Automatic Battery Maintainer – Done and Dusted

Hello Everyone

I hope that you all are still doing great. I finally had the time to fault find my battery charger/maintainer circuit. Through fault finding the circuit I came to better understand it since I took the design from another website. So the first part of the circuit is the LM317 power supply section which consist out of two LM317 chips, the first is used to limit the current while the second is used to adjust the output voltage. The dip switches is to adjust the current limiting value. I chose dip switches since a 3W portentiometer would have been too expensive in comparison to the DIP switch alternative and the amount of times that you will be adjusting it will be minimal.

The second part of the circuit involves the “sensing part” which detects when the battery level drops below a certain set value so as to enable the circuit by biasing a transistor which in turn drives a relay. The relay then connects the power supply section to the battery terminals and when the the battery voltage reaches another set level the “sensing part’s” output voltage drops causing the transistor to not be biased anymore and the relay to open, disconnecting the power supply section / charger section (Constant voltage / selected current limiting). Once the battery voltage drops to the preset voltage again the process repeats itself again.

At the hart of the “sensing part” is a TLC271 opamp that is configured as a differential amplifier. This in essence means that when the voltage applied to the positive pin of the opamp is more than the voltage applied to the negative pin of the opamp, the output voltage will go low. As soon as the voltage on the negative pin is more than that of the positive pin then the output will go high which will be more or less the voltage that is applied on the positive and negative pins.

The original circuit design didn’t really specify components but I picked up which components were used from the pictures. From the pictures I could see that zener diodes were used in the “sensing part” with 1% accurate resistors. I then went and used Schottky diodes since I had some already. In addition to breaking the rules on the diodes I also used 5% accurate resistors in stead of the 1% accurate resistors. The original circuit also specified a 500 Ohm potentiometer to adjust the “switch on” voltage (Voltage at which the relay is switched on to enable charging).

I found that with the 500 Ohm potentiometer I couldn’t adjust the “switch on” voltage to about 12.6 V or lower so I changed this potentiometer first to a 5 kOhm potentiometer. After doing this I was able to adjust the “switch on” voltage to what I wanted it to be. The other issue that I picked up then was that the “sensing part” only disengaged the power supply part of the circuit at above 14V. This was a big no no since a lead acid battery won’t ever reach this voltage. So I had to figure out how to change the circuit so that I could adjust the “switch off” set point as well. To accomplish this I changed one of the resistors of the voltage divider circuit that is connected to the battery to a potentiometer so that I could adjust the voltage that is fed into the negative input pin of the TLC271.

This allowed me to adjust the “switch off” voltage to 13.6 V which is the fully charged value of a lead acid battery that is still in good condition. The downside is that as soon as I adjusted the “switch off” set point the “switch on” set point changed as well. So the I realized that I had to change the 5 kOhm potentiometer again. I changed it to a 22 kOhm potentiometer. After I changed it I played around with the values of both potentiometers of the “sensing part”. I firstly adjusted the “switch on” set point to 12.6 V again and then initiated a charge on a battery. I also connected a multimeter in series with the battery to measure the current. I then  continued to charge the battery until the battery wasn’t pulling any current from the power supply section anymore meaning that the battery was fully charged. I then changed the value of the “switch off” potentiometer until it disengaged the power supply section. I then measured the battery voltage and is was at 12.8 V. This might be because I accidentally shorted the battery out once. I then connected the charger to my power supply and tested where the “switch on” point now was and it was at 11.4 V. This minimum voltage set point was good enough for me since I’m only going to use the charger to charge a backup battery for my Wendy house alarm so that when the power fails that the alarm in the Wendy house don’t trip.

So enough of the details, time for some pictures. Here is the revised board with the two replaced potentiometers, it didn’t fit into the original holes but I revised the circuit should you wish to build it so that you can fit it. You can find a link to download the circuit diagram from the bottom of the post.

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Here is the whole setup in the test phase after the potentiometers were adjusted:

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So now I required an enclosure. Being a slight hoarder I had this meal replacement holder which I decided would work perfectly:

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I cut the top off and tested whether everything would fit and eureka it did:

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Connected everything up:

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Checking that it will work one final time:

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Looks nice 🙂

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And here is the final result, it fits nicely with the help of some cable ties and it works yippee 🙂

(I turned the battery upside down so that it would be right side up if the enclosure is upside down):

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Circuit Diagram:

Automatic Battery Charger Circuit Diagram

Board with Components:

Automatic Battery Charger Board

In conclusion, this circuit works but is a bit sensitive to changes and won’t be able to adopt to a degrading battery so it will require some supervision every once in a while by my estimation. But if you feel like you would like to build it then you can get the circuit design from my Dropbox folder over here. Alternatively you can download an A4 pdf page with the board layout on already to duplicate from here. The Bill of materials you can get from here. If you have any questions drop me a comment and I will try to respond as soon as I can.

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