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

Last Updated on by 39 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.

 

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Forum Discussion - Ask a Question

  1. Hi Swagatam,

    Your blog and explaining technique is very good. I learnt a lot about powering LEDs. Thank you.

    I need some help from you about my emergency light project which powered from 18650 x 2 batteries (3.7 v + 3.7v). I would like to glow 20 led (white bright) using parallel circuit. Please help.

    Reply

    • Thank you Sunil, glad to know you liked my site.

      Here’s the formula and calculations:

      Assuming you want to illuminate 5mm LEDs high bright type LED with 20mA current each, 20 of them in parallel would consume 20 x 20 = 400mA or 0.4 Ampere.

      The formula for calculating the current limiting resistor is:
      R = (Supply voltage – LEd forward voltage)/LED current
      R = (3.7 – 3.3) / 0.4 = 1 Ohm
      wattage will be (3.7 – 3.3) x 0.4 = 0.16 watts or simply a 0.25 watt standard resistor could be used

      for the emergency light circuit you could tr the following concept

      https://www.homemade-circuits.com/how-to-make-efficient-led-emergency/

      Reply

  2. I think one drawback when you’re connecting LED in parallel is if one burn out the rest not emitting light.

    Reply

    • In parallel connection even if one of the LEDs burn others will remain illuminated, however today the LED quality has improved a lot and they will not get damaged as long as the specifications are correctly maintained, regardless of whether they are connected in series or parallel.

      Reply

  4. Hi, I wonder if you can help me out as I am having problems making an led hula hoop that stays lit up. I have a single 3.7v aaa battery 600mah and 24 LEDs- 4 blue, 4 pink, 4 red, 4 yellow, 4 orange and 4 green. I have 68ohm resistors on the red, orange and yellow and no resistors on the blue, pink and green. The LEDs all light up fine, but then, the blue, pink and green ones dim and fade until they have gone out (not long after they start to fade). They do this if i add another battery too. Any ideas on what the problem may be?
    Thanks
    Jen

    Reply

    • Hi, the problem is caused due to their higher FWD voltage drop rating than the other LEDs in the group.

      You must add resistors to each and every LED through proper calculation, as per the following formula:

      R = Supply voltage – LED optimal FWD drop rating / LED's safe current rating

      for example, for blue the above formula would go in the following way

      R = 3.7 – 3.3 / 0.02 = 20 ohms

      Reply

  5. Hello, Swagatam. I like your blog very much.
    But you are wrong, the resistor is not at 3.3V, but it is under 3.7 – 3.3 = 0.4V, and the power it manages is 0.4 x 0.1 = 0.04 W
    Then, the resistor may be 1/8 W or less.
    Regards.

    Reply

    • thank you Harry, yes that's a typographical error by me.

      The wattage is always equal to potential difference across the resistor multiplied by the current flowing through it, that's simple Ohms law

      I'll correct it soon

      Reply

    • Hai iam have 3.7v nokia battery and also have 3v bulb iam light a bulb easily but please tell what problem cause if i not vonnect a ohm resistance and also tell what type v resistor i need to light a 3 v bulb using 3.7v battery please help me thank you

      Reply

    • Hi, you won't require a resistor for a bulb, resistor is required only for LED….still for better safety you can use a 1 ohm resistor in series

      Reply

  7. Hello Sir,

    Thanks for your response,

    I said the same thing to the shop guy its 1/2 watt resistor, He told in 1/2 watt there are many resistors with different powers.

    Can you tell me what can I ask

    Thanks & Regards,
    Soma.

    Reply

    • Hi Soma,

      In the article it is clearly mentioned as 4 ohms…so i think you should tell the shopkeeper about this value.

      It's 4ohms 1/2watt

      Reply

    • Hi, who much voltage and current a 10 mm led require. I want to connect 4 10mm led to 12v bike battery in parallel. How much ohms resistance should I give. Whether parallel or series connection is optimum.

      Reply

    • Hi, here are the details:

      3.0-3.4VDC Forward Voltage
      80mA Forward Current
      30 degree viewing angle
      16,000-20,000 MCD output

      connect them in series, use a single 10 ohm resistors, 1/2 watt

      Reply

  8. Hello Sir,

    When I ask 1/2 watt resistor , He is asking how much power you want,

    Whether is it 100 ohms 1/2 watt resistor or 10 ohms 1/2 watt resistor.

    Can you tell me which 1/2 watt resistor I can use for the above circuit of 5 LEDS,

    Thanks In Advance
    Regards,
    SOma.

    Reply

  9. Thanks a lot sir, i have some queries sir, since I'm a beginner in the circuits world, if u can spare some of your time in explaining the circuit briefly, it would be of great help sir. about what would be the discharge voltage and all. I have been learning electronics from your site and its amazing sir. I have even built the LED lamp circuit from your website which is now illuminating my table. Thanks a lot sir.

    Reply

    • Ideally discharge rate can be calculated by dividing the AH value of the cell by the load current.

      here the AH = 800mAH while the load current is 100mA, so dividing them gives 8, which should be in hours, therefore the discharge rate is 8 hours but in practical world this would be a lot lesser..may be just 5 hours or so.

      circuit is very simple and self explanatory, a single resistor is used which limits the current to safe levels, I have explained the formula in the article.

      Reply

  10. sir,
    what are other types of LEDs other than 5mm? also their current ratings? please specify.Also what is a 1 watt LED & LED driver module?
    iam having a 2.4v 600mA battery should i need a resistance for any no of LEDs connected?

    thank you sir for this post

    Reply

  11. Sir i tried this circuit and it works really good, and sir i have a query, i want to build a circuit which can be used in farms, on the ground so that it would shock the rats off from the field, i have done some research and found that the working is similar to the mosquito bat we usually use. But I'm unable to design the circuit for it. I plan to lay the wire mesh at around 2-3 inches off the ground all around and in between the field so that the whole field is protected from rats. The circuit should basically act as a discharge circuit when the wire mesh is shorted.

    Please help me with this circuit sir, My mail ID is shriramstrong@gmail.com

    Reply

    • Shriram, you can try this circuit:

      users.silenceisdefeat.net/~lgtngstk/Sites/Circuits/Electric_Fence_Charger/Electric_Fence_Charger.gif

      Reply

  12. Hello Sir,

    First of all thank you very much sir. your blog is really very usefull.

    I learnt how to calculate battery backup time form above post.

    I have a doubt sir. (Eg.I have a 2 Ah 3.7 V Li-Ion cell phone battery). If i use 5MM leds as explained above post how many leds can i use maximum and how to calculate. pls explain sir.

    Thanks in advance.

    Naresh

    Reply

    • Sir as you advised above i tried to find the resistance, but i am not cleare.

      as per above formula i calculated for 50 LED's

      R = (Supply voltage – LEd forward voltage)/LED current

      = (3.7-3.3)/1000=0.0004/1 = 0.0004

      i am getting 0.0004

      what is the actual resistance.

      Thanks

      Naresh

      Reply

  13. Hello Sir,

    I have a doubt. I have a LED i don't know that is 5 MM or 10 MM and i don't know how much of power it consume. But i need to glow that LED. how to know the power rating and led input voltage? if we know any two values we can find the other one using ohms law. But i don't know any value of the LED, we have any option to find this?

    Thanks,

    Naresh

    Reply

    • Hello Naresh,

      measure the base diameter of the LED, you will get the mm range of the LED.

      most leds operate at 3.3V, current will depend on their type and size.

      a 5mm led will require about 20mA, 3.3V for illuminating optimally.

      Reply

  14. HI Swagatham Sir,

    In the diagram you have mentioned R1 as limiting resistor R1, But what is the power of the resistor.

    How can I ask the shopkeeper to purchase it.

    Can you tell me in Layman language.

    Thanks In Advance,
    Regards,
    SOma.

    Reply

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