Why no charging on nissan almera ?
So, you did not have the welding done with gas, but with electric arc.
Anytime a person welds on a car with anything electric, the car's owner must disconnect ALL things electrical, or everything may be toasted, as yours is.
The alternator is a self-contained unit, and it either works or doesn't.
The alternator has the voltage regulator inside as a part of it.
I believe you may be needing an ECU, which is the brain of the car, however, because of the damage caused by an uninformed person, your car may never be fixed, because some wires may have shorted out (melted together) inside on of the wire harnesses that are all over your car. I think the best you can hope for is that it runs, and the alternator charges the battery as needed. I think the alternator is supposed to put out about 13.1 volts.
If you look up an ECU on e-bay, you will notice there are repair services who can repair your ECU. I have used them several times, and this has worked well for me.
Sorry.
Be blessed.