Can I use a 22 Ohm 7 watt resistor to power two SMD LEDs? Please help!!?

22 ohms and 10 volts (13 - 3) is 450 mA

But no, you cannot wire LEDs in parallel, unless they happen to have identical Vf voltages. You need two separate resistors. Each should be about (13–3.5)/0.25 = 38 ohms, 3 watts (preferably 5 watts).

And you need to be very careful about power dissipation. At 250mA they will be dissipating typically 0.9 watts, max of 1.0 watts.

These seem to be designed for use on a pulse basis, for cameras. To operate them at 250mA DC, you need to mount them on a special metal core PC board, as noted in the datasheet. It also lists lots of special handling precautions.

edit: "two exact same LEDs made from the same manufacturer" doesn't matter, they can and will, still be different. Spec says max 4v, min 3v, and what that means is that one could be at 3, the other at 4 volts.

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I would start by putting the LED's in series, using less power and smaller resistor.

At about 14V when charging, 3.5V typ diode drop leaves about 7V across the resistor. By Ohm's law,

Voltage Drop = Current * Resistance. 250mA max and 7V gives E/I = 28Ohms minimum. Resistor power is E*I = 1.75 Watts. A slightly higher resistor will drop the current lower and drop light output some, but considerably add to life. You need to be confident that the junction temperature is limited to the max allowed, even if that means using bigger resistors to limit current below the max allowed.

Use a multimeter to verify the drop across the resistor. Ohms law lets it directly indicate current. Check the drops across the diodes, they should be about the same. If not, you might want to buy 1 or 2 more LED's, and select the two that are closest.

There are some variations in voltage in the battery/charger system, but design for the max voltage, and you probably don't care if the LED's are dim when the engine isn't running.

Make sure current is 250mA or less when charging. Resistor power would be I^2R or 1.75W. Again, check the voltages across the resistor and diodes to make the current and the diode voltages are in spec.

Parallel LED's are not a good idea because of current sharing problems.

If the two LED's are very similar in characteristic (really need a test to determine this - don't rely on makers info) then should be OK. Otherwise use two resistors and feed separately.

no

in most of time one led of parallel leds be on.

2 parallel leds need 500 ma current

res volt==> 500ma * 22ohm=11 volt

max leds volt ==> 14-11=3 volt

min leds volt ==> 12-11=1 volt

1~ 3 volt cant turn on that leds

With a total supply of 3.6 volts and a forward voltage of 3.2 volts, your 22 Ohm resisters will be dropping approximately 0.4 volts or .018 Watts. a 1/8W resister will be fine.

Yes!