I have the D 34/78 in my XJ about 3 years. The XJ have been sitting since August of 2009 and have not been started. I have a 1.6 AMP solar charger as a battery maintainer because I have been out of the US all that time. February I walked into the driveway and using the remote start the XJ started right up. The secret is maintianing the battery at or near full charge.
Here is some information from Optima site:-
Battery Storage
Because of the high purity lead grid in the OPTIMA battery, it has a self-discharge rate much lower than conventional flat-plate batteries. This means the OPTIMA can sit for longer periods retaining enough charge to start your vehicle. Depending on storage temperature, the OPTIMA can usually sit for 8 to 12 months and start most vehicles.
When possible, store your battery in a cool, dry location. Check the battery voltage every 6 months and charge if it falls below 12.6 volts.
Remember, newer vehicles with on-board electronics such as computers, LCD screens, game systems, GPS units, clocks, etc., require battery power to retain system memory while the vehicle is parked. If the vehicle is to be stored for long periods you should use a maintenance charger to compensate for this drain. This charger should be voltage regulated between 13.2 - 13.8 volts, 1 amp maximum. On older vehicles, without electronics, disconnect the battery cables when the vehicle is not being used for extended periods.
