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