Hi
In a simple circuit 12V Battery, a resistor 820Ω .25W, LED 4V 40mA.
The LED will consume .16W (4V x .04A), Is that all the wattage the battery is spending or the resistor is consuming some wattage from the battery?
and How can I minimize the watt spent on the resistor?

thx

 

Your resistor value is wrong. It should be 200Ω and it would be wasting 320mW (twice what the LED is using since it's dropping twice the voltage at the same current) and it would need to be a 1W resistor.

The simplest, easiest way I know to reduce the waste is to use a switch mode voltage converter to get the supply voltage down much closer to the requirement of the LED. The quickest, cheapest way to do that is to buy a surplus cell phone car charger for $2 at a thrift store. Try to get a name brand like Nokia, Motorola or LG so you know it's a switcher. Make sure it has a 5V output and try to find one that's easy to disassemble (put together with screws or a threaded ferrule). Take the circuit board out and attach your input and output wires in the obvious places. A substantial side benefit to using a converter is that it regulates the voltage, keeping it steady at 5V as your battery voltage drops which in turn keeps your LED current constant.

With a 5V supply, the calculated resistor value is only 25Ω and it's only wasting 40mW. That's not a standard value so use a 27Ω ¼W resistor which drops your LED current to 37mA and wastes 37mW. There will be about 56mW of power wasted by the converter assuming 80% efficiency (it may be a little better) so now your total wasted power is 93mW or less, down from 320mW.

Now I have to ask if you're sure about your LED specifications because 4V is a little high even for white and 40mA is an unusual current (20mA is typical)?

 

the values I took from the data sheet of an LED which puts out 3600 "cmd" or something like that, sorry I forget what it stands for, I am thinking if it would be better to use at home instead of the normal house light bulbs!

 

3600 mcd (millicandelas) is not going to be enough for more than a bright night light. Millicandela ratings are very deceptive. You need to convert to lumens. There are formulas and online calculators on the internet.