I typically run mine in parallel with separate resistors.
However, for lots of LED, it can be simpler to run them in series till the
number of LED, in volts, matches the voltage of your battery.
So, 7 serial wired pairs of white LED run in parallel
If you wanted to wire all 15 of the white LED in parallel, and add one resistor
to the head of the line, you would likely need an 2.0 watt resistor to handle
the current. Maybe go with a 5 watt just to give it some allowance.
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The resistors help the match the voltage 3v, 6v, 9v, 12v, of the battery to the voltage limits of the LED
The Ohms is the resistance. You need ohms to reduce the voltage from 6v to 1.8 - 3.4 volts for the LED.
If the resistance, ohms, is too small, you will burn out the LED,... too much voltage.
If the resistance. ohms, is too large, the LED will not get enough volts, and refuse to light up.
The watts is the current. The resistors can only handle a certain amount of current. If the LED or set of LED
draws more current, watts, than the resistor can handle, then the resistor will burn out.