Voltage Required for Battery Charger

Hi, I'm trying to make a car battery charger from a microwave transformer. I removed the high voltage secondary windings and now
want to add windings for the 12V secondary. I was wondering how to calculate the required voltage needed for a 25A charger? I know the voltage is typically from 13.7-14.4V maybe up to 15V.
Brock
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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 or depleted.
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. http://www.battery-chargers.com/charging_instructions.htm
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durabol wrote:

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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