In this post we learn regarding a simple 6V battery charger circuit with over current protection, which could be used in conjunction with a solar panel, or an AC/DC adapter input.

The circuit also includes a 4 stage battery status indication feature, an over current controller stage, automatic switch OFF for the load and battery charging, and also a separate cell phone charging outlet. The idea was requested by Mr. Bhushan Trivedi.

Technical Specifications

Greetings, I trust you are well. I am Bhushan, and I am working on a hobby project currently. I am very impressed by the knowledge you share at your blog, and was hoping if you would like to guide me a bit with my project.

My project is around charging a 6V 4.5 Ah sealed battery with grid and solar panel.

This battery will supply power to led lights and a mobile phone charging point. Actually, the battery will be kept in a box. and box will have two inputs for battery charging. These two inputs are solar (9V) and AC (230V) for charging the 6V Battery.

There will not be any automatic switchover. Its like the user has an an option to either charge the battery from solar or grid. but both the input options shall be available.

For example, if on a rainy day or for some reason the battery can't be charged from a solar panel, then grid charging should be done.

So I am looking for an option of both the inputs to the battery. Nothing automatic hereThe battery level indicator LED should indicate in red yellow and green on the battery level.

Automatic battery cut off after voltage goes down certain limits to ensure long battery life. I am attaching a short problem statement along this email for your reference.

I am looking for a circuit for the arrangement shown in it. I am keen to hear from you on this

Kind Regards,



The Design

The required 6V solar battery charger circuit can be witnessed in the diagram presented below.

Referring to the diagram, the various stages may be understood with the help of the following points:

The IC LM317 which is a standard voltage regulator IC is configured to produce a fixed 7V output determined by the resistances 120 ohms and 560 ohms.

The BC547 transistor and its base 1 ohm resistor ensure that the charging current to the 6V/4.5AH battery never exceeds the optimal 500mA mark.

The output of the LM317 stage is directly connected with the 6V battery for the intended charging of the battery.

The input to this IC is selectable via a SPDT switch, either from the given solar panel or from an AC/DC adapter unit, depending whether the solar panel is producing sufficient voltage or not, which could be monitored through a voltmeter connected across the output pins of the LM317 IC.

The four opamps from the IC LM324 which is a quad opamp in one package are wired up as voltage comparators and produce a visual indications for the various voltage levels at any instant, during the charging process or during the discharging process through the connected LEd panel or any other load.

All the inverting inputs of the opamps are clamped to a fixed reference of 3V through the relevant zener diode.

The non-inverting inputs of the opamps are individually attached to presets which are appropriately set to respond to the relevant voltage levels by making their outputs high sequentially.

The indications for the same could be monitored via the connected colored LEDs.

The yellow LED associated with A2 may be set for indicating the low voltage cut-off threshold. When this LED shuts off (white lights up), the transistor TIP122 is inhibited from conducting and cuts off the supply to the load, thereby ensuring that the battery is never allowed to discharge to dangerous unrecoverable limits.

A4 LED indicates the upper full charge level of the battery....this output could be fed to the base of the LM317 transistor in order to cut-off the charging voltage to the battery preventing overcharging (optional).

Please note that since the A2/A4 do not have hysteresis included could produce oscillations at the cut-off thresholds, which won't necessarily be an issue or affect the battery performance or life.

Adding an Auto-cut OFF on Batery Battery Full Charge

The modified diagram with over charge auto-cut of can be implemented by connecting A4 output with the BC547.

But now the current limiting resistor formula will be as follows:

R = 0.6 + 0.6 / max charge current


Feedback from Mr. Bhushan

Thank you very much for your continued support and the above circuit designs.

I have a few minor changes to the design now, which I would like to request you for incorporating in the circuit design.  I would like to express that cost of the PCB and components is a big concern, but I do understand quality is also very important.

Hence, I request you to strike a fine balance between the performance and cost of this circuit. So to begin with, we have this BOX, in which will house the 6V 4.5 Ah SMF Lead Acid Battery and the PCB too.

The 6V 4.5 Ah Battery will be charged either through the followingn options from one single input:

a) A 230 V AC to 9V DC Adaptor (I wish to go ahead with a 1 amp rating charger, your views?) ‘OR’

b) A 3-5 Watt Solar module (Max Voltage: 9 V (6V nominal), Max Current: 0.4 to 0.5 Amps)

Block Diagram


The battery can be charged by only one supply at a time hence will only have one input on the left side of the box.

For the time when this battery is being charged, there will be small red led light which glows on the font face of the box (Battery Charging Indicator in diagram) Now, at this point, the system should also have a battery level indicator (Battery level Indicator in diagram)

I wish to have three levels of indications for the battery state. These tables state the open circuit voltage. Now with the very little electronic knowledge I have, I am assuming this is ideal voltage and not the actual conditions, right?

I think I will leave that on you to decide and use any correction factors if required for calculations.

I wish to have the following indicator levels:

  1. Charge level 100% to 65% = Small Green LED is ON (Yellow and Red LED off)

  2. Charge level 40% to 65% = Small Yellow LED is ON (Green and Red LED off)

  3. Charge level 20% to 40% = Small Red LED is ON (Green and Yellow LED off)

  4. At 20% Charge level, battery disconnects and stops supplying output power.

On the Output side now (Right Side View in diagram)

The system will supply power to the following applications:

a) 1 Watt, 6V DC LED Bulb – 3 No’s

b) One output for Mobile Phone Charging I wish to incorporate a feature here. As you see, the DC loads connected to the battery are of relatively less wattage. (just a mobile phone and three 1 watt LED Bulbs). Now, the feature to be added in the circuit should kind of work as a fuse ( I don’t mean an actual fuse here).

Assume if a CFL bulb is connected here or some other application of higher wattage rating, power supply should be cut off. If the total power drawn is in excess of 7.5 Watts DC connected to this system, the system should cut off supply and shall only resume when the load is below 7.5 Watts.

I basically wish to ensure that this system is not misused or drawn excessive energy from, thereby damaging the battery.

This is just an idea. I do however understand this can potentially increase the complexity and cost of the circuit. I will look for your recommendation on this on whether to include this feature or no as we already are cutting off the battery supply once the state of charge reaches 20%.

I hope you find this project exciting to work on. I look forward to receiving your much valued inputs on this.

I am thanking you for all your help till now and in advance for your extended cooperation on this.

Kind Regards,


The Design

Here's a brief explanation of the various stages included in the proposed 6V battery charger circuit with over current protection:

The left side LM317 is responsible for producing a fixed 7.6V charging voltage across its output pin and ground for the battery, which drops to around 7V via D3 to become an optimal level for the battery.

This voltage is determined by the associated 610 ohm resistor, this value can be reduced or increased for changing the output voltage proportionately if required.

The associated 1 ohm resistor and the BC547 restricts the charging current to around a safe 600mA for the battery.

The opamps A1---A4 are all identical and perform the function of voltage comparators. As per the rules if the voltage at their pin3 exceeds the level at pin2, the corresponding outputs become high or at the supply level..... and vice versa.

The associated presets may be set for enabling the opamps to sense any desired level at their pin3 and make their corresponding outputs go high (as explained above), thus A1 preset is set such that its output becomes high at 5V (Charge level 20% to 40%)....A2 preset is set to respond with an output high at 5.5V (Charge level 40% to 65%), while A3 triggers with a high output at 6.5V (80%), and finally A4 alarms the owner with the blue LED at battery level reaching the 7.2V mark (100% charged).

At this point the input power will need to be switched off manually since you did not demand for an automatic action.

Once the input is switched off, the 6v battery level sustains the above positions for the opamps, while the output from A2 ensures that the TIP122 conducts keeping the relevant loads connected with the battery and operative.

The LM317 stage at the right is a current controller stage which has been rigged to restrict the output amp consumption to 1.2 amps or around 7 watts as per the requirements. The 0.75 ohm resistor may be varied for altering the restriction levels.

The next 7805 IC stage is a separate inclusion which generates a suitable voltage/current level for charging standard cell phones.

Now, as power is consumed the battery level begins receding in the opposite direction, which are indicated by the relevant LEDs....

blue is the first one to shut off illuminating the green LEd, which shuts off off below 6.5V illuminating the yellow LEd which identically shuts off at 5.9V making sure that now the TIP122 no longer conducts and the loads are shut off....

But here the condition may oscillate for some moment until the voltage finally reaches below 5.5V illuminating the white LEd and alarming the user for an input power switch on and commence the charging procedure.

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