Just dismantled a PSU which is marked "12V 900mA DC", but I know was to charge a lawnmower starter 12V battery.
It outputs about 14.3V DC, with about 50mV ripple off-load.
The circuit is essentially an iron-core standard transformer, bridge rectifier, smoothing, then the 8-pin DIL chip driving an output transistor, which goes via an inductor to the output.
The design is close to that shown on various datasheets for the chip - example here
https://www.ventronchip.com/pdf-f5/RT34063AGS.pdfWhat I'm particularly interested in is why a basically linear design uses this chip - is it specifically as it is used to charge a Pb-acid battery?
Does the voltage output somehow regulate down as the battery reaches the more nominal 13.2V?
I'm assuming the external tr is for current boost?