There is a difference in the batteries.
The optima Spiral Cell batteries cannot be charged with a battery charger direct. You have to run it through another battery, and run jumper cables to the optima battery from the battery that is hooked up to the charger to have it be able to be left on charge or charged.
This is what my Friend who is an Optima Battery Dealer has told me. He was told this by his optima rep.
I think most batteries only last 5-6 years anyway.
Have you thought of just buying a trickle charger for the battery for when it is stored, and going with a battery that is the right size for your car, rather than the optima which you say is taller, and wont fit.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.