RE: Garden Tractor Battery
As stated - The battery charger transformers are current limited by their internal impedance. The high internal impedance is built in intentionally as stated above, where in a normal filament transformer, the internal impedance is made as low as possible by design. A lead acid batery has extremely low internl impedance - it looks like a short circuit to any voltage source higher than the battery terminal voltage. That being the case - something has to give and likely it will be a burned up transformer.
The published charger circuit by AS EE is an accident waiting to happen depending on what a novice builder might select as a transformer.
Most standard filament transformers are not even close to being current limiting and the transformer will overheat, melt out the potting and possibly catch fire.
The old selenium rectifier chargers use the internal resistance of the selenium rectifier as a current limiter. While the technology is dated - it worked very well in that application.
I have built maintenance type chargers that worked fairly well using a wall wart type supply.
For current limiting, I used a series resistor for current limiting and a resistor and LED in series shunting the limiting resistor as a charge indicator. It works, but a cheap charger from a discount store is probably a better choice.
BTW - Being and electrical engineer doesn't mean jack when it come to this type of stuff!
It all boils down to how you use what you learn in school.
I know -
Pat Bunn
BSEE
PE SC # 9801
licensed in over 15 states