By Stefan Vorkoetter
The electric model aircraft and car industries have produced a bewildering array of
field chargers for NiCd motor battery packs. These range from simple 6 or 7 cell
chargers consisting of a resistor and mechanical timer, to more complex chargers with
peak detection, cycling, and the ability to handle 36 cell packs. The resistor/timer type
of charger is cheap, but it is has two drawbacks: it might not fully charge the pack in the
allotted time, or it might overcharge the pack. The more complex chargers have none of these
drawbacks, but they are very expensive.
The charger described in this article can charge packs of 4 to 7 cells with capacities
ranging from 600mAh to 2Ah. The charger automatically begins charging when a pack is
connected. It charges at a (nearly) constant current (adjustable), and terminates the
charge when the pack begins to get warm (a NiCd pack begins to warm up when it has reached
full charge). An LED indicates that charging is in progress.
The circuit for the charger (Figure 1) is simple. Z1A is a comparator which compares
the voltage on its two inputs and produces a high output when the + input (pin 5) is
higher than the - input (pin 6), and a low output otherwise. R1 and R2 form a voltage
divider, presenting a fixed voltage (about 7V) to pin 5. A pair of thermistors (TR1 between
the PAK+ and TEMP terminals, and TR2 between the TEMP and GND terminals; see Figure 2) form
another voltage divider which presents a voltage to pin 6. This voltage is proportional to
the temperature difference between TR1 and TR2. When TR1 is within 10°C of TR2, this voltage
is below 7V, and the output of Z1A turns on Q1. This causes current to flow through Q2, R5,
and R4, turning Q2 on. This in turn causes Q3 to conduct, resulting in current flow through
the NiCd pack connected between the PAK+ and PAK- terminals. The amount of current flowing
through Q3 (and thus the pack) is determined by the current flowing through Q2, which in turn
is determined by the setting of R5. The current can range from about 2A to 5A. While Q1 is
on, current will also flow through LED1 and R6, thus illuminating LED1 to indicate that
charging is in progress.
As the temperature of TR1 rises, the voltage at pin 6 rises. When the temperature of TR1
exceeds that of TR2 by 10°C or more, pin 6 will exceed 7V, and Z1A will turn off Q1, Q2,
and Q3, terminating the charge. Z1B performs the same comparison as Z1A, but its output
is used to provide hysteresis (the current flowing into the base of Q1 pulls Z1A’s output
too low for it to perform this function). When Z1B goes low, current flows through R7,
lowering the voltage at pin 5 to about 1.4V. This ensures that the charger will not switch
back on as the pack cools off (unless it cools to about 50°C below ambient). The only way to
restart the charge is to disconnect the pack being charged. This will disconnect TR1,
causing pin 6 to go to 0V, which will turn the charger back on. Since there is now no pack
connected, no current will flow through Q3, even though the CHARGE LED is lit. When a pack
with a sufficiently cool TR1 is plugged in, charging will recommence.
TR1 and TR2 are identical thermistors. Their resistance is 10KW at 25°C, and the resistance
increases or decreases by about 4% for each 1°C fall or rise in temperature (the actual rate
of decrease and increase varies with temperature). TR2 is installed in the charging cable
near the charging plug to measure ambient temperature. TR1 is installed in the battery pack
to measure pack temperature. By using two thermistors, the charger will shut off based on the
temperature rise instead of the absolute temperature (otherwise the pack will be overcharged
on a cold day or undercharged on a hot day).
Connectors and Cables
I use 4-pin computer power supply connectors for charging my packs. My packs are all wired
permanently into my planes, so this connector does not need to handle the motor current.
Even if you connect your packs to your planes with connectors (eg. Sermos), I suggest each
pack have a separate charge connector, thus reducing wear and tear on the more critical
power connectors. I use the connector with the male housing (and female pins) in the plane,
and the female/male connector on the charger. Computer power supply splitter cables are a
good source of these connectors. I use the red and yellow leads for the battery +
and - connections, and the two black leads for the thermistor (TR1).
Figure 2 illustrates how the cable, connectors, and battery pack should be wired. The PAK+
and PAK- conductors should be 18ga or 16ga, since they must handle up to 5A. The GND and
TEMP conductors can be thinner since they handle less than 3mA.
For the 12V+ and 12V- inputs to the charger, use a two conductor 18ga or 16ga cable
terminated with large clips suitable for connection to a car battery. Lamp cord is good for
As the diagram implies, I have a separate thermistor in each of my packs. If you wish,
you can use a temperature probe that is permanently connected to the charging cable, and
insert this probe into the pack being charged. If you do this, you will need to install a
normally-open START push-button between the TEMP and GND terminals, since the charger will
not reset automatically with TR1 connected.
The circuit is designed to be installed in a Radio Shack project case (see
parts list). Any suitably sized enclosure with a metal lid (or an all-metal enclosure)
will do. R5 is glued to the component side of the board, with short lengths of wire
connecting it to the appropriate pads. The lid of the case is drilled for R5 and LED1.
The potentiometer is then installed in the appropriate hole, and this holds the board
in place inside the case.
Figure 3 represents the full-sized printed circuit pattern for the charger.
Figure 4 illustrates component placement on the board.
Solder short lengths of wire to the appropriate terminals of R5. Glue R5 to the board,
ensuring that R5’s shaft is in line with the holes for LED1, and solder the leads. Install
LED1, paying attention to polarity. The negative lead (usually indicated by a dot, flat spot,
or shorter lead on the LED) is furthest from R5. The LED should be installed so it is high
enough above the board to protrude through the corresponding hole that you’ll make for it in
Transistor Q2 should be laid flat on the board. A piece of aluminum channel, about 1.5
long and the width of Q2 should be placed on the copper side of the board, extending past
the end of the board. Hold Q2, the board, and the aluminum channel together with an
appropriate sized bolt. Ensure that the channel does not short circuit any traces.
Install the remaining components, ensuring that none of them stick up high enough to
interfere with the case once the board is installed. Transistor Q1 should have its
rounded side facing R3. I suggest you use a socket for Z1, because it is easily damaged
by soldering, and hard to remove if it is damaged. Install the socket, with pin 1 at the
top left corner.
Transistor Q3 should be installed on a hefty heatsink on the outside of the case and
connected to the rest of the circuit with wires. The emitter of Q3 connects to the point
marked E and the base to the point marked B. Use 16ga or 18ga wire for the emitter-to-E
Connect the charging cable to the circuit. Connect the PAK+, TEMP, and GND leads as marked
on the board. Connect the PAK- lead to the collector of Q1 (the case). Connect the supply
leads to the circuit at the points marked 12V+ (red) and 12V- (black).
Install everything in the case. The circuit
board occupies the left half of the case. The heat sink for Q3 is installed on the right
half of the front panel. The heat sink for Q2 extends past the circuit board, underneath
the top-right quarter of the case. LED1 protrudes through a vinyl grommet, providing more
Testing and Calibration
Connect the power leads to a 12V source (eg. a car battery). The LED should light immediately.
Connect a 50KW potentiometer between the PAK+ and TEMP leads, with the resistor set at the
half way point. The LED should stay lit. Slowly decrease the resistance. When the resistance
reaches approximately 10K (assuming 20°C room temperature), the LED should go out. The LED
should stay off even as you increase the resistance again. Temporarily disconnecting and
reconnecting the potentiometer should cause the LED to light once again.
To calibrate the charging current, use an ammeter in line with the PAK+ lead (an extra pair
of 4-pin connectors is handy for this). Monitor the current when charging a depleted pack
(the current will reduce towards the end of the charge, especially when charging 7 cell
packs). Note the settings of R5 required for different currents and mark them on the case
if you wish.
When using the charger for the first time, monitor it carefully. Feel the pack from time
to time (don’t touch the thermistor though or your body heat will terminate the charge).
The pack should barely start to warm up before charging stops.
I'd Like to Hear from You
If you build this circuit (or not), let me know what you think. If you have problems,
I may be able to help you, but be sure to supply a detailed description. I can be reached at:
8150 Concession 4
or on the Internet at:
The following table lists all the parts needed. Radio Shack part numbers are provided
for those parts available there. The thermistors used in the prototype are from Radio
Shack, although almost any thermistor with a 3% to 5% resistance drop per 1°C temperature
rise will do.
||50kW small potentiometer*
||2N3904, 2N2222, or equiv.
||TIP42, MJEF34, or equiv.
||2N3055 or equiv.
||LM393 dual comparator
||4 x 2 1/8 x 1 5/8 case*
||2 x 2 TO-3 heat sink*
||knob to fit R5
||vinyl grommet to fit LED
*Notes: If you cannot obtain an appropriately sized potentiometer, Radio Shack 271-1716
will do, but you’ll need a larger case. You can also use a 100kW potentiometer, which
will provide a wider current range (about 1A to 5A). If you cannot find an appropriate
heat sink, you can get away without it if you use an all-metal case such as Radio Shack