The following 6 volt 4 AH battery charger circuit has been designed by me and posted here in response to the request from Mr. Raja, let's learn the whole conversation.
"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 Circuit Design
The following circuit shows a simple automatic 6 volt 4 to 10 AH battery charger circuit using a 12 volt relay.
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.
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.