Stop in for a cup of coffee

Man that 270 poly has one very nice rumble. With stock style mufflers. Followed my buddy the 35 miles home. Ran great, hit 70 mph and it didn't blink. But get to the gas station, battery is dead. Jump it, takes off instantly. Ammeter appears to be working. Headlights and turnsignals both have a slight draw down for a second then right back to normal. 6volt systems