Is the LED a DC LED? Just checking to be sure.

Seller on amazon did not say ac or dc led

 

JFTR, that LED looks very much like a standard parallel/series matrix COB without any resistors.

Seller say don't need resistor just need heat sink,so sound like the seller saying there is a resistor in the led.

 

This led is rated for 12 volts ,20 watts,1800ma.
Question is if want to run this led 12 volts at 900ma instead would resistor be 13 ohm?
If run the led with less current draw ,would battery run longer time since not not using the full current1800 ma?
mrel

 

If you connect a 13 ohm resistor to a 12.0V battery without an LED then the current would be 923mA.
Maybe the LED has very poor quality (is cheap) so when the power supply is 11V it barely lights up and when the power supply is a fully charged 12V battery at 13V then the LED burns out.

 

If you connect a 13 ohm resistor to a 12.0V battery without an LED then the current would be 923mA.
Maybe the LED has very poor quality (is cheap) so when the power supply is 11V it barely lights up and when the power supply is a fully charged 12V battery at 13V then the LED burns out.

No, that is wrong! COB LED assemblies s require a higher voltage when compared to a normal LED. A 12V drop is not unusual (there are four white LEDs in series). Actually, white LEDs can go up to 3.6V each. This is normal LED behavior, and you should expect a large current variation without any headroom and a very small dropper resistor, when the voltage varies a little bit. The LEDs are not low quality.

Ideally, the supply should be, at least, 15V, and the dropper resistor should be of a larger value, and thus the current will not vary as much. In this case, since the OP is limited to a 12V supply, he should opt for a different LED arrangement, having three or less LEDs in series.

Kind regards, Samuel Lourenço

 

Perhaps it would be best to design according to the LED assembly's datasheet.

 

Perhaps it would be best to design according to the LED assembly's datasheet.

Indeed! The OP should either raise the available voltage, or use LEDs that require less voltage.

Kind regards, Samuel Lourenço

 

I think 6 ohms would be closer @ 5 W or higher. Or maybe an adjustable CC or buck converter ?

 

See