Hello 
,
I'm looking for help to get a stable voltage from the car battery to the Arduino board because I'm working on a car related project and everything works fine except when i crank the ignitio, the battery voltage drops for a moment due to the starter motor load so the board restarts
Will a voltage regulator fix this issue??
Likely not. Better stuff is needed.
Please post a wiring diagram. I've solved such issues before.
For many years, the automobile key switch turns OFF all but the connection to the starter solenoid in order to avoid the problem you are experiencing. You should be powering your circuitry from the wire from the key switch that goes to all the other accessories.
Paul
Will not make any difference where the Op picks up his supply line.
Fact is the battery voltage is dropping and he is probably experiencing brown out.
If a circuit diagram of the connections are supplied there is likely to be an answer.
As others have explained, your board is resetting due to brown out. One very simple way to avoid this would be to place a large capacitor, around 4.7 uF or 10 uF on the power supply to your Arduino board and ground. Something like this:
Essentially, attaching this to the power supply of your board will ensure that any sudden drops in the power supply are smoothed out. As long as the dip in voltage you are talking about is quick, i.e. around a second or less, then this will cause a significant delay in the voltage drop which will ensure that your board won't reset
Check out an Arduino battery shield, you can search for a sales link on google.
It let's you put a battery in the line and automatically charges and switches it for you.
Run it off the battery with relay triggered by the keyed switch leg.
Then, use a buck to regulate voltage.
From the buck to the battery shield and Bob's your uncle.
Three words.
SMOOTHING, FILTERING and SUPPRESSION.
CAPACITOR, VK200, P6KE BIDIR 
A capacitor by itself won't help because when the voltage drops the capacitor will "instantly" discharge as it tries to help power the starter and the whole car, etc.
However, if you put a diode in series with the power to the Arduino, with a capacitor on the Arduino-side, the capacitor can't discharge backwards through the diode.
I would try at least a 1000uF capacitor (rated for at least 16V for some safety margin). It's hard to say how long you can power the Arduino from the capacitor so you might need a bigger capacitor.
With the capacitor charged to 12V it can discharge down below 5V before your Arduino shuts down, so that's working in your favor.
I do this as a habit for several reasons…
The caveat is 600mV drop of a silicon diode, or use a suitably rated schottky diode to minimise that drop.
I've measured as low as 6 volts on a starting vehicle with valleys going a little lower. The voltage waveform is generally similar to a sine wave.
I would try a 5V to 9 - 12V boost converter. A capacitor (with schottky diode) may help but may not work on a cold morning.
Any capacitors you use need to be at least 35V as there are transients on the DC line of a vehicle.
I faced the same problem, computer browned out due to a motor consuming a terrible lot of current when starting.
We solved it by using a serial diode capable of some amps and a "good" capacitor. During the motor start, punching down the battery voltage, that capacitor worked as a battery for computer.
As already adviced, connect "directly" to the battery.
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