The following two versions of 6 volt 4 AH battery charger circuits has been designed by me and posted here in response to the request from Mr. Raja, let's learn the whole conversation.

Technical Specifications

"Dear sir, please post a circuit to charge 6 volt 3.5 ah lead acid battery from 12 volt battery. The charger should automatically stop charging as the battery is fully charged.

Please use transistor instead of relay to stop charging, and also tell me how to use 12 volt relay for the same circuit.

Explain Which is safe and durable either relay or transistor to cut off charging. (At present i am charging my above said battery by simply using LM317 with 220 ohm and 1 kilo ohm resistors and a couple of capacitor) i'm awaiting your article, thank you".

The Design

The following circuit shows a simple automatic 6 volt 4 to 10 AH battery charger circuit using a 12 volt relay, designed to automatically cut off the supply to the battery as soon as the ful charge level for the battery is reached.

How it Works

Assuming no battery is connected with the circuit, when power is switched ON, the relay contact will be at the N/C and no power will be able to reach the IC 741 circuit.

Now when battery is connected, the supply from the battery will actuate the circuit, and assuming the battery to be in a discharged state, pin#2 will be lower than pin#3 causing a high at pin#6 of the IC. This will switch ON the transistor relay driver, which in turn will shift the relay contact from N/C to N/O connecting the charging supply with the battery.

The battery will now begin charging slowly and as soon a its terminals reaches at 7V, pin#2 will tend to become higher than pin#3, causing pin#6 of the IC to become low, switching OFF the relay and cutting off supply to the battery.

The existing low at pin#6 will also cause pin#3 to become permanently low through the linked 1N4148 diode, and thus the system will be latched, until power is switched OFF and ON again.

If you don't wish to have this latching arrangement, you can very well eliminate the 1N4148 feedback diode.

Note: The LED indicator section for all 3 following diagrams were recently modified after a practical testing and confirmation

The following circuit shows a simple automatic 6 volt 4 AH battery charger circuit without using a relay, rather directly through a transistor, you can replace the BJT with a mosfet also to enable high Ah level charging also.


The above transistorized 6V charger circuit has a mistake. At the full-charge level as soon as the battery negative is cut-off by the TIP122, this negative from the battery is also cut-off for the IC 741 circuit.

This implies that now the IC 741 is unable to monitor the discharging process of the battery, and will be unable to restore the battery charging when the battery reaches the lower discharge threshold?

To correct this we need to make sure that at the full-charge level, the battery negative is only cut-off from the supply line, and not from the IC 741 circuit line.

The following circuit corrects this flaw and makes sure that the IC741 is able to monitor and keep track of the battery health continuously under all circumstances.

How to Set Up the Circuit

Initially, keep the pin6 feedback resistor disconnected and without connecting any battery adjust R2 to get exactly 7.2V across the output of the LM317, for the IC741 circuit.

Now simply play with the 10k preset and identify a position where the LEDs just flip/flop or change or swap their illumination.

This position within the preset adjustment may be considered as the cut-off or the threshold point.

Carefully adjust it to a point at which the RED LED in the first circuit just lights up......but for the second circuit it should be the green LED that is supposed to get illuminated.

The cut-off point is now set for the circuit, seal the preset in this position and reconnect the pin6 resistor across the shown points.

Your circuit is now set for charging any 6V 4 AH battery or other similar batteries with an automatic cut-off feature as soon as or each time the battery becomes fully charged at the above set 7.2V.

Both the above circuits will perform equally well, however the upper circuit can be altered to handle high currents even up to 100 and 200 AH just by modifying the IC and the relay. The lower circuit may be made to do this only up to a certain limit, may be up to 30 A or so.

The second circuit from above was successfully built and tested by Dipto who is an avid reader of this blog, the submitted images of the 6V solar charger prototype can be witnessed below:

Adding a Current Control:

An automatic current control regulator function can be added with the above shown designs by simply introducing a BC547 circuit as shown in the following diagram:

The current sensing resistor can be calculated through the simple Ohm's law formula:

Rx = 0.6 / Max charging current

Here 0.6V refers to the triggering voltage of the left side BC547 transistor while max charging current signifies the maximum safe charging for the battery, which may be 400mA for a 4AH lead acid battery.

Therefore solving the above formula gives us:

Rx = 0.6 / 0.4 = 0.24 Ohms.

By adding this resistor will ensure that the charging rate is fully controlled and it is never exceeded the specified safe charging current limit.

Test Report Video Clip:

The following video clip shows the testing of the above automatic charger circuit in real time. Since I did not have a 6V battery, I tested the design on a 12V battery, which does not make any difference, and its all about setting the preset accordingly for the 6V or a 12V battery as per user preference. The above shown circuit configuration was not changed in any manner.

The circuit was set to cut off at 13.46V, which was selected as the full charge cut off level. This was done to save time because the actual recommended value of 14.3V could have taken lot of time, therefore to make it quickly I selected 13.46V as the high cut off threshold.

However one point to be noted is that the feedback resistor was not employed here, and the lower threshold activation was automatically implemented at 12.77V by the circuit, as per the IC 741's natural hysteresis property.

6V Charger Design#2

Here's another simple yet accurate automatic, regulated 6V lead acid battery charger circuit which switches off the current to the battery as soon as the battery reaches full charge. An illuminated LED at the output indicates the fully charged condition of the battery.

How it Works

The CIRCUIT DIAGRAM can be understood with the the following points:

Fundamentally the voltage control  and regulation is done by the versatile, work horse IC LM 338.

An input DC supply volt in the range of 30 is applied to the input of the IC. The voltage may be derived from a transformer, bridge and capacitor network.

The value of R2 is set to get the required output voltage, depending upon the battery voltage to be charged.

If a 6 volt battery needs to charged, R2 is selected to produce a voltage of around 7 volts at the output, for a 12 volt battery it becomes 14 volts and for a 24 volt battery, the setting is done at around 28 volts.

The above settings take care of the voltage that needs to be applied to the battery under charge, however the tripping voltage or the voltage at which the circuit should cut off is set by adjusting the 10 K pot or preset.

The 10K preset is associated with the circuit involving the IC 741 which is basically configured as a comparator.

The inverting input of the IC 741 is clamped at a fixed reference voltage of 6 via a 10K resistor.

With reference to this voltage the tripping point is set via the 10 K preset connected across the non inverting input of the IC.

The output supply from the IC LM 338 goes to the battery positive for charging it. This voltage also acts as the sensing as well as the operating voltage for the IC 741.

As per the setting of the 10 K preset when the battery voltage during the charging process reaches or crosses the threshold, the output of the IC 741 goes high.

The voltage passes through the LED and reaches the base of the transistor which in turn conducts and switches off the IC LM 338.

The supply to the battery is immediately cut off.

The illuminated LED indicates the charged condition of the connected battery.

Circuit Diagram

This automatic battery charger circuit can be used for charging all lead acid or SMF batteries having voltages in between 3 and 24 volts.

The above circuit was found not so satisfactory by some of the readers, so I have modified the above circuit for a better and guaranteed functioning. Kindly see the modified design in the below given figure.

PCB Design for the above finalized 6V, 12V, 24V automatic battery charger circuit

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