I would like to make a car battery-based spot welder using an Arduino Nano to control the time of the pulse of a large relay (turned on using a BC337 transistor) and display the pulse duration setting on a seven-segment display. The Arduino would be receiving its power from the car battery which at full chage would be 14.4 V. The thing is, when the car battery is shorted, I expect the voltage to drop and I don't know how much. Would this be likely to affect the running of the Arduino? Would a capacitor be needed in parallel with the battery (I think it would also need a diode to stop the current flowing out of the capacitor away from the Arduino when the pulse happens). If necessary, how large a capacitor would it need to be?
I don't expect the voltage of the battery to sag below the voltage needed by the Arduino Vin; I just expect it will drop suddenly and have no idea how much that will affect the output of the regulator. I don't yet know how long the pulses will need to be but definitely less than 300 ms.
Thanks for any help.