Hello,
I got a power source which is a 18650 battery (3350mAh, 3.6 - 4.2v)
My question is how many LEDs I can use in series with this battery without needing to use a resistor?
LED's specifications are: Current: 20mA, Voltage: 3-5v
And my second question is how many LEDs I can use if I use 2 of those batteries in series, to have double the voltage and capacity?
Thank you!
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Hi,
Why don't you want to use a resistor?
How else will you limit the current though the LEDs
The link from @Idahowalker should prove helpful.
Tom.. 
no one. The use of a resistor is mandatory for any connection.
Or some other current limiting ...
Ok, then does anyone know what is the maximum numbers of these LEDs I can use with this battery and what resistor do I need for them?
Thank you!
FIRST! You need to tell us the color of the LEDS. Are all the LEDs in a string the same color? Different color LEDs require different currents.
He said that the LEDs are 20mAh, so we can assume that all of them are the same.
Yes, they are all same specs and use 20mAh.
Thank you.
And they all use 5v to work properly.
Do I still need a resistor even though the LEDs need 5v and the battery cannot exceed 4.2v even when is full?
20mAh is a (very, very small) battery capacity and nothing to do with LEDs. I think you mean 20mA.
The LEDs do not need 5V. They need something between 3.0V and 5.0V, and each led can be different. If the led you connect needs 3.0V and you connect it to 4.2V with nothing to limit the current, it will be damaged. In practice, most will be closer to 3.5V.
So, yes, you need a resistor.
If what you say @PaulRB is true, then why the spec sheet sais all of their color models need the same voltage? 3.5v and 20mA
Because the amount of light from each color will have different intensity. If you want identical intensity, then the current will be different for each color.
I don't know, it is strange. I would expect red, orange and yellow to have a lower typical voltage than blue and white. I wonder if that data sheet contains errors.
These are more like the forward voltages I would expect:
It may be because they are intermitent LEDs that are automatically turning on an off?
Ah, yes, I completely missed that on the data sheet you linked to. It says:
5mm Round Flashing LED
2.4Hz 1/12 Duty Cycle
So maybe the voltage requirement is that if the built-in flashing circuit/chip. Since the voltage range is the same for all led colours, that may mean that the circuit includes a constant current source. If so, no external current limiting is required.
Now I think I can answer your original question: you cannot connect these LEDs in series, their built-in driver circuits would interfere with each other and would not work. You must connect all your LEDs in parallel across the battery.
Again, you cannot connect these LEDs in series, they must be connected in parallel. If you connect two batteries in series, the voltage would be too high and damage the LEDs (their Max voltage is 5V).
Connecting 2 batteries in series does not increase their capacity, only their combined voltage. Connecting 2 batteries in parallel increases the capacity.
Thank you @PaulRB , so if I connect these LEDs in parallel to 1 battery without a resistor, how many can I use? (I need the light to work for about 10 min.) before I recharge the battery.
