How to Set IC 741 for Auto Cut-oFF

In this post I have explained how to set or adjust an opamp 741 IC based battery charger circuit for implementing an automatic cut-off for the connected battery once it reaches the full-charge level.

Since I have many such IC 741 battery charger circuits posted here, the instructions will be helpful for correctly setting up most of those relevant circuits. The query was put forth by Mr. Chaw.

How to Setup Charging Thresholds

I have finished with the universal Automatic 741 battery charger circuit prototyping. In order to test, can u explain me briefly about the settings for input and trimmers etc before i charge the battery ?

Let's say now I am going to charge Ni-MH 400mAh 7.2V battery:

(1) What is the power supply input voltage and current ? According to what u have mentioned, any voltage between 15-24V and 5A setting will do regardless of the battery voltage ?

(2) How can I predict and calculate my charging time to the battery ?

(3) What are the modification to 4k7 and 10k trimmer before charging ?

(4) Last but not least, if I want to add the LED showing the charging status, where should i add this to ?

I am sorry for asking too many questions, but thanks for all the way helping me out with my internship project. May god bless u for ur hardwork. Really appreciate it !! 😀

Video Explanation:

Circuit Diagram

PCB Design

Analyzing the Circuit Query

Hi Chaw,

The instructions given below will help you to understand regarding how to quickly set up an opamp 741 battery charger circuit for an auto cut-off operation.

1) First disconnect the 1N4148 diode link connected at preset center arm and also the LED 4k7 resistor end from the base of the transistor. Connect this 4k7 end to ground temporarily.

2) Do not connect any battery as yet.

3) Feed an input voltage that may be at least 3 to 5V higher than the battery voltage.

4) Adjust the 4K7 pot to get a 8.5V across the terminals which is to be connected with battery.

5) Now adjust the trimmer such that the red LED just lights up.

That's all, your circuit is all set now

Reconnect the 1N4148 end with the preset center terminal as before and restore the LED 4k7 connection with the BC547 base.

You can now charge the battery from this charger and witness an auto cut off when the 7.2V reaches 8.5V.

The back up time will depend on the input current, if it's at the full 1C rate then you can expect the cell to charge within 1 hour or time.

you can add another LED across positive rail and pin#6 (for IC 741) with a series 1K resistor for the charging ON indicator

Estimating Charging Time

Yes noted, I have followed ur instructions to make a setting before charging. I have mentioned that I will made use of a 7.2V battery to test. In this case, may i know the purpose of why u advice me to get a 8.5V at the battery terminal ?

Like for example, if I were to charge another battery (12V,24V,etc...) with different voltage, how do I know the voltage that I should achieve ? Additionally, adjusting 4k7 trimmer to get 8.5V at the battery terminals (cathode of 1N5408 and ground), do I also need to get this voltage at the Vout pin of LM338?

Since my battery is 400mAh, I try to use 40mA for my input current because I got read from some of the comments that u suggested to use the input current which is five or ten times lesser than the battery mAh.

Thus, the time taken will be up to 4 hours to charge my battery, am I correct ? Is there any way to shorten my time taken like increase the input current in the other way ?

One more thing to clarify is that if I test with my multimetre to the battery terminals while charging, the voltage will keep on increasing after some time and when it reaches 8.5V, it will auto cut off the current and said to have a full charging ? I do apologize for so many questions every time sir.

Since I am still doing an internship, I want to understand in depth about all this project. I really do appreciate your patience to me 😀 With regards, Chaw

Charging a 12V Battery Optimally

For a 12V battery the recommended full charge level is 14.3V, so with this yardstick we can easily calculate the full charge levels of other batteries using the following simple cross multiplication formula:

12/V(Batt) = 14.3/V(full)

Here V(batt) is the normal batt voltage of any battery, and V(full) is the required full charge result.

the charging voltage should be measured and set after the diode cathode so that the diode FWD drop is countered appropriately, meaning the LM338 terminal voltage will be 0.6V higher than the above level.

For a lead acid batt the 1/10th charging current becomes a crucial factor and is strongly recommended but for Li-Ion this may not be the case, these batts can be charged even at their full AH rates for achieving a quick 1 hour or 2 hour charging period (temperature of the batt may go significantly high and must be monitored in such cases).

Yes the battery voltage will gradually keep rising while it gets charged, and as soon as it reaches 8.5V, will be auto cut-off by the opamp circuit.

Wish you all the best!