Ultracapacitors Boost Battery in Power Tools

Oct 1, 2006 12:00 PM
By John Dispennette, Application Engineer, Maxwell Technologies, San Diego

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.

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