[alanw]

Yea dude with a high (12.8) starting voltage it sounds like you got a nice healthy battery there. Checking this battery life calculator using your 10.5ah battery and 250mah power consumption for the mark-4 shows you might get around 12 hours before the voltage drops to 12.5 or 12.6. Take it out on another trip without recharging and you'll find out.

Just because your voltage is high after 6 hours of use doesn't mean it's still fully charged. You can use over 10% of the battery capacity before you'd even see a 1/10v drop. You'd be able to see better if you used a voltmeter with higher accuracy to 1/100ths of a volt or more.