Reuse Old Mobile Phone Battery for LED lighting

Do you have old unusable Mobile battery?

Normally mobile batteries have a lifespan of 2-5 years under normal usage. But then we have to replace them.

Nowadays there are cheap replacement batteries which cost no more than 1$. But these low cost batteries run only 6-12 months. What to do with the used batteries, which can’t be reused in mobiles?

Well, easy solution is to use the batteries in a circuit that requires less current. We can use them for LED lighting.

So, just connecting the LED to the battery will light them? Yes, but we need a switch also.

For switching the LED light on or off, here we are not going to use a mechanical switch, instead we’ll be using a darkness detector circuit(just reverse application of light detector circuit). And since the battery needs also to be recharged in intervals, we’ll connect it to mobile charger to charge it.

Here’s a tricky part. The battery voltage is 3.7V, and the output from mobile charger is 5V, so we’ll use a resistor in series to charge the battery.

We need to make potential difference of  5.0-3.7=1.3V across the resistor so we are using here a 1? resistor. Note that ay mobile charger can be used with any mobile battery in this setup.

For the battery, find out which 2 pins correspond the +ve and -ve terminals by using a voltmeter, or simply connecting a LED across the pair combinations of battery terminals. The indicative image shows normal pin configuration of Nokia/Samsung battery.

In the darkness detector circuit, you can control the sensitivity of it by altering the value of the resistor used for base biasing, i./e, 10K. The more the resistance, it will require more dark, and vice-versa.

In the output unit, there are eight(8) white LED in parallel connection and it is connected to the controlled output of darkness detector by a series resistor of 1? 2Watt. The resistor is chosen big as it may generate too much heat.


Thus your automatic lighting emergency light is ready which runs by your waste Mobile Battery.

Got a question or suggestion ? Just leave a comment, I’ll be happy to discus about it.

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10 Responses

  1. Chinmoy Mitra says:

    Hi Arup,
    The idea of using a used mobile battery as LED power source is very good, since almost all of us have a few used mobile batteries lying around.
    But I have a few doubts about the implementation of this circuit.
    1) Firstly, it is NEVER recommended to charge a Li-Ion battery by simply connecting it to a power source, even if it is current controlled. A lithium battery needs a lot of monitoring while charging, like the terminal voltage, temperature etc. These functions are normally done by the charge controller built inside the mobile phone. The battery MUST be charged by putting inside the mobile phone. There are available external Li-on battery chargers which can also be used for this purpose. Uncontrolled charging can result in explosion and fire.

    2) LED’s should not be connected in parallel as shown in the circuit. The reason is simple; every LED has a slightly different forward voltage drop, so the led with the least forward voltage is going to draw most of the current, leaving the rest of the led’s starved. This will result in unequal brightness among the LED’s.

    3) The circuit is very power inefficient, since you have yourself mentioned “The resistor is chosen big as it may generate too much heat”! The more the heat, the lesser the light, since the power provided by the battery is finite, and we need more light isn’t it? I suggest you use some sort of a constant current source after the switching transistor (2SD880), it may be a simple constant current device using a single transistor or any other suitable chip available freely in the local market.

    These are my humble opinion, and please feel free to point out if I am wrong. We all have to work together to make a better DIY environment in our country.

    Best regards

    • Arnab says:

      Thanks for such valuable feedback.
      If the charging resistor is replaced with a 1N4007 diode and current capacity of the charger is limited to 0.2C of the battery, apparently charging will not be a problem.

      And yes, driving LEDs like such is just a waste of power, even the simples boost converter and LEDs in series will be more efficient.

      I’ll try to build a such circuit and write about it here.

      • Chinmoy Mitra says:

        Thanks for your positive response to my message.
        I have just now noticed a very serious mistake in the circuit. Build as shown, this circuit will never work. The collector of the transistor and the emitter are both going to the negative terminal of the battery! So effectively, the LED’s are not getting any power supply to work!! The collector of the transistor should go to the positive supply line via the LED’s, otherwise it will not get any power to operate.
        Please correct the circuit and republish it. Of course, experienced hobbysts will be able to notice the mistake and correct it themselves, but the beginners will be at a loss.

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