I cannot find a data sheet for the SD109. It is probably a reference code used between the board manufacturer and his supplier. As it's in a charging circuit, the SD would seem to mean 'silicon diode' with the '109' being its ratings. The 'D5' etc on the board would appear to confirm that it's part of a bridge rectifier circuit.
See if you can establish what voltage and current the transformer gives, then look for a suitable diode in any of the many component supplier's sites.