Choosing leds for batteries power

Question

Can I use 3V (2*AA or 2*C batteries in series) as power for 10/20 leds connected in parallel?

LED specs:
Diameter: 3mm / 0.1"
Emitting color: Red/Green/Blue/White/Yellow
Material: GaP
Forward voltage: 1.8-3.4V
Forward current: 20mA
Backward voltage: 5V
Wavelength: 460-630nm
Frequency characteristic: Low frequency

I will connect only 1 collor lets say 20 red or 20 green leds... need them to work at 70-100% of intensity... the batteries model and number is not a limit I am just searching now...and yes I can make a connection of 3*C batteries in series

Answer 1

If the LEDs are to be directly powered by the batteries, the blue, white, and possibly the green LEDs (depending on which type of green LED it is) will not work at all on 3 Volts: They require a forward voltage of 3+ Volts, 3.4 Volts as specified in the question.

Also, once the voltage starts dropping due to depletion, even the marginally lit ones (e.g. the green, may be the orange) will no longer work.

Therefore ideally only red LEDs should be used: Red LEDs typically have the lowest operating voltage amongst visible colors. Alternatively, as noted, 3 cells would be needed in series.

Now about power:

LEDs can be operated typically at half the nominal current or less. So 20 LEDs operated in parallel at around 10 mA each will require 200 mA current, well within the capabilities of typical AA Alkaline cells.

For instance, the Energizer E91 Alkaline AA battery is rated for around 3 to 4 hours at 200 mA current. Put three of those in series, add in an appropriate current limiting resistor for each LED, and it will all work.

Answer 2

You don't really give useful information in your question. What are your requirements, how long do you need the leds to be on? Do you need them to be at a semi-constant brightness? Do you need them all of them to be on (because once the battery voltage starts dropping, the blue, green and white will turn off first since they need more voltage, and leave the orange, yellow and red on)?

If you just want to know if its possible, sure, its possible, but the white, blue and green leds will dim very fast once the batteries start to drain.

However, if you add an extra battery in series, for a total of 4.8V (1.6 per battery), you could have all the leds on for a longer time. Just provide the correct resistors for each led to limit the current to 20mA when you have the max voltage of 4.8V.