Make sure you are getting a real battery tender and not just a trickle charger. You want something setup for AGM batteries and something that will shut off and float charge when the battery is full. Some cheap trickle chargers just jam X amount of power in all the time regardless of state of charge.

If you are constantly having problems with Optima batteries you have to look in the mirror for the problem. I sell 1000+ of them at work a year and they are very trouble free. We get several people bringing them in that say they are bad but they just need properly charged. It is always the race car or show car guy having problems with them.