Here is a low current 6V battery charger circuit. We applied them from cheapest a mobile phone charger. Inside is simple DC switching regulator circuit.

This circuit we can apply 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 include D3,D4,C3,C4 and R6, will control the bias.

In Figure 1 is 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 voltage of the zener diode

In 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 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.

Part you will need
Q1_____________MJE13001_____600V 0.2A NPN Transistor____________________________ = 1 pcs
D1,D2,D5____1N4007_________1000V 1A Diodes______________________________________ = 3 pcs
D4_____________1N4148____75V 150mA Diodes_______________________________________ = 1 pcs
D3_____________1N5237__8.2V 0.5W Zener Diode____________________________________ = 1 pcs
LED____________As you want______________________________________________________ = 1 pcs
C1_____________2.2uF 400V__Electrolytic Capacitors_____________________________ = 1 pcs
C4_____________22uF 50V_____Electrolytic Capacitors_____________________________ = 1 pcs
C5____________470uF 25V_____Electrolytic Capacitors_____________________________ = 1 pcs
C2___________0.01uF 50V_____Ceramic Capacitors__________________________________ = 1 pcs
C3__________0.0022uF 50V____Ceramic Capacitors__________________________________ = 1 pcs
R1___________1 ohms__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______ 390 ohms__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 battery is long time and can recharge several times. When battery is full, we can use it for 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.