor. While different battery chemistries provide equivalent life and capacity for primary cells, their behavior at high currents is significantly different. At 4 A, the AA alkaline cells produce a small fraction of their rating. In rechargeable NiMH cells, both capacities per charge and life cycles decline as current increases. Lithium cells tend to be internally protected against high power demands to avoid excess cell heating.
Conventional Li-ion rechargeable cells such as those used in laptop computers, which are rated below 2 A, are also limited in power. Recently, some of the major professional power tool manufacturers have come out with new technology Li-ion cells. These are adequate in environments where power and energy storage are needed, but are not economical in a tool that runs 5 min per day.
The tube cutter is a good example of the kind of power design challenge often faced. Alkaline cells are unable to supply the peak power, and Li-ion cells are too expensive. But can NiMH batteries provide a good compromise solution? Superior Tool found that a six-cell NiMH battery configuration was required to provide the needed peak power. The six-cell configuration was arrived at experimentally. First, a deregulated dc power supply was connected to a prototype of the tube cutter to determine the required operating voltage (approximately 7 V). Then, it was determined that six NiMH cells in series would provide that voltage under load.
However, this arrangement meant that they had to reduce their original cut time specification from 10 sec to 5 sec. In practice, this was considered acceptable. However, the battery life (determined experimentally) was marginal for achieving the 100 usable cuts-per-charge requirement. As a result, NiMH cells by themselves were deemed unsuitable.
By connecting one or more ultracapacitors in parallel with the batteries, the ultracapacitors can provide the peak power demands of the application and can be recharged from the battery when the power demands are lower. The low impedance of the ultracapacitor means that it can provide high power from a relatively small device, and can be recharged quickly or slowly as required.
In the cutting design, Superior Tool tested ultracapacitor/battery parallel arrangements with alkaline cells. Initially, alkaline cells tested alone produced under 10 cuts, and applying ultracapacitors in parallel meant the design goal of 100 cuts with 3 sec to 5 sec cut times could be met.
Similarly the NiMH solution was tested in combination with ultracapacitors, which produced somewhat less dramatic (but still impressive) improvements. The number of cuts between recharges was increased by more than 30% compared to the NiMH battery alone. At a minimum, this also would translate directly to a 30% increase in battery life, as the life of a rechargeable chemistry is related to the number of recharging cycles. It is anticipated that the life may be at least doubled due to the reduced peak current demanded by the battery if ultracapacitors are put in parallel.
The final tool cutter design included three 10-F, 2.5-V ultracapacitors in series (see the figure). This arrangement allows six alkaline cells rated at 7.2 V and the three ultracapacitors rated up to 7.5 V to be placed in parallel. Although the alkaline cells are nominally rated at 1.5 V, their voltage under load drops quickly as a result of their internal impedance. In addition, a diode is placed in series with the cells (in the battery pack) to prevent damage in the event battery cells are installed backwards against charged capacitors.
This design illustrates several important considerations with ultracapacitors. While these should be incorporated, they are relatively straightforward and do not add significantly to the cost or complexity of the design (see the figure).
First, load-balancing resistors (R1-R3) are needed in parallel with each ultracapacitor. (For guidance on how to select this resistor value, see “Cell Balancing in Low Duty Cycle Applications” at www.maxwell.com.) This is provided to ensure equal voltage distribution across the ultracapacitors. The value of the resistor is chosen to ensure that any variations in internal leakage current of the ultracapacitors will be overwhelmed by the balancing resistor. The battery life extension is due to the fact the ultracapacitors buffer the high current from the batteries.
Finally, a DPDT on/off switch (S1) should be used to disconnect the ultracapacitors from the battery and the load. This extends the life of the ultracapacitors and eliminates the possibility of the tool being started inadvertently, because the run switch is designed for easy access. Decoupling the ultracapacitor from the battery allows the ultracapacitors to discharge and prevents the batteries from feeding the continual current draw that would result due to the balancing resistors.
Several other components are shown in the schematic. D1 is used to brake the motor on stopping. The “Home” limit switch causes the tool to automatically return to the position where it is ready to cut again after a cut is made. F1 protects the tool against motor faults (like jamming) that would cause the motor to overheat. Meanwhile, F2 ensures that the 5-W resistor doesn't overheat and burn the tool when it operates in its slow mode. D3 is an LED that lights the cutter wheel position and acts as a pilot light for the tool. P1 and P8 are the connections to the motor.