Brock, I believe in general that battery charger transformers are specially
made to have a high internal resistance (Rint) so that the entire voltage
can drop across the Rint in the event the battery you clip on to is shorted
This is in addition to internal circuit breakers and thermal breakers.
Also at 25 amps I believe you would boil most types of auto batteries.
I would advise great care and thought before proceding with this project.
I recommend this link to your attention.
You need to either regulate the voltage to 14.8V, or limit the current
to something safe and let the voltage float to wherever the battery
wants it. I believe most inexpensive chargers use magnetic shunts to
limit the current, as do microwave transformers, so you may be able to
make something work. Really though given the potential dangers (these
things are normally run unattended) I would just buy a charger, or build
a smart charger using one of the many purpose designed IC's out there.
Microwave transformers are normally built with the minimum amount of
iron and windings to get the job done and are not really intended for
continuous duty. They can be useful, both original and rewound for
various things but be careful.
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