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How to Connect 5mm LEDs to a 3.7V Li-Ion Cell

Last Updated on May 22, 2019 by Swagatam 51 Comments

The article explains regarding how to connect and illuminate a few 5mm LEDs using a 3.7V Li-Ion cell, normally used in cell phones.

I keep receiving requests from the readers who seem confused with the connection details of 5mm LEds with a 3.7V Li-ion cell. The requests inspired me to write this post, hopefully it would answer the many related queries.

Using a Cellphone Li-ion Cell

Since standard 3.7V Li-Ion cells which are normally used in cell phones are rated at around 800 to 1100mAh, are quite capable of supporting a few 5mm LEDs, and would be able to keep them illuminated for quite sometime.

A normal 5mm white LED requires about 20mA current at 3.3V for getting illuminated optimally.

The circuit involved for illuminating 5mm LEds through a 3.7V Li-Ion cell is actually too simple, primarily because the parameters are closely matched with each other.

Here, connecting the 5mm LEDs in series wouldn't be feasible because the maximum volts from the cell is just 3.7V while even two LEDS in series would call for above 6V.

Therefore the only option left is putting them in parallel.

Ideally when parallel connections are involved, a series limiting resistor becomes imperative with each LED in the array. This helps ensure uniform light distribution or emission from the LEDs.

However it's not an absolute requirement, especially when the driving voltage is close to the forward voltage of the LEDs.

Also taking the simplicity factor into account, a single limiting resistor may be used in such cases and therefore here too we have eliminated individual resistors.

How to Connect the LEDs

The circuit diagram below shows a simple configuration comprising of a 3.7V Li-ion cell, 5nos 5mm LEDs and a limiting resistor R1. The procedure shows how simply a Li-ion cell may be used for illuminating 5mm LEDs for a reasonably long period of time.

Each LED is supposed to consume 20mA current, therefore 5nos would together consume around 100mA, therefore R1 may be calculated as follows:

The Formula

R = (Supply voltage - LEd forward voltage)/LED current
= (3.7 - 3.3)/100 = 0.4/0.1 = 4 ohms.
The required wattage would be 0.4 x 0.1 = 0.04W, so a 1/4 watt resistor would be more than enough.

Assuming the cell to be rated at 800mAH, with 5 LEDs, the approximate back up time available from the cell could be calculated using the following cross-multiplication.

800/100 = x/1100x = 800x = 800/100 = 8 hours ideally.

However practically you would find the above calculated back up time to be considerably less due to many inherent inefficiencies associated with the system or the circuit.

More LEDs can be added, if you are ready to compromise the backup time proportionately.

connect 20mA LEDs with 3.7V Li-Ion cell
caution electricity can be dangerous

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About Swagatam

Swagatam is an electronic engineer, hobbyist, inventor, schematic/PCB designer, manufacturer. He is also the founder and the author of the website: https://www.homemade-circuits.com/, where he loves sharing his innovative circuit ideas and tutorials.
If you have any circuit related queries, you may interact through comments, and get guaranteed replies from the author.

Reader Interactions

Comments

  1. mike whitney says

    August 20, 2023

    If you will I am trying to conform to existing legalities with existing lighting. Thank you
    Mike

    Reply
  2. mike whitney says

    August 20, 2023

    Hi Swagatam, I was inspired by what I have read. I have a particular situation I have been unable to solve. If you could help it would be appreciated more than you can imagine. Picture if you will, 4 red leds in parallel powered by 2 aa batteries. I need to add a switch and circuit to the existing that will allow a regular flash of said leds. if you see a second switch causing flash the first switch a constant light

    Reply
    • Swagatam says

      August 20, 2023

      Hi Mike, I think you can try the following design and see how it works:

      https://www.homemade-circuits.com/wp-content/uploads/2023/08/LED-flasher-with-a-switch.jpg

      Reply
      • Mike says

        August 20, 2023

        Thank you for your prompt reply. I assembled this circuit this morning. Unfortunately I could not make it work as intended. I will continue to ponder the question as to where I went wrong.

        Reply
        • Swagatam says

          August 21, 2023

          Are the LEDs flashing? It is a simple transistor astable oscillator circuit and is a tested design, it should work.

          Reply
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