Costco 6 volt batteries- both died

[duponts]

I recently purchased 2 6 volt batteries from Costco for my travel trailer and wired them in series. I used them for 2 trips and when I picked up my trailer last weekend found that both were dead (I forgot to unplug the negative battery lead and accidentally left a light on the travel trailer for a few weeks). When I got home I attempted to charge the batteries and got the 'Dead' message on my Schumacher charger. I next attempted to charge each battery individually but my charger did not even recognize and try to charge one of the batteries and the other battery read 'Dead' after 10 minutes of charging. Do you think I received 2 bad batteries from costco? I tried using a different battery charger and have run into the same issues with the batteries.

[wcjeep]

Lead acid batteries do not appreciate zero voltage. They are most likely permanently damaged even if you were able to exert some charge into them.

[rrupert]

I would say that running the batteries completely dead was the fatal mistake. Deep cycle batteries are designed to not be discharged below about 50%.