Optima Yellow Top failures

[68LS1]

The car in question is my 68 Camaro, with LS1 & 6 speed. Not driven much recently. Lives with a Ctek battery charger attached. Has an immobiliser & E-Stopp brake. No electrical changes in years.

Last May the battery had suddenly died completely, this battery was around 3 years old. Replaced it with another Yellow Top, it was dead in 4-5 months, during that time it had fully charged and all signs on charger were correct. So it was replaced. I purchased another battery charger in case it was the culprit. Yesterday I go to car and again another battery has died completely, again 4-5 months old. This one I had done a couple of drives, and charger showed all was good until I checked yesterday.

The last 2 batteries have been replaced under warranty, but what are my possible options to look for, or a different brand of battery?