Scheme diagram of automatic 9V battery charger circuit

This is a schematic diagram of the automatic 9V battery charger circuit, the parts list is provided below the schematic.

Circuit details are based on European standards: 120E, 150E, etc. "E" just stands for ohms, so 120 ohms, 150 ohms. The original circuit specified a HEF type CMOSIC which is not readily available in most national markets around the world. So just buy any other type of CMOS chip like MC4011, MC4020, MC4047 from Motorola. Any other type will work fine too. The BC548B can be replaced with an NTE123AP (note: make sure it is the "AP" type, a typical NTE123A is a completely different transistor), ECG123AP and 2N3904 will also work. Pay attention to the correct pin location as the BCE may be reversed with this European type. LM317T is TO-220 type and can be replaced by ECG956 or NTE956. LM339N can be changed using ECG834 or NTE834

Although this battery charger circuit looks very impressive and may be a bit complicated, it's actually not that difficult to understand. This circuit needs to be connected to a DC supply voltage between 16.5 and 17.5 volts maximum, otherwise the CMOSIC will malfunction. Simply because I didn't want to build a separate power supply for this circuit, I connected it to my fully variable bench power supply.

First, we connect the "to-be-charged" 9-volt NiCd battery to the appropriate connections. Then connect it to the power supply. Once connected, the 1nF capacitor activates the two RS flip-flops formed by IC1a, IC1b, IC1c, IC1d and pulls pins 3 and 10 "high" and pins 4 and 11 "low". The clock pulses are created by the free-running multivibrator IC4. The frequency of IC4 is determined by the 10uF capacitor, 220K resistor and 100K trimpot. The clock continues to run, but the counter IC5 behind it does not count because pin 11 (master reset) remains high. When the "START" button is pressed, the output pin 4 of IC1a goes high and biases TR4 and the red LED (D9) stays on making it visible. NiCad now discharges through this transistor and the 100 ohm resistor. The 10K trimpot (on the right side of the picture) is adjusted in such a way that when the battery voltage drops below 7 volts, the output of IC3 goes low and output pin 11 of IC1a goes high. At the same time, the output pin 10 of IC1d goes low and the red LED goes out.

As output pin 11 goes high, the green LED (D8) lights up and at the same time the voltage level rises, causing the battery to charge. The charging current is determined by the 120ohm, 150ohm and 1K trimpots on the right side of IC2. Actually we can use 1 resistor, but the output voltage of IC2 may be different for different brands, around 1.25 volts.

Simply because the charging current is divided by the value of the resistor, using a trimmer can adjust the current to the correct value for your own 9 volt NiCad. (In my case the battery is a 140mA type, so the current charge should be adjusted to 14mA (c/0.1).

At the same time, a low level on output pin 10 of IC1d starts the clock counter. A pulse appears on pin 9 of IC5 that lights up the red LED. This is achieved for two factors, the clock frequency can be adjusted to the correct value with the 100K trimmer; the red LED must be on for 6.59 seconds and off in the same time, besides that the green LED indicating the charging current can be checked Is the total charging time correct? When the counter reaches 8192 pulses (x6.59 = 53985.28 seconds = 14.99 hours) output pin 3 of IC5 goes high again, transistor Tr1 activates and resets both flip-flops to the starting position.

The charging process is stopped and trickle charging is done through the 10K resistor and D2 diode and keeps the battery fully charged. Adjustments to this project are really simple and nothing to worry about. Turn the walker of the 10K potentiometer in the direction of the 12K resistor. The ground point of the 10K resistor/diode D2 is the same as the adjustment pin of IC2. Add 7 volts to the battery connection terminal, turn on the power, and slowly turn the pot backwards. , until the green LED starts to light up. Turn off the power and remove the connections you made to make adjustments. Insert an amp meter between the battery and the output connection and turn the power back on. The battery will, if it is not completely empty, be fully discharged (to a safe level) and then enter a charge cycle once it reaches a 7 volt margin. The charge present at this point is adjusted exactly to the desired value by a 1K trimmer potentiometer (in series with a 150 ohm resistor and in parallel with a 120 ohm resistor).