430 ohm needs half watt is enough.
If these are newer white LEDs they likely need 3 to 3.5 volts each which means 4 light in series are ok with 86 ohms or less. 12 volt lead acid batteries can be as much as 13 volts trickle charging, and 15 volts fast charging, so you should be sure your LEDs are off if you fast charge (vehicle alternators do fast charge the battery) or you could use 430 ohm resistors when fast charging. A 430 ohm resistor drops 8.6 volts at 20 ma, so 2 or more LED's in series are very safe up to almost 15 volts, but rather dim at 12.6 volts. 8.6 volts at 0.02 amps is 0.17 watts, so 1/4 watt resistors are good.
The 100 milliamp LEDs are likely ok 4 in series with a ten ohm 1/4 watt resistor.
86 ohms drops 8.6 volts at 100 milliamps, so two LEDs are very safe up to almost 15 volts, but a bit dim at 12.6 volts. One LED with 86 ohms is likely to fail at 12 volts, so you need almost 100 ohms, minimum. 100 ohms at 10 volts needs to be a one watt resistor with good ventilation or a heat sink, otherwise use a 2 watt resistor. One watt is ok for the 86 ohm resistors with two or more 100 milliamp LEDs in series.
LEDs from some sources may be poorly matched, so it may be wise to test with an extra LED in the series string or at reduced voltage, then remove any LEDs that are significantly brighter or dimmer than the others. I have not had good luck with the misfits, but perhaps you can group dimer ones with with dimer ones and brighter ones with bright ones.
Sorry but the guys above are not even close.
Resistor power rating is not even half the story. You need to also know the resistors thermal transfer characteristics.
For example a resistor may be rated as 1W but that is 1W below 200C usually. If the resistor is in a situation were its Rth is 500c/W then it really is for that application a 200mW resistor as anything over that will cause it to over heat.
So you have a situation where you could be dropping 10V at 100mA across a single resistor. If the resistor is more than 0.5 inch (12.5mm) away from any other hot resistors (to stop mutual heating) then to get rid of heat at a reasonable rate assuming no lifting spacers (to get the resistor body away from the PCB) then you need realistically a 3W resistor. With lifting spacers you could get away with 2W. With close proximity to other resistors you would need 5W or even 7W.
Resistor value;
R = (Vsupply - Vled)/Iled
Resistor wattage;
Power = R* Iled^2
I have 2 types of led's I want to use with 12 volts.
V=Ixr
One is 20 MA 0.020 and the other is 100 MA 0.100.
Anyways I need some 430 ohm and 86 ohm registers but not sure of wattage to use?
led is 1/2 watt but would a 1/4 watt resistor.matter/work fine?
Thank you
R.N.
