what ohm resistor will i need to power it from a standard 12v neg ground car battery safely without frying anything.
simply a dummy 12v flashing l.e.d.
dmb06851
I am guessing that you mean 5Vdc at 80mA.
From a 12V source a resistor of 87.5Ω would be needed. However, you can't get one of that value. A 91Ω one would allow 77mA to flow.
Once the engine starts, the system voltage will be higher, at about 14V, and the current with a 91Ω resistor would rise to 99mA.
You could, instead, use a 110Ω resistor which would give you about 64mA with the engine stopped, or 82mA when the engine runs.
Deric
Don't worry about the battery voltage fluctuating.
Skip the resistors and just use an inexpensive 7805 regulator IC.
3 Pins/connections - simple
Nominal +12.6 volt input pin
Common negative ground pin
+5 Volt output pin - stable - and 80 Ma is no sweat.
Regulator dissipation is less than 1 watt for you