The post discusses a simple NiCd charger circuit with an automatic overcharge protection and a constant current charging.
When it comes to correctly charging a Nickel-Cadmium cell, it is strictly recommended that the charging process is halted or cut off as soon as it reaches the full charge level. Not following this may adversely affect the working life of the cell, reducing its backup efficiency significantly.
The simple Ni-Cad charger circuit presented below effectively tackles the overcharging criterion by including facilities like a constant current charging as well as cutting off the supply when the cell terminal reaches the full charge value.
Main Features and Advantages
- Automatic cut off at full charge level
- Constant current throughout the charging.
- LED indication for full charge cut off.
- Allows the user to add more stages for charging up to 10 NiCd cells simultaneously.
Circuit Diagram
How it Works
The simple configuration detailed here is designed to charge a single 500 mAh 'AA' cell with the recommended charge rate of close to 50 mA, nonetheless it could conveniently be customized cheaply to charge several cells together by repeating the area shown in dotted lines.
Supply voltage for the circuit is acquired from a transformer, bridge rectifier and 5 V IC regulator.
The cell is charged with a T1 transistor which is configured like a constant current source.
T1 on the other hand is controlled by a voltage comparator using a TTL Schmitt trigger N1. During the time the cell charges the terminal voltage of the cell is held at around 1.25 V.
This level appears to be lower than the positive trigger threshold of N1, which keeps the output of N1 high, and the output of N2 becomes low, enabling T1 to get the base bias voltage through the potential divider R4/R5.
As long as the Ni-Cd cell gets charged the LED D1 remains illuminated. As soon as the cell gets close to the full charge status its terminal voltage climbs to approximately 1.45 V. Due to this, the positive trigger threshold of N1 rises causing the output of N2 to go high.
This situation instantly turns off T1. The cell now stops charging and also the LED D1 is shut off.
Since the positive activation limit of N1 is approximately 1.7 V and it is controlled by a specific tolerance, R3 and P1 are incorporated to alter it to 1.45 V. The negative trigger limit of the Schmitt trigger is around 0.9 V, which happens to be lower than the terminal voltage of even a completely discharged cell.
This implies that connecting a discharged cell in circuit will never trigger the charging to initiate automatically. For this reason a start button S1 is included which, when pressed, takes the input of NI low.
To charge more number of cells the portion of the circuit revealed in the dotted box may be repeated separately, one for each battery.
This ensures that, regardless of the discharge levels of the cells, each one of them is individually charged to the correct level.
PCB Design and Component Overlay
In the PCB design below two stages are duplicated for enabling two Nicad cells to be charged simultaneously from a single board set up.
Ni-Cad Charger using a Resistor
This particular simple charger could be constructed with parts that could be seen in just about any constructor's junk container. For optimum life (number of charging cycles) Ni-Cad batteries must be charged with a relatively constant current.
This is often accomplished rather easily by charging via a resistor from a supply voltage many times higher than the battery voltage. Change in the battery voltage as it charges will likely then have minimal influence on the charge current. The proposed circuit is made up just a transformer, diode rectifier and series resistor as indicated in figure 1.
The associated graphical image facilitates the necessary series resistor value to be determined.
A horizontal line is drawn through the transformer voltage on the vertical axis until it crosses the specified battery voltage line. Then, a line pulled vertically down from this point to meet the horizontal axis subsequently provides us the necessary resistor value in ohms.
For instance, the dotted line demonstrates that if the transformer voltage is 18 V and the Ni-Cd battery to be charged is 6 V, then the resistance value will be around 36 ohms for the intended current control.
This indicated resistance is calculated to deliver 120 mA, while for some other charging current rates the resistor value will need to be reduced down appropriately, e.g. 18 ohms for 240 mA, 72 ohms for 60 mA etc. D1.
NiCad Charger Circuit using Auto Current Control
Nickel-cadmium batteries generally require a constant current charging. The below shown NiCad charger circuit is developed to supply either 50mA to four 1.25V cells (type AA), or 250mA to four 1.25V cells (type C) connected in series, eventhough it could simply be modified for various other charging values.
In the discussed NiCad charger circuit R1 and R2 fix the off-load output voltage to approximately 8V.
The output current travels by means of either R6 or R7, and as it rises transistor Tr1 is gradually switched on.
This causes point Y to increase, switching on transistor Tr2 and enabling point Z to become less an less positive.
The process consequently decreases the output voltage and has a tendency to bring down the current. A balance level is ultimately attained which is determined by the value of R6 and R7.
Diode D5 inhibits the battery which is being charged, providing supply to the IC1 output in case of the 12V is removed, which could otherwise cause serious damage to the IC.
FS2 is incorporated to protect against damage to the batteries which are under charge.
Choice of R6 and R7 is done through some trial and error, which means you will need an ammeter having a suitable range, or, if R6 and R7 values are genuinely known, then the voltage drop across them could be calculated through Ohm's Law.
Ni-Cd Charger using a Single Op Amp
This Ni-Cd charger circuit is designed for charging standard AA size NiCad batteries. A special charger is mostly recommended for NiCad cells for the reason that they possess an extremely low internal resistance, resulting in an increased charging current even if the utilized voltage is just slightly higher.
The charger should therefore include a circuit to restrict the charge current to a correct limit. In this circuit, T1, D1, D2, and C1 work like a traditional step-down, isolation, full-wave rectifier, and DC filtering circuit. The additional parts offer the current regulation.
IC1 is employed like a comparator with a separate buffer stage Q1 providing a appositely high output current functionality in this design. IC1's non-inverting input is supplied with a 0.65 V: reference voltage presented through R1 and D3. The inverting input is connected to ground through R2 within quiescent current levels, allowing the output to get completely positive. Having a NiCad cell attached across the output, a high current may make an effort to via R2, causing an equivalent amount of voltage to develop across R2.
It might merely increase to 0.6V, nevertheless, an increasing voltage at this point reverses the input potentials of the IC1 inputs, causing the output voltage to be reduced, and lowering the voltage around R2 back 0.65 V. The highest output current (and also the charge current received) is as a result the current generated with 0.65 V across 10 ohms, or 65 mA put simply.
Most AA NiCad cells possess a optimum preferred charge current of no more than 45 or 50 mA, and for this category R2 must be increased to 13 ohms so that you can have the appropriate charge current.
A few rapid charger varieties may work with 150 mA, and this demands lowering R2 to 4.3 ohms (3.3 ohms plus 1 ohm in series in case a ideal part cannot be procured).
Furthermore, T1 needs to be improved to a variant with a current rating of 250 mA., and Q1 must be installed using a tiny bolt-on finned heatsink. The device can easily charge up to four cells (6 cells when T1 is upgraded to a 12 V type), and all these should be attached in series over the output, and not in parallel.
Universal NiCad Charger Circuit
Figure 1 exhibits the full circuit diagram of the universal NiCad charger. A current source is developed using the transistors T1, T2 and T3, that offer a constant charging current.
The current source becomes active only when the NiCad cells are attached the correct way round. ICI is positioned to check the network by verifying the voltage polarity across the output terminals. If the cells are rigged properly, pin 2 of IC1 is not able to turn as positive as on pin 3.
As a result IC1 output gets positive and resources a base current to T2, which turns on the current source. The current source limit could be fixed using S1. A current of 50 mA, 180 mA and 400 mA could be preset once the values of R6,R7 and RB are determined. Putting S1 at point 1 shows that the NiCad cells can be charged, position 2 is intended for C cells and position 3 is reserved of D cells.
Miscellaneous Parts
TR1 = transformer 2 x 12 V/0.5 A
S1 = 3 position switch
S2 = 2 position switch
The current source works using a very basic principle. The circuit is wired like a current feedback network. Imagine S1 to be at position 1 and IC1 output is positive. T2 and 13 now begin getting a base current and initiate conduction. The current via these transistors constitutes a voltage around R6, which triggers T1 into operation.
An escalating current around R6 signifies that T1 can conduct with greater strength thus minimizing the base drive current for transistors T2 and T3.
The second transistor can at this point conduct less and the initial current rise is restricted. A reasonably constant current by means of R3 and the attached NiCad cells thus gets implemented.
A couple of LED's attached to the current source indicate the operational status the NiCad charger at any instant. IC1 resources a positive voltage once the NiCad cells are hooked up in the right way illuminating the LED D8.
If the cells are not connected with correct polarity, the positive potential at pin 2 of IC1 will be higher than pin 3, causing the op amp comparator output to become 0 V.
In this situation the current source will remain switched off and LED D8 will not illuminate. An identical condition can transpire in case no cells are connected for charging. This may happen because pin 2 will possess an increased voltage compared to pin 3, due to the voltage drop across D10.
The charger will only activate when a cell comprising of a minimum of 1 V is joined. LED D9 shows that the current source is operating like a current source.
This might appear quite peculiar, however an input current generated by IC1 just isn't adequate, the voltage level also needs to be large enough to reinforce the current.
This implies that the supply should always be greater than the voltage across the NiCad cells. Only in this situation the potential difference will be sufficient for the current feedback T1 to kick-in, illuminating the LED D9.
PCB Design
Using IC 7805
The circuit diagram below demonstrates an ideal charger circuit for a ni-cad cell.
This employs a 7805 regulator IC to deliver a constant 5V across a resistor, which causes the current to be dependent on the value of resistor, instead of on the cell potential.
The value of the resistor should be adjusted with regard to the type that is used for charging; any value between 10 Ohm to 470 Ohm could be used depending on the cell mAh rating. Due to the floating nature of the IC 7805 with respect to the ground potential, this design could be applied for charging individual Nicad cells or series of a few cells.
Charging Ni-Cd Cell from a 12V Supply
The most fundamental principle for a battery charger is that its charging voltage must be more than the nominal battery voltage. For example, a 12 V battery should be charged from a 14 V source.
In this 12V Ni-Cd charger circuit, a voltage doubler based on the popular 555 IC is used. Because output 3 of the chip is connected alternately between the +12 V supply voltage and earth, the IC oscillates.
C3 gets charged through D2 and D3 to almost 12 V when pin 3 is a logic low. The moment pin 3 is logic high, the junction voltage of C3 and D3 boosts to 24 V due to the negative terminal of C3 which is plugged at +12 V, and the capacitor itself holds a charge of the same value. Then, diode D3 becomes reverse biased, but D4 conducts just enough for C4 to get charged over 20 V. This is more than enough voltage for our circuit.
The 78L05 in the IC2 positions acts as a current supplier which happens to hold its output voltage, Un, from appearing across R3 at 5 V. The output current, In, can be simply calculated from the equation:
Iη = Uη / R3 = 5 / 680 = 7.4 mA
The properties of the 78L05 include drawing current itself as the central terminal (usually earthed) gives ours around 3 mA.
The total load current is about 10 mA and that is a good value for constantly charging NiCd batteries. To display that charging current is flowing, an LED is included in the circuit.
Charging Current Graph
Figure 2 depicts the properties of the charging current against battery voltage. It is quite evident that the circuit is not entirely perfect as the 12 V battery will be charged with a current measuring only around 5 mA. A few reasons for this:
- The circuit’s output voltage seems to drop with the escalating current.
- The voltage drop across the 78L05 is around 5 V. But, an additional 2.5 V must be included to ensure the IC operates precisely.
- Across the LED, there is most likely a 1.5 V voltage drop.
Considering all the above, a 12 V NiCd battery with a rated capacity of 500 mAh could be charged uninterruptedly using a current of 5 mA. In total, it is only 1% of its capacity.
Sr. in section “Ni-Cd Charger using a Single Op Amp” you show an scheme circuit with is wrong connected. Electrolytic Capacitor will blow up, and O.P. will not work at all since it is not rightly power biased. Please consider revising.
Thanks Colo, for pointing out the mistake, I have now corrected and updated the diagram accordingly.
Thank you for your comprehensive information. I use a gas detector powered by six NiCad “D” cells. The charger for the device has a red and green LED. The green illuminates; the red does not. There is a circuit on the gas detector that lets me know when the batteries are charged, but I always am concerned about overcharging. I haven’t been able to find a 6-cell charger that I can adapt to my gas detector, allowing the batteries to be charged without opening the device and removing the batteries, so I am considering building one using your diagrams and instructions. My electronic knowledge is limited, but I can follow directions and use a soldering iron. Since the total voltage of the six cells in series is approximately 9 V, I presume I would build a charger for 9 V. What I am wondering is if the charger power rating is that of one D cell times 6. Can you comment?
Hi, I think you can take the help of the second concept shown in the above article. It is by far the easiest and the safest Ni-Cd charger that you can get. The associated graph provides all the details regarding the input supply, battery voltage and the resistor value for the current control.
Thank you for the reply. I shall follow your recommendations.
— Jay
If I want to charge single 600 mAh battery, what is the formula, plz reply sir?
the formula has been already discussed in the previous comments
Hi Swagatam. Thanks for your explanations!
If you could help… I have four ni-cd cells (1.2v each) in series from old screwdriver. I think they are 3000mAh. I would like to make the simplest charger, this one with a diode and a resistor only. The original B&D power supply give 8.5VAC (yes, AC). Which value resistor? 10R? And how long time to full charge?
I think I lost something… the formula to this calculation… :-/
Regards and thanks a lot!!
No problem Rob, the idea is actually provided in the second design. If the transformer is rated at 8.5 V would mean that the peak is 8.5 x 1.41 = 11.98 V or 12 V.
Therefore applying ohms law, the resistor value should be = 12 – 6 / .05 = 120 ohms
What will be the output current after 7805 IC?
Same as the input…
Ok Thankyou Sir.
So If I am having 5V 1A regulated supply, Can I remove 7805 IC and proceed the further circuit?
Sure Daniel, you can remove the 7805 IC, if the input is already 5 V. It is the 50 mA current limit for a standard AAA NiCad that really matters, an increased voltage may be of little significance.
If we want to increse tp 60 mAh in 1st circuit, what to do?
You will have to reduce the 15 ohm resistor accordingly.
You mean to reduce 15 ohm resistance from potentiometer P1?
I am referring to R1 resistor, reduce its value appropriately to get 60 mA…anyway the circuit is already set for delivering 50 mA so need of any change.
If we are having 5volt, 800 mAh supply from adapter. Can we proceed after 7805 IC?
You will just need a current limiting resistor after the 5 v supply to charge your NiCad battery safely…
formula is
R = 5 – Battery voltage / charging current
assuming there are 2 AAA NiCad cells in series, then the formula could be solved as:
R = 5 – 1.2+1.2 / 50 mA
= 5 – 2.4 / 0.05
= 52 ohms
wattage = 2.6 x 0.05 = 0.13 watt 0r 1/4 watt
Actually I want to make auto cutoff circuit, with the above idea it is just charging and will not halt the charging process after full charge, Right?
At the full charge level the battery will stop accepting any further charge if the current is adjusted to 50 ma, however for auto cut off you can apply the other relevant circuits after the 5V input.
Yeah That’s what I am telling, If I have 5V adapter, how much mAh is required?
Just tell me Minimum and Maximum for the first auto-cutoff circuit?
The input supply current can be around 100 mA
You mean from 5V adopter after 7805 IC portion?
Yes, the 5V adapter can be 100 mA, a 7805 may not be needed if the 5v is constant.
What I will do is I am having 5V 2A supply, from current limiting resistor I will convert into 1A ,That’s it. Rest circuit after IC is same.
Is that fine?
Yes, you can use a current limiting resistor to limit the current to 100 ma and then use the 3rd design with it.
I think you are wrong, it suppose to be,
R=5+Battery Voltage/ Charging Current.
Sir In 1st circuit 7805 pinouts are correct?
Bcz supply is going from output to input?🤔
Shalin, the 7805 connection in the first circuit is only symbolic, in practical assembly you must confirm the pinout orientation and then do the connections.
But the transistor and IC Gates pinouts are perfect only right?
And all the resistor values are 1/2 watts,?
Yes they are perfectly OK. All resistors are 1/4 watt
Sir above diagram is very confusing, is it possible for you to email me a good schematic or change it here itself because R6 resistor is written as twice near 7805 also, What to understand?
Plz help me out. Moreover I have started your another design but not able ti finish due to unavailability of LTC4060 and that IC is costly also. But you had helped me alot. I will never forget your appreciation. And also recommended everyone to follow your projects.
Shalin, the diagram was taken from an old magazine, so may be it is a printing mistake from their end, but no problem you only have to check the actual values of the resistors, and implement them in your practical circuit.
Soon I would be adding one more NiCad circuit in the above article. Actually I have many more variants of NiCad charges which I will try to add one by one.
I am glad my circuits are helping to learn more about electronics, please keep up the good work.
Sir please post ASAP, we are waiting for your efforts. God bless you.
Thanks Shalin, I have posted a new design, more will come soon…
Thankyou so much sir, and have you tried above circuit, will it works fine?
Because I am going to order some if the components from online.
You are welcome Shalin, I have not yet tested it, but it is tested by the Elektor electronics engineers.
What is the Triggers N1,N2?
I mean which component is this?
I am having less experience in electronics.
Those are the gates from the IC74132. These are called NAND gates, there are 4 NAND gates inside the IC, only 2 are used.
You can also use the IC 4093 IC, it also has 4 gates, out of which 2 can be used. Make sure to ground the inputs of the other two unused gates
And also it can charge Ni-Mh batteries also..
Correct?
yes, surely it can.
Ground the inputs means to joint the input togather?
Connect the inputs of the all the unused gates with the negative line (ground).
Sir, How to connect IC in this circuit, because I am a first year B tech student, I don’t know much.
First, 2 inputs from pin 1 & 2 then pin 3 is output, that output will goes to pin 5 & 6 as a input and pin 4 is final output, then another unused two gates inputs have to be grounded means pin 8 &9 will have to connect and pin 12 & 13 will have to connect and their outputs are left that means the pin 10 & 11.
Is that correct?
Yes that is correct Shalin.
for the unused gates, connect all the inputs pins to the negative line (or the positive line), and the outputs can remain open.
Negative line means pin 7( Ground)?
yes, and this pin7 must also connect with the negative line of the circuit. These are extremely basic things, if you do not know these things then you are supposed to attempt this circuit.
Never mind my comment.
I’ve seen it in the PCB layout.
Ok
Nélio
OK, thanks for the feedback!
Hi,
In order to charge 4 cells, I duplicate the circuit inside grey-dot area and have a single push-button to start the charging process in all 4 cells?
Thanks.
Nélio
My wife has become a huge fan of the multi-LED string lights powered by a small solar panel charging a single AA NiCad. Several of these strings have failed due to faulty charging and more seem to require battery replacement more often. I am pretty handy with project boxes and a soldering iron. Do you have a circuit design that I could use and replaces the charging and control?
Thanks!
Mike
Hi, You can try the following simpler design, I’ll update the explanation sometime later.
The R1 selection table is given below:
Hello Sir, You are amazing.
I have followed many ideas from you, but I am having a doubt about this circuit. Is it Autocutoff circuit after battery becomes full?
Thank you Shalin, the first design is auto cut off, the second one is not, but due to the limiting resistor the design is still safe to use.
Sir its giving 20 V output, I am shocked ,How it’s possible?For charging 1.2 volt cell.
Shalin, the series resistor drops the current to a value which ultimately becomes harmless for the battery. You can compare this with an LED, which illuminates safely even with much higher voltages due to a series resistor. However, the drawback of this circuit is that the charging process will be very slow.
Sir will this LED glow off after full charge?
In the first circuit the LED will shut off when the battery is fully charged.
Sir can u pls make a circuit for 7.5v with 500 mA output for my Ni-Cd cell .
Vikra, you can try implementing one of these concepts:
https://www.homemade-circuits.com/simple-voltage-regulator-circuits-using-transistor-and-zener-diode/
Hello good afternoon I am a faithful follower of the website. I want to know if you have a circuit to charge 1.5V and 9v alkaline batteries. Thank you.
Carlos of Buenos Aires Argentina.
Hello Carlos, sorry I do not have a circuit to charge dry batteries.
Dear Sir. Now I got a great teacher like you. Thank you very much. As I am a hobbyist and not a well trained person in electronics, I need assistance from you to understand the working and construction of the circuits. For the last few days I came to contact with you and regularly seeking help to make the circuits and you quickly solved my problems. Now I am a big fan of yours.
Your NI-CD charger is good I will certainly make it. But now a days cell 18650 is being widely used in solar based and circuits. Please publish a solar panel charger charging the cell 18650.
Please I tell that to charge a cell 18650 (2100mah) what should be parameters for solar panel
that is how much volt and amp/watt should be given by the solar panel to charge the cell 18650 and what is time required to charge the cell 18650. If you provide the information regarding solar panel, it will be very easy to purchase a suitable and correct solar panel.
One more suggestion and I am sure that your subscriber and hobbyist will be very much benefitted. There are large No of places which are not well connected with the electronics components markets and it is very difficult to get the desired components and and due to non availability of the components we could not make the circuit. If you can provide the components, on price, required for the circuits to the subscriber if they demand, it will be great
help to them. Thanks again.
Thank you Dear AK Goel,
I have an article published in this website regarding the mentioned cell, you can read it here:
https://www.homemade-circuits.com/18650-2600mah-battery-datasheet-and-working/
You can easily charge a 18650 cell from a solar panel through an LM317 regulator circuit by setting its output voltage to 4.1 V.
The solar panel should be rated 7 V/ 1 Amp minimum.
There are already many online stores from where the parts can be purchased cheaply. You can buy them from popular stores like amazon, aliexpress etc quite reasonably. I myself purchase the parts from these sellers, and so far there hasn’t been any issue with them.
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