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To keep it simple lets just say you have three 3 Watt LEDs, an R, G, and B.
I assume you want to adjust the current in each LED to get the same functionality as a Phillips Hue light bulb.
It is easiest if you find green and blue LEDs with a forward voltage below 3V which is just under the 3.2V cutoff discharge voltage of an Li-ion battery.
Rather than an RGB I would recommend using the Luxeon C Color line LEDs. They are small, have excellent color mixing, and very wide viewing angle (170°). Some good alternatives are the OSRAM Oslon SSL 150 and Cree XP-E and XP-E2
If you are stuck on a single RGB then I would recommend the LED Engin LZ4 which is an RGB and White. You do not have to use the white.
You will need three LED drivers, one for each color. Here you have to design your own or use an off the shelf DC-DC constant current buck step down driver. If your forward voltages get too high then you'll need a buck-boost constant current driver.
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The only off the shelf driver I would use would be the Mean Well LDD (buck) or LDB (boost-buck) but they have a minimum input voltage of 9V. You could use 3 batteries in series. The efficiency of the LDD driver is 97% so you will still be using most of the battery's energy to drive the LEDs.
The big problem with LED drives is the overhead voltage required. Some buck drivers need 4V over the forward voltage. You need a driver with a very low overhead or use serial batteries.
There are two methods you can use to dim the LEDs. PWM or a varying voltage level.
If I knew more about your application and you had the desire to design your own driver I could recommend some driver chips.
If you were to go with a NiMH or NiZn rather than Li-ion you could use a very simple boost driver. The Nickel-zinc (NiZn) is an excellent LED battery with a very flat discharge curve but best of all it is 1.65V rather than 1.5V.
Microchip Boost Driver
Unlike LEDs that are wired in series, LEDs wired in parallel use one wire to connect all the positive electrodes of the LEDs your using to the positive wire of the power supply and use another wire to connect all the negative electrodes of the LEDs your using to the negative wire of the power supply. Wiring things in parallel has some distinct advantages over wiring things in series.
If you wire a whole bunch of LEDs in parallel rather than dividing the power supplied to them between them, they all share it. So, a 12V battery wired to four 3V LEDs in series would distribute 3V to each of the LEDs. But that same 12V battery wired to four 3V LEDs in parallel would deliver the full 12V to each LED - enough to burn out the LEDs for sure!
Wiring LEDs in parallel allows many LEDs to share just one low voltage power supply. We could take those same four 3V LEDs and wire them in parallel to a smaller power supply, say two AA batteries putting out a total of 3V and each of the LEDs would get the 3V they need.
In short, wiring in series divides the total power supply between the LEDs. Wiring them in parallel means that each LED will receive the total voltage that the power supply is outputting.
And finally, just some warnings...wiring in parallel drains your power supply faster than wiring things in series because they end up drawing more current from the power supply. It also only works if all the LEDs you are using have exactly the same power specifications. Do NOT mix and match different types/colors of LEDs when wiring in parallel.
OK, now onto to actually doing the thing.
I decided to do two different parallel setups.
The first one I tried was as simple as it could be - just two 1.7V LEDs wired in parallel to a single 1.5V AA battery. I connected the two positive electrodes on the LEDs to the positive wire coming from the battery and connected the two negative electrodes on the LEDs to the negative wire coming from the battery. The 1.7V LEDs didn't require a resistor because the 1.5V coming from the battery was enough to light the LED, but not more than the LEDs voltage - so there was no risk of burning it out. (This set up is not pictured)
Both of the 1.7V LEDs were lit by the 1.5V power supply, but remember, the were drawing more current from the battery and would thus make the battery drain faster. If there were more LEDs connected to the battery, they would draw even more current from the battery and drain it even faster.
For the second setup, I decided to put everything I had learned together and wire the two LEDs in parallel to my 9V power supply - certainly too much juice for the LEDs alone so I would have to use a resistor for sure.
To figure out what value I should use I went back to the trusty formula - but since they were wired in parallel there is a slight change to the formula when it comes to the current - I.
R = (V1 - V2) / I
where:
V1 = supply voltage
V2 = LED voltage
I = LED current (we had been using 20 mA in our other calculations but since wiring LEDs in parallel draws more current I had to multiply the current that one LED draws by the total number of LEDs I was using. 20 mA x 2 = 40 mA, or .04A.
And my values for the formula this time were:
R = (9V - 1.7V) / .04A
R = 182.5 Ohms
Again, since the variety pack didn't come with that exact value resistor I attempted to use the two 100 Ohm resistors bundled together in series to make 200 Ohms of resistance. I ended up just repeating the mistake that I made in the last step again though, and wired them together in parallel by mistake and so the two 100 Ohm resistors only ended up providing 50 Ohms of resistance. Again, these LEDs were particularly forgiving of my mistake - and now I have learned a valuable lesson about wiring resistors in series and in parallel.
One last note about wiring LEDs in parallel - while I put my resistor in front of both LEDs it is recommended that you put a resistor in front of each LED. This is the safer better way to wire LEDs in parallel with resistors - and also ensures that you don't make the mistake that I did accidentally.
The 1.7V LEDs connected to the 9V battery lit up - and my small adventure into LED land was completed.
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