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Electronic Design

Nanophosphate Batteries Create High-Energy, Rechargeable Source

This design shows how we combined four 36-V DeWalt DC9360 nanophosphate battery packs in a series-parallel configuration to create a robust, high-energy power source. Originally intended for the prototype Neodymics Cyclemotor electric bicycle kit, this power source may be used in other applications. Output power was at least 1.6 kW at 66 V, energy capacity was about 300 W-hr, and recharge time was one hour. Also, the claimed cycle life exceeds 2000. The system was successfully tested over 70 cycles.

The battery packs weren’t modified, and the internal battery maintenance system (BMS) remained functional. The batteries were charged using DeWalt DC9000 chargers. This made it easier to use the latest battery technology—the DC9360, which employs lithium-iron-nanophosphate cells developed by A123 Systems.

Initially, a DeWalt DC900 drill was dismantled and its control circuitry studied. Voltage-controlled pulse-width modulation (PWM) varied with the drill trigger potentiometer (Fig. 1). The drill load is a brushed dc motor, which will produce 1 hp, according to DeWalt’s specifications. The drill power leads were connected to a resistive load, which was varied between 2.5 ? and 1 k?. The 4-kHz PWM power waveform varied between a 10% and 90% duty cycle as the drill trigger was advanced.

Further tests revealed that decreasing V5 from 2.4 V to below 1 V produced a 31-kHz PWM signal. This suggests that a single DC9360 could also efficiently generate other voltages, with the internal PWM control circuitry forming part of a buck or boost switching regulator.

An independent power control was built around a 50-k?, panelmount linear-taper pot, and the drill was reassembled. Duty cycle varied linearly with the position of the pot’s wiper. The fixed resistor network shown in Figure 1 yielded a continuous output for the resistive loads studied above.

In the series-parallel discharge circuit, pushing SW2 turns on pack B3, which sends current through the protection diode in B1 and energizes the optically coupled MOSFET relays U1 and U2 (Fig. 2). The energized relays enable all packs simultaneously. D1 is the on/off indicator. Pushing SW1 turns off the battery packs.

D2 is a dual Schottky diode with a low forward-voltage drop. These diodes form an ORing network to prevent a weaker pair of series-connected packs from draining a stronger pair. D3 and D4 provide a similar function for the switching circuit. D5 is a reversebias protection diode. Inductor L1 prevents the capacitive load of the motor controller from actuating the BMS protection circuitry and shutting a pack down when the system is turned on.

In the power system, each battery pack used a separate connector, which was cut from the base of a DeWalt 509 flashlight. We built the resistor network shown in Figure 1 into each connector. The operational load was a Crystalyte 406/409 brushless electric bicycle motor with a 72-V, 20-A PWM controller built into a 16-in. bicycle wheel.

We have operated the bike for about 700 miles, enabling sustained speeds of 25 mph. Range per charge at this speed was about 11 miles. After accounting for typical power generation, transmission, and transformation efficiencies, this electric bicycle compared quite favorably with the thermal energetic performance of other personal vehicles.1

It should be noted that this circuit produces dangerous voltages and appropriate precautions should be taken.


1. Radtke, J.L., “The Energetic Performance of Vehicles,” The Open Energy and Fuels Journal, March 28, 2008.

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