13th July 2016 Cat: Circuit Diagram with No Comments »
In this post we learn regarding a simple 6V battery charge controller circuit, 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.
The Request

Dear Swagatam,

Thank you very much for your continued support.

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 following 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)
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:

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

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

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

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 automatic battery charge controller circuit:

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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