[Chhanda Das]

Running a wall/table clock with a 18650 model battery

As most of you may know, most basic wall/table clocks run on 1.5 volt AA/AAA sized non-rechargeable batteries which get depleted every 6 months to a year. Recently, we have started to feel that finding such batteries in the shops in our locality is getting increasing difficult since they don't stock these batteries for some unfathomable reason.

Meanwhile, we still had quite a few 18650 model rechargeable 4.2 volt lithium cells left over from our DIY 12 volt portable power station and our DIY 5 volt USB power bank projects. We just needed to convert or step down 4.2 volts to 1.5 volts efficiently.

Turning to Google gave us quite a few pointers and we chose to follow the tried-and-tested way involving a BC 547 model transistor and 2 numbers of 10 kilo-ohm 0.25 watt resistors. And to recharge the single 18650 model battery, we would need a TP 4056 module with current control. This is available with both USB type C and micro USB input ports but we chose the latter. And we needed a battery holder for the 18650 battery as well. The total cost of the parts came to ₹ 26.

Here is the pin diagram for the BC 547 transistor where C stands for collector, B stands for base and E stands for emitter :

Now let us do the connections using a few thin wires, a soldering iron and some lead free solder as follows :

1) Take one of the aforementioned resistors and solder its one end to the pin C of the transistor and the other end to the pin B of the transistor.

2) Take the other resistor and solder its one end to the pin B of the transistor.

3) Solder the appropriate battery holder wires red to B+ and black to B- terminals on the TP 4056 module.

4) Solder the pin C of the transistor to the positive terminal of the load terminals on the TP 4056 module.

5) Solder the exposed end of the second resistor to the negative terminal of the load terminals on the TP 4056 module.

6) Solder one end of a thin wire preferably with black insulation to the negative load terminal on the TP 4056 module and the other end of the wire to the original negative battery terminal of the clock.

7) Solder one end of another thin wire preferably with red insulation to the pin E of the transistor and the other end of the wire to the original positive battery terminal of the clock.

Now you can stick the 18650 model battery holder and the TP 4056 module to the back of the clock. Install the 18650 model battery with appropriate polarity into the holder, set the time & date (if applicable) and you are all set.

Here is an image taken during the first charging test :

I hope you all found this informative and useful

Disclaimer : This project was executed solely by my son. If you want to replicate this project then please do so solely at your own risk.

[Amrik Singh]

Seems interesting. Nominal voltage of single 18650 cell is 3.7 V while 4.2 V is maximum voltage when fully charged.
Could you please share Wiring Diagram (Circuit Diagram) for us to understand and follow.

[Chhanda Das]

Originally Posted by Amrik Singh

Seems interesting. Nominal voltage of single 18650 cell is 3.7 V while 4.2 V is maximum voltage when fully charged.
Could you please share Wiring Diagram (Circuit Diagram) for us to understand and follow.

Sadly, I couldn't find a circuit diagram but here is the best wiring diagram that I could find :

In the image above, the 1.5 volt output terminals are connected to the respective 1.5 volt battery terminals of the clock. Additionally, the 18650 battery is connected to the battery terminals of the TP 4056 module. I hope that this helps

[Amrik Singh]

I cannot imagine a place where AA or AAA size Cells are not available. I am using rechargeable AA /AAA cells for most of my Wall clocks.
My relative in Punjab posed me the problem of not finding 'C' Size cell for her favourite Clock. Incidently, length (height) of C size cell is similar to AA size cell. Carefully slid in AA cell where C cell should go, did the trick. She has replaced the cell herself since then.

[Chhanda Das]

Originally Posted by Amrik Singh

I cannot imagine a place where AA or AAA size Cells are not available. I am using rechargeable AA /AAA cells for most of my Wall clocks.

I have no idea why the shops rarely have AA or AAA batteries here in my locality in Kolkata. I tried using rechargeable AAA nickel metal hydride (NiMH) cells from Duracell, Panasonic and Eveready in the particular table clock shown in my initial post above but the display was substantially dimmer probably because those rechargeable cells have a lower voltage at 1.2 volts. And they would barely last for 5-6 months since their capacity was around 750-900 milli ampere hours (mAh). In stark contrast, the 18650 model cell that my son used for this project has a rated capacity of 2800 mAh from Toshiba. So I am guessing that it should last for around 1-1.5 years before it needs recharging.

[srini1785]

Originally Posted by Chhanda Das

Sadly, I couldn't find a circuit diagram but here is the best wiring diagram that I could find :

In the image above, the 1.5 volt output terminals are connected to the respective 1.5 volt battery terminals of the clock. Additionally, the 18650 battery is connected to the battery terminals of the TP 4056 module. I hope that this helps

My understanding is that the BC547 is used as a voltage divider (pot) to provide a 1.5V from a 3.7V Source. The current flowing from Base to emitter is restricted by the Resistance between the Base and Emitter to provide 1.5VDC. Anyway it is good to know that your son has got into electronics. My (2paise) worth of advise is to ask him to first get acquainted with basic circuit components like Capacitor, Transistor and Resistor before getting into the Arduino non sense. For that ask him to construct the same circuit using a charging capacitor to provide the 1.5V using PWM.

All the best.

[Chhanda Das]

Originally Posted by srini1785

... Anyway it is good to know that your son has got into electronics. My (2paise) worth of advise is to ask him to first get acquainted with basic circuit components like Capacitor, Transistor and Resistor before getting into the Arduino non sense. For that ask him to construct the same circuit using a charging capacitor to provide the 1.5V using PWM. All the best.

Thank you very much for the kind words of appreciation. I have conveyed them to my son.

Here is something my son requested to ask you. When you suggested using a charging capacitor, did you mean connecting a capacitor between the 1.5 volt positive and negative output terminals ? Would a 47 micro Farad 25 volt capacitor be sufficient here ? He also begged your pardon for his ignorance here since he has a rudimentary background in Physics while his primary academic background is Mathematics

[srini1785]

Originally Posted by Chhanda Das

Here is something my son requested to ask you. When you suggested using a charging capacitor, did you mean connecting a capacitor between the 1.5 volt positive and negative output terminals ? Would a 47 micro Farad 25 volt capacitor be sufficient here ?

Here's a circuit that i pulled off the Net. (I don't want to hijack this thread)


It is a DC-DC converter circuit using a single switched capacitor. The switches can be replaced with a BJT or MOSFET. The basic idea is to use the charging curve of the capacitor to step down the DC Voltage. The 47Mic, 25 V should be fine. You can experiment with the charging time using a timer circuit (555) which trigger the BJT's or MOSFETs.

He also begged your pardon for his ignorance here since he has a rudimentary background in Physics while his primary academic background is Mathematics

There are no pardons here madam. As I told you, I am happy that your son has gotten into Electronics (and math). So again i wish him the best.

[Janus]

Ikea makes good rechargeable batteries. Try these if there is a store in your city or if your pin code is available for online delivery.


AA 2450mAh
https://www.ikea.com/in/en/p/ladda-r...1-2v-70506529/

AA 1900mAh
https://www.ikea.com/in/en/p/ladda-r...1-2v-50509816/

AAA 750mAh
https://www.ikea.com/in/en/p/ladda-r...1-2v-50509816/

AA & AAA Battery Charger
https://www.ikea.com/in/en/p/stenkol...rger-00506448/

[Chhanda Das]

Update 1 :

Added a capacitor to the circuit

We found that despite using the same circuit as earlier in an analog wall clock, it was slowing down very slightly over the course of a few weeks. And my son could not figure out why. We didn't face this problem with the digital wall/table clock.

Originally Posted by srini1785

... ask him to construct the same circuit using a charging capacitor to provide the 1.5V using PWM.

But then thanks to our fellow Bhpian srini1785, he got the idea of adding a capacitor to the circuit. After some research, he added a 10 volt 220 micro Farad capacitor to the 1.5 volt output terminals of the circuit and that seems to have fixed the problem of the clock slowing down. srini1785, you have both of our profuse thanks and gratitude for this.

Here is what the new circuit design looks like :

For added safety, my son incorporated the same design into the earlier digital wall/table clock as well :

The total cost for this project is now ₹28. I hope you found this informative. Wishing you all happy and safe drives ahead