Straight battery chargers use different styles of transformer than normal, as we want totally different behavior.
Toroids are electrically stiff..., you ask for 24v you get 24v and loading makes little difference... this is useless for battery charging, as it will blow the fuses at lower SOC and wont finish the battery at higher SOC.... so we are looking for the opposite of this behavior.. ie loads of leakage... we want good current at low SOC, and small currents at high SOC.
Dedicated transformers for this regime are ones where the secondary is loosely coupled to the primary. One favorite way was to wind the primary at one end of the former, and the secondary as far away as you could up the other end... so the magnetic coupling was loose.... some even went as far as using shunts between the windings, to short out some of the field, and shield the secondary from some of the flux.
This way when the battery was only 22v, the current would be fairly strong as the voltage differential is great, and when we get up to the 30v mark, it is still charging, but at a lower rate, as it nears the top end that the transformer can do.... but at low SOC, the current was decent without being brutal, because of the soft magnetic circuit.
Your 23v AC will translate to 23x1.414=32v less diode losses.. so yes it has the voltage for charging 24v bank. It will depend on your magnetic coupling as how well this will work out. ( 23vac as measured was rms. with peaks of 32v )
If it is tightly coupled, you will struggle with heat and over power at low SOC.
You can use a ballast ( inductor) to limit the current on the primary, or a capacitor on the primary will do the same thing but different methodology.
Caps are a cheap way of controlling the power of the charger, and I have used torroids and caps to this effect.
Your welding transformer will be nowhere as tight as a torroid, so may be ok as is. It was designed to be shorted out without taking out the fuse as you tried to strike the arc, so it must have been a bit loose by design.
I expect it has windings separated by a gap, and this will be a good start. If too strong at low SOC you would be able to place steel laminate strips between as a shunt to limit the current if you need to.
Or.. to answer you question directly.... there is no optimum, it depends on how you wish to tackle it, and what style of transformer and winding you use... nothing is simple with batteries is it...
Consider this.. for a 48v system, the transformer needs to operate sensibly from 46v to 60v..... thats a fair dynamic range to deal with, only loose coupling will do it without resorting to electronics.
..............oztules