I have salvaged a old microwave transformer that i am going to use for a SLA battery charger i am building. I have rewound the secondary with 50amp wire and i get an AC output of 12v what is what i want but im confused with working out how many amps it puts out. I have run it through a bridge rectifier and a capacitor and i get about 18V DC. I was testing the amps by adding 12v 50watt light bulbs to it 1 at a time and when i get to 200watts the voltage starts to drop to about 10v DC. My question is does that mean the transformer is being overloaded? and if so is there a way to increase the amp output by winding secondary with different wire? I have seen online videos of Microwave transformers being rewound and they give out 50amps at 12v.