Wiring 1.8V and 2.1V SMLEDs in Series as 1S LiPo Voltage Indicator—Is Resistor Needed?

I searched and found enogh adequate information to fit 6 multi-colored low voltage surface mount LED's to my helicopter blades for night flying.
I am using a single lipo cell with a storage voltage of 3.8v and a full charge of 4.2vMy LEDs vary slightly in forward voltage, but seem to run efficiently enough using the 100 ohm resistors for each LED wired in parallel. The battery is 300 mah and running the set for 25 minutes resulted in about 25/30 mah recharge.
I figure I'm good, but ..... I question.I started to read more and understand more and thought hey- what if I put a 1.8 and 2.1 v LED in series without a resistor as a voltage indicator of storage charge and time to stop flying.I found a pair wired in series that lights (not bright) with 4.15v, but is very dim almost not lit at 3.85v.Then I read the LED might not be consistent and might still require a resistor. So I worry it would discharge the battery too quickly with no resistor, or is 2 leds in series enough resistance together to not require a resistor.?I do not know the exact specs of the LEDS as I bought from Amazon and I have 5 or 6 different colors and notice different light output from each color.I am using smd 0603 100 ohm paired to 0603 smd super bright LEDs found on Amazon.
Thanks for your timeSteve

There can be quite a spread in characteristics of LEDs. The light output is roughly proportionl to current, and is also  dependant on the eyes sensitivity to the visible light wavelengths.If you want some consistancy, the LEDs need to each have there own resistor, which can be tweeked to get the best apparent performance from the LEDs in terms of  brightness.LEDs are a current operated device, a series resistor  used to drive an LED typically has a  relatively low value, so it behaves as a rather poor current source. If you don't have data for the LEDs, then your best bet is to put a small series resistor and potentiometer (the small resistor will limit the current if you take the pot acidentally to zero Ohms) in series with the LED, put the whole thing across the battery you are using, and check the brightness. If you have a bench top power supply with good resolution in the current metering, you can put the LED straight across the terminals if the unit is in constant current mode.Cheers,Richard

Thanks for the comment.Interesting I never thought of it as a function of current. So that explains all the comments I read about 20mah.
I don’t have a fancy voltage supply and I just use the LiPo batteries for the testing. I don’t want to add more weight or mass to the rotor blades so a potentiometer is not what I want to do.I guess A better question would be if I run 2 leds in series with no extra resistor will it draw too much current and deplete the charge quickly or does the led have enough resistance in itself to handle/ limit the current. (& heat)

Basically all I am trying to do is have an led or two light with 4+ v and not light at 3.85vThis is what I bought from amazon 
https://www.amazon.com/gp/product/B01LYX8FBF/...x_yo_dt_b_asin_title_o04_s00?ie=UTF8&th=1

The pot is just to find a suitable value for the fixed resistor, once testing is complete you measure the total reistance and then pick a value from the nearest E12 series of fixed resistors. There is every chance that without a series resistor to limit the current that you will destroy the LEDs. When you say a helicopter, I take it you are talking about the real thing, and not a model? CheersRichard

Hi Richard,
 Thanks for the reply. That makes total sense.No this is a model helicopter spinning the blades the leds are mounted on at 2000 rpm.this set of blades

Can't see the picture in the link. So how big is the model, how are you getting power to the LEDs.The additional mass could cause problems, changing the dynamics of the blades considerably,cheersRichard

I just thought of something I could use as well. I have a variable resistor tool I used to use for testing the old analog gauges on cars. I can find the proper resistor using that. Thanks for help

If you are interested in extending the life of the batteries, you must lower the current flowing through the LEDs. If you accept a slight lower brightness you musy increas the resistance in series. You could try some results by driving the LEDs with the PWM and varying the duty cycle.CheersGiovanni

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I considered mentioning PWM, but if you want a continuous band of light it won't work, you will get all sorts of strange effects since the blades are revolving at considerable speed, PWM only works with a staionary LED,cheers,Richard