6V Solar Battery Charger Circuit

In this post we discuss a simple 6V solar battery charger circuit with an automatic cut-off feature using 4 way LED indication, and an overcurrent protection. The system can be operated through a solar panel or via an AC/DC mans adapter unit. The idea was requested by Mr. Bhushan Trivedi.


The Request


Dear Swagatam,

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 here

The 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,

Bhushan

The Design

The required 6V solar battery charger circuit can be witnesed 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.

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