Battery voltage may not be the issue. The issue may be what the amps/wattage of each battery is under load. Have your dealer run a load test on each battery to see if they are up to spec. A battery can show its rated voltage but fail under load. Today I was splitting logs in my pasture and pulled my log splitter and a full load of split oak logs in the bed up a steep bank in 4wd and in low without any problems.