Here is a low current 6V battery charger circuit. We applied them from cheapest a mobile phone charger. Inside is a simple DC switching regulator circuit. Also, it can be a 6v 4.5ah battery charger for 10hr charging. It is good for battery low current no overheat, long life battery.

This circuit we can apply in many ways. For example, a power supply for general load and a low current battery charger circuit.

This circuit we can use for charge a 6 volts lead-acid batteries, and also useful for others: nickel-cadmium batteries,nickel-metal hydride battery(also known as NiMH), etc.

The output current more or less depends on the voltage selected. It will discuss how to adapt later.

How this circuit works

First of all, the AC-voltage for both series(AC120V and AC220V) to the R1-1 ohms. It is the resistor fuse to cut off the circuit when overloaded.

While the AC220V to the diode D1-1N4007 to rectified from AC (Alternating Current) to a pulsating DC (Direct Current) about 300V.

After that, The DC300V will via the filter capacitor-2.2uF.

Then, the Q1-transistor acting together with R5, C2, D2 and coil on top of T1. Before, the Q1 has a bias at pin B via R4 and R2. Then the feedback circuit includes D3, D4, C3, C4, and R6, will control the bias.

Figure 1 is a Schematic diagram of the battery charger adapter.

Finally, both D4 and C5 are a rectifier circuit. They are special because it will work at frequencies around 30KHz. But the frequency of a general transformer is 50Hz only.

The output voltage is controlled by the zener diode D3.

Choosing the voltage of the Zener diode

In the circuit, we use number 1N5237 (8.2V 0.5W), causes the output voltage about 7.4V, for charge the 6V battery.

We will see that:

1. The output voltage of this adapter should be higher than the battery’s voltage of about 20%. If we use a 12V battery, so the output voltage is about 14 volts.

2. The breakdown voltages (Vz) of Zener Diode should also higher than the output voltage is around 20%.
Suppose, the output voltage is about 14 volts. Therefore, we should use the 18 volts Zener diode.

The Zener diode in the circuit used 0.5 watts.

Parts you will need

Q1: MJE13001, 600V 0.2A NPN Transistor
D1,D2,D5: 1N4007, 1000V 1A Diodes
D4: 1N4148_75V 150mA Diodes
D3: 1N5237, 8.2V 0.5W Zener Diode = 1 pcs
LED: As you want_ = 1 pcs
C1: 2.2uF 400V, Electrolytic  = 1 pcs
C4: 22uF 50V, Electrolytic = 1 pcs
C5: 470uF 25V, Electrolytic = 1 pcs
C2: 0.01uF 50V, Ceramic  = 1 pcs
C3: 0.0022uF 50V, Ceramic  = 1 pcs
R1: 1Ω, 0.5W Resistor = 1 pcs
R2,R5: 47K, 0.25W Resistor = 2 pcs
R3: 15 ohms 1W Resistor = 1 pcs
R4: 1M 0.25W Resistor = 1 pcs
R6: 390Ω 0.25W Resistor = 1 pcs
R7: 1K 0.25W Resistor = 1 pcs
T1: Transformer = 1 pcs

Caution
1. For the device has various polarity includes the Electrolytic capacitors,Zener diode,all diode,transistor,LED we should be careful in the assembly circuit.
2. If the output voltage is at 12V you need change C5-470uF 25V up.

Advantages of this adapter
While is charging, the battery is not hot. Thus, the life of a battery is a long time and can recharge several times. When the battery is full, we can use it for a longer time than the normal high current charging.

The disadvantage is it takes about 3 hours to charge because it is the low current charging.

We can easily use this adapter as the power supply circuit at about 500mA, 12V.