Here is a simple transistor battery charger it can charge batteries up to 12v in series. It has an LED and a few parts.
To use it you must decide on a power supply voltage, current limit resistor, and
|Battery series voltage(B1)||minimum(V1)||maximum(V1)|
Just pick a number of batteries in series that matches its output voltage. The power supply voltage must not go below minimum voltage or over the maximum voltage(as seen in the table above)
Now that we figured power supply voltage and battery count we must figure out the current limit resistor(R2). This charger will take 15 hours to charge. To figure out we need to know amp-hour of one battery. All batteries must have the same amp-hour rating. The current should be limited to ten times less than the amp-hour rating. So if it is 1000mAh the resistor should limit it to 100mA.Amp-Hour rating:mAh
The calculated times small should be close to 10.
To use more than 10 cells multiply the number of parts. D1 is a standard 1A or higher rectifier diode. The charger should be turned off when it is done charger. The LED should change its state. Also, currents add up. The circuit originally came from ElectronicsYard