Automatic 9V Battery Charger

This is the scheme diagram of automatic 9V battery charger circuit, the parts list provided below the schematic image. The circuit was designed by Jan Hamer, translated by Tony Van Roon dan republished in this circuit diagram site.

Automatic 9V battery charger

The circuit details are based on european standards: 120E, 150E, etc. The ‘E’ just stands for Ohms so 120 ohm, 150 ohm. The original circuit specified the HEF type of CMOS IC’s that are not readily on the market in most of worldwide country. So just get any other kind of CMOS chip like the MC4011, MC4020, MC4047 from Motorola. Any other type will work fine too. The BC548B is replaceble by a NTE123AP (NOTE: ensure it’s the ‘AP’ type, the typical NTE123A is a total different transistor), ECG123AP, and also the 2N3904 will be work. Watch for the proper pin locations because the BCE may be reversed with this european type. The LM317T is a TO-220 type and replaceble with a ECG956 or NTE956. The LM339N could be changed using a ECG834 or NTE834

Even though this battery charger circuit looks pretty impressive and perhaps a bit complex, it’s actually not difficult to understand. The circuit needs to be hooked-up to a DC supply voltage of between 16.5 and max 17.5 volt, otherwise the CMOS IC’s will go defective. Simply because I didn’t feel like to build a seperate power supply for this circuit, I connected it to my fully variable bench top power supply.

To start with, we connect a ‘to-be-charged’ 9-volt nicad battery to the suitable connections. Then hook it up to the power supply. Upon connection the 1nF capacitor starts up 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. IC4’s frequency is determined by the 10uF capacitors, the 220K resistor and the 100K trimpot. The clock runs continuesly but the counter behind, IC5, is not counting however because pin 11 (the master-reset) is kept high. When the ‘START’ button is pressed, output pin 4 from IC1a goes high and biases TR4, which is made visible by the Red LED (D9) which remains lit. The NiCad is now becoming discharged via this transistor and also the 100 ohm resistor.The 10K trimpot (at the right of the diagram) is adjusted in such a way that when the battery voltage dips below 7 volt, the output of IC3 goes LOW and the output pin 11 of IC1a HIGH. At hte same time the output pin 10 of IC1d goes LOW, and also the red LED turns off.

Because output pin 11 went HIGH the green LED (D8) lights up and at the exact same time the voltage level rises causing the battery to be charged. The charge-current is determined by the 120 ohm, 150 ohm, and the trimpot of 1K, at the right side of IC2. Really we could have used 1 resistor, but the output voltage of different brands for IC2 may differ, by about 1.25 volt.

Simply because the charging present is devided by value of the resistors, with the trimpot the current could be adjusted to the correct value of your own 9-volt NiCad. (In my case, the battery is a 140 mA type, so the charge present should be adjusted for 14 mA (c/0.1).

At the exact same time the LOW of output pin 10 from IC1d starts the counter of the clock. On pin 9 of IC5 appear pulses which light up the red LED. This is implemented for two factors, the clock-frequency can, with the 100K trimpot, be adjusted to the correct value; the red LED has to come ON for 6.59 seconds and for the same duration going OFF and except for that fact the green LED, who indicates the charge current, can be checked if the total charge-time is correct.When the counter has reached 8192 pulses ( x 6.59 = 53985.28 sec = 14.99 hours) the output pin 3 of IC5 goes high again, transistor Tr1 activates and resets the two flip-flops to the start position.

The charging process stops and goes over to trickle charge via the 10K resistor and the D2 diode and keeps the battery topped-up.The adjustments of the project are truly extremely simple and nothing to be concerned about. Turn the walker of the 10K pot in the direction of the 12K resistor, ground connection point of 10K resistor/diode D2, like the adjustment pin of IC2, apply a voltage of 7-volt to the battery connection terminals, switch the power ON and slowly turn the pot backward until the greeen LED starts to light up. Switch OFF the power and take away the connections you created to make the adjustment.Insert an amp-meter between the battery and also the output connection and again switch the power ON. The battery will, in case it is not totally empty, totally discharged (to a secure level) and as soon as the 7 volt margin is reached goes over to the charge cycle. The charge present is at this time adjusted via the 1K trimpot (which is connected in series with the 150 Ohm resistor and in parallel with the 120 ohm resistor) accurately to the desired value.

Addendum: It’s strongly suggested to include little 100nF ceramic capacitors over the power supply lines feeding Every CMOS IC to keep feasible interference to a negliable value.

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