Simple Ni-Cd Battery Charger Circuits Explored

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 essential battery voltage line. A line pulled vertically down from this point to meet the horizontal axis subsequently provides 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 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.

About Swagatam

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials.
If you have any circuit related query, you may interact through comments, I'll be most happy to help!