12.5 v is 90% charged. 12.7 is fully charged.
I believe that if you leave the charger on for too long the cranking battery voltage will eventually exceed that of the auxiliary battery and then the system will start to suck power out of the cranking battery to charge the auxiliary. Sorry, my knowledge is very general but I now disconnect my charger as soon as the cranking battery shows fully charged. So it seems what you are doing is correct. I’d be going back to where you bought it and having it checked out.
David


				
				
				
					
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