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the circuit attached works fine when connected to 12V battery. The question is what would happen to the performance when connected to the car battery with the alternator charging.
Would I require any additional filtering and would the output of the comparitor alter with the voltage fluctuations?
When running at a fast idle or faster, modern cars regulate the battery/bus voltage to a narrow range between 13.8 and 14.4V. There are times when the voltages are different: with key-on but engine stopped, the voltage will be ~12V. During cranking, the voltage will drop as low as 8V.
With engine running, the alternator ripple is "filtered" by the battery. The battery is equivalent to a capacitor of hundreds of Farads, and an effective series resistance of a few mΩ. The peak-to-peak ripple voltage at the battery is usually only a few tens of mV.
Most of the problems that people have with alternator ripple and other noise in automotive systems (particularly with audio systems) are due to resistance and common-mode voltages developed along the vehicle/ grounding system, which is usually the car's frame and body.
There are times when large short-duration inductive spikes (both positive and negative) can damage electronics. The spikes are usually triggered by high current inductive loads, like the starter motor, the starter solenoid, the A/C clutch, electric fuel pumps, etc. Most ICs developed for automotive applications are specially designed to tolerate +-50V short duration transients on their power inputs...