A weak battery in the pack

So the cart is up and running again thanks to all of you who helped me sort it out. Today I noticed a new problem. For reference, the cart is a 2000 E825, long box. Previous owner had installed a Delta-Q Quiq charger. And installed 6 new interstate batteries last May.

Just pulled it off the charger this morning showing full charge on the LED. When I turned the car on my battery percentage was 70%. Weird, so I checked the individual batteries. All were in the 12.5-12.6V range except the one under the drivers seat. 8.61V while connected to the pack, 10.4V disconnected by itself. I put it on my Optima 12V charger to see if I can revive it.

When I bought the car this spring the owner was having electrical trouble and couldn’t get the car to charge. I noticed a few loose connections and after charging each battery individually had no trouble until recently. When I bought it 2 batteries were deeply discharged, this is one of them.

I doubt Interstate will warranty them, a possible discount at best. But any ideas on why this would happen? We never discharge more than 50% and I regularly check the water levels. This is the first time since buying it that I’ve metered each battery.

Thanks for any suggestions!