Temperature Controlled NICD Charger
This circuit is for a temperature controlled constant current battery charger.
It works with NICD, NIMH, and other rechargeable cells.
The circuit works on the principle that most rechargeable batteries show
an increase in temperature when the cells becomes fully charged.
Overcharging is one of the main causes of short cell life, hot cells pop
their internal seals and vent out electrolyte. As cells dry out, they lose
capacity. This circuit can quickly charge a rechargeable battery pack
without any negative effects.
This circuit uses a 22VDC power supply, my
version 2 circuit
can run from a 12VDC supply such as a solar-powered or automobile power system.
The transformer, bridge rectifier, and 1000uF capacitor provide around
22 Volts of DC power to run the rest of the circuit. The 7812 regulator
drops this to 12V to run the 311 comparator and 4011 nand gates.
The start switch is pressed to start the charging cycle.
This causes the two 4011 nand gates, which are wired as an r-s flip-flop,
to go into the charging mode.
The Red LED is lit, and the VMOS FET current switch is turned on.
Charging current runs though the battery pack.
If the battery starts out warmer than the reference temperature, the
circuit will not switch into charging mode. Let the pack cool down.
When the battery pack reaches a full state of charge, the differential
temperature sensor causes the flip-flop to switch off, turning off the
VMOS current switch, and lighting the Green LED.
The 7805 voltage regulator is wired as a constant current regulator.
This provides a safe maximum charge current for a number of different
The 500 ohm resistor across the VMOS FET sets the trickle charge current
which flows through the battery pack after the bulk charging is finished.
The 1N5818 diode prevents the pack from discharging if the AC power is
The resistor, diode, and capacitor around the start switch cause the
circuit to auto-start when power is first applied.
The differential temperature sensor circuit works by presenting
two voltages to the input of the 311 comparator. The comparator
output switches on or off depending on which input is at a higher
voltage than the other. As the thermistors warm up, their resistance
drops, lowering the associated comparator input.
Since there are two sensors, the room temperature can vary and
the circuit will only react to the difference in temperature between
Parts placement is non critical with the exception of the thermistors which
should be epoxied to separate aluminum plates and connected back to the main
circuitry via a cable with two wires and a shield. The 7805 regulator should
be mounted on a heat sink.
The current setting resistor may be a high power potentiometer, switch
selectable resistors as shown in the schematic, or a single fixed resistor
if you only need one current setting.
Be sure to thermally separate the
battery and sensors from the other electronics so the heat from the circuitry
doesn't affect the sensors. In my prototype, I glued the both thermistors
to separate aluminum sheets, approximately 2" X 2" and set the battery
sensor on top of the pack being charged, usually with a weight on top.
It is important to make sure the batteries have a decent thermal contact
to the sensor.
Allow the two temperature sensor plates to reach the same temperature,
place a volt meter across the 311 chip's + and - inputs (pins 2 and 3).
Adjust the sensor balance trimmer for a reading of -0.02 volts on the meter.
Press the reset button and make sure the "charging" LED lights. Warm the
battery temperature plate up with your hand and observe that the "done"
light comes on.
- Connect a rechargeable battery pack to the charger "+" and "-" connectors.
The pack may need to cool down to the ambient temperature before charging.
- Place the "battery temperature" sensor under the battery pack and hold
it in place with a rubber band or a heavy object.
- Place the "reference temperature" sensor in a location that is not too
close to the charger, the battery, or any other source of heat.
- Press the "reset" button, observe that the "charging" light lights.
Note: if the battery was recently discharged at a high rate, or it was
moved from a warmer place, it may be warmer than the ambient temperature
sensor and the circuit won't go into charging mode.
Let the battery cool down to ambient temperature, or temporarily
warm up the reference sensor if you are in a hurry.
- Note that some cells in a series string will always be first to get warm.
After several cycles it would be a good idea to leave the pack on the
charger for a few hours to trickle charge the lower cells up to a full
state of charge. This process is called "equalizing" the pack.
It is also possible to press the start button again, the weak cells will
get more charge for a while, then the full cells will warm up and turn
of the current.
The Thermistors and most of the other parts in this circuit may be obtained
from DigiKey (1-800-DIGI-KEY). Also see: http://www.digikey.com/
I have no connection to DigiKey other than being a satisfied customer.
Printed Circuit Patterns
Fernando from Brazil has generously submitted the following printed circuit
board artwork for this project:
Discharging NiCd Packs
Many people discharge their NICD packs to prevent the memory effect.
Memory effect is a controversial topic, the reader may want to research
it on their own.
Discharging is, in the least, fairly harmless if it is done correctly.
Discharging may also be beneficial for equalizing the charge level in a group
of cells. It may prevent the situation where a partially charged cell reaches
full charge before more fully discharged cells in a series string.
Cells can undergo a finite set of charge/discharge cycles,
discharging reduces the life of the cell.
A good way to kill NiCd cells is to put a reverse voltage on them.
If you incorrectly discharge your NiCd packs as a string, the weak cells
will go to zero volts and will then go negative as the stronger cells continue
discharging, thus damaging the reversed cells.
Over-discharging to a negative voltage can cause conductive
dendrite growth within the cell, that makes the cell become prone
If want to discharge your cells, it is best to discharge them individually,
the cells should not be drawn down to zero volts.
An easy way to accomplish this goal is to put a silicon diode in series with
a resistor for each cell, this sets the minimum battery voltage
to about 0.7 Volts.
For multi-cell packs, a discharger can be built with a multi-cell battery
holder and resistors and diodes for each cell.
A typical discharge current is C/5 where C is the amp hour capacity of the cell.
For a 500ma AA NiCd a 1N4001 or similar 1A diode in series with 5 ohm 1/2W
resistor should give a C/5 (100ma) discharge rate assuming a 1.2V cell,
and an 0.7V final resting voltage.
This charger circuit has been working quite nicely for several years now,
I have recharged many NICD and NIMH packs repeatedly with no problems.
The 1K thermistors may exhibit a small amount of self heating which
may result in less sensitivity to battery temperature changes. Changing to
5K thermistors would greatly reduce the effect. The balance pot should
be changed to 2K and the resistors on either end of the balance pot
should be changed to 3.9K.
to FC's Electronic Circuits page.