i put the battery charger on the other day and it let the smoke out with a bang. so i'm having a fun time pulling it apart and working out why it failed.

this is an older repco 8 stage rbc168s model, which is also available under the power train brand, no doubt the same Chinese manufacture. 

what failed is one of the transformer circuits went bang. whats interesting is its not the main power supply. this is an inverter design that uses a single mosfet to pulse the multi coil transformer. on the primary side there is a 2ndary coil, (sense coil?) but its not connected to the mosfet, its connected to the voltage rails, one side direct the other with diode. this looks like it pumps current into the dc rail using back emf. this smaller coil is whats shorted out against main rail thats right next to it in the transformer. its interesting they use a flywheel diode for the main coil, but use this 2ndary coil to provide a pulse down the main dc rail to pulse a secondary coil. however those pins are right next to each other on the transformer.

the 2ndary coil went bang and shorted across the pins with enough current through to explode the current limiting resistor, and the fuse.

however what i think is actually the cause, is a faulty connection on the main load/battery side. a leg of the main diode and mosfet had been accidently unsoldered by the worker pouring a ton of solder onto the tracks, and not even cleaned the flux off. so the main diode has been a poor connection and finally let go, the feedback circuit saw the drop in output voltage and cranked up the transformers pulse rate, resulting in the 2ndary circuit going to high and shorting across to the main rail.   

if you have one of those 8 stage models you might want to check the soldering.