Just be aware that a voltage of 12.6 may mean the batteries are fully charged or fully discharged. Say you charge at 13.6 volts, then stop charging. The voltage will be 13.6, gradually diminishing over the next 8 Hours or so with no current being used as the fresh charge is fully absorbed.
Battery monitors start at $25 on eBay. They measure the voltage and current, integrate to get the actual charge going out and in and display the number of amp-hours of charge in the battery and % of full charge. I have one of these and really enjoy seeing the per cent of full charge at all times. It is more work to install than a voltmeter, though.
http://www.ebay.com/itm/DC-120V-100A-Battery-Monitor-State-of-Charge-Remaining-Capacity-AMP-Voltage-AH-/182147916411
