At 12.5 v it isn't actually charging at all , low battery voltage and a slight high resistance in the lamp unit earth are enough to slow the
flashers down.
An electronic flasher unit will fix it but fix the alternator and check for voltage drops in the wiring.
RealFixesEalFast Alternator & Battery test.
[Edited on 11/6/17 by britishtrident]
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]