A DC inverter is well suited, for an AC welder you need at least one diode, but it is better to twist a rectifier bridge.
This way I regularly charge the battery, no problem. To select the correct resistor, you need to know the open circuit voltage of the welding machine, I have it 65-64V.
For my welding machine, a 1.5 kW boiler is suitable as a resistor, when connected in series, the current receives no more than 1.15A, which is enough for humane charging of a battery, especially a deeply discharged one.
You can also use an electric kettle, an electric stove and other devices with a heating element such as Heating element and not only.
The boiler hardly heats up, but just in case I put it into the water anyway. The higher the power of the device, the higher the current of the improvised charger will be.
P is the power (W, kW), and U is the voltage.
For example, with a boiler I = 1500/220 = 6.8A (this is when powered by 220V !!!)
And since our voltage is not 220V, but 65V, this means that the current will be less than 4 times less. Even if the idle speed of the welding machine is 110V, the current will be only ~ 3.4A, which is not much for charging a battery with a capacity of more than 50Ah.
Be sure to use a fuse that will protect against erroneous voltage supply directly from the inverter to the battery !!! I have a couple of strands of stranded wire.
Not a call to action, you don't know and you are afraid – don't.