The IC 555 is so versatile, it can be considered the single chip solution for all circuit application needs. No doubt it's been utilized here too for yet another useful application.
A single IC 555, a handful of passive component is all that's needed for making this outstanding, fully automatic lead acid battery charger circuit.
The proposed design will automatically sense and keep the attached battery up to date.
The battery which is required to be charged may be kept connected to the circuit permanently, the circuit will continuously monitor the charge level, if the charge level exceeds the upper threshold, the circuit will cut off the charging voltage to it, and in case the charge falls below the lower set threshold, the circuit will connect, and initiate the charging process.
The circuit may be understood with the following points:
Here the IC 555 is configured as a comparator for comparing the battery low and high voltage conditions at pin#2 and pin#6 respectively.
As per the internal circuit arrangement, a 555 IC will make its output pin#3 high when the potential at pin#2 goes below 1/3 of supply voltage.
The above position sustains even if the voltage at pin#2 tends to drift a little higher. This happens due to the internal set hysteresis level of the IC.
However if the voltage continues to drift higher, pin#6 gets hold of the situation and the moment it senses a potential difference higher than 2/3rd of supply voltage, it instantly reverts the output from high to low at pin#3.
In the proposed lead acid battery charger circuit design, it simply means that, the presets R2 and R5 should be set such that the relay just deactivates when the battery voltage goes below say 11.3V (for 12V batts) and activates when the battery voltage reaches above 14.2V.
Nothing can be as simple as this.
The power supply section is an ordinary bridge/capacitor network.
The diode rating will depend on the charging current rate of the battery. As a rule of thumb the diode current rating should be twice that of the battery charging rate, while the battery charging rate should be 1/10th of the battery AH rating.
It implies that TR1 should be around 1/10th of the connected battery AH rating.
The relay contact rating should be also selected as per the ampere rating of TR1.
How to set the battery cut off threshold
Initially keep the power to the circuit switched OFF.
Connect a variable power supply source across the battery points of the circuit.
Apply a voltage that may be exactly equal to the desired low voltage threshold level of the battery, then adjust R2, such that the relay just deactivates.
Next, slowly increase the voltage up to the desired higher voltage threshold of the battery, adjust R5 such that the relay just activates back.
The setting up of the circuit is now done.
Remove the external variable source, replace it with any battery which needs to be charged, connect the input of TR1 to mains, and switch ON.
Rest will be automatically taken care of, that is now the battery will start charging and will cut off when its fully charged, and also will get connected to power automatically in case its voltage falls below the set lower voltage threshold.
How to Set Up the Circuit.
The setting up of the voltage thresholds for the above circuit may be done as explained below:
Initially keep the transformer power supply section at the right hand side of the circuit completely disconnected from the circuit.
Connect an external variable voltage source at the (+)/(-) battery points.
Adjust the voltage to 11.4V, and adjust the preset at pin#2 such that the relay just activates.
The above procedure sets the lower threshold operation of the battery. Seal the preset with some glue.
Now increase the voltage to about 14.4V and adjust the preset at pin#6 to just deactivate the relay from its previous state.
This will set up the higher cut off threshold of the circuit.
The charger is now all set.
You may now remove the adjustable power supply from the battery points and use the charger as explained in the above article.
Do the above procedures with lot of patience and thinking
Feedback from one of the dedicated readers of this blog:
Hi, you have make mistake on preset 2 and 5 not 10k but 100k, I just make one and succesfull...thank you.
As per the suggestion, the above diagram may be modified as shown below: