Hi,
If you want to charge at 50 amps (which is pretty high really) then you need diodes that are rated for continuous 50 amps or better (maybe 100 amps), and the secondary voltage should be adjusted as you go, by taking turns off until you get the right current, starting at say 18vrms output.
It would be better if you could put turns back on, but that's probably harder to do. Because we are taking turns off that means we might have a current that is too high to begin with, so some caution has to be exercised. Maybe use some resistance in series to limit current for the first try, then eventually reduce that resistance to zero or nearly zero as you take turns off.
What comes into question is the initial voltage of the battery and the drop of the diodes as well as the series resistance of the primary and secondary, and the inductance also. We can predict to a certain degree, but it's better to test for the right value rather than try to predict this exactly because we are dealing with higher than usual current (50 amps) and some of the variables are not easy to measure.
You can start by measuring the primary and secondary resistance, that might help get started.
The current is limited by several things:
The primary voltage, leading to the secondary voltage,
the primary resistance,
the secondary resistance,
the equivalent series inductance,
the wire resistance,
the battery initial voltage,
the diodes initial and final voltage drops,
connection resistances.
If the resistances are all low and the output voltage is too high, we might pump too much current into the battery and cause it to heat up and even blow up. These things are very nasty when they blow, so you really have to be careful to limit the current to a safe level. That's the whole goal, and since we are taking turns off, that means we will be starting with a current that is too high to begin with uless we limit it by some other means like extra resistance. Once we get closer to the right value, we can decrease the extra resistance and that will eventually leave us with the right output voltage.
There also could be variation with input voltage if the line varies.
You also have to realize that diodes this big will need heat sinks. Also the battery can not be left on charge for too long as it will overcharge because there is no active charge control.
There is also the question of whether or not the welder is rated for continuous duty or intermittent duty. It will overheat and shut down if it is for intermittent duty and has built in protection if it is left operating too long at high current.
What is the current rating of the welder?
Is it rated for continuous or intermittent duty?
Can you measure the primary and secondary resistance?
What is the input line voltage?