The 2 batteries at 7.8v 30 minutes after a full charge are goners
. I guess the others are around 8.3 or 8.4 (8.49 is 100%) There is probably nothing wrong with your charger it is not seeing enough voltage to shut off and it is correct. The real problem here is the charger is overcharging the good batteries trying to reach a full charge level and then when you run 15 miles you are over taxing the good batts because they are carrying the weak ones. All of this overcharging and overtaxing will result in a shortened life for the good batts... Do not replace the 3 yr old batteries with new you must find 2 used good batteries to add to this pack or you will be replacing all of them soon .....please post the readings for all of the batteries 30 minutes after 6-8 hrs. of charging.