From years of boating, one learns to keep batteries fresh over the off-season. Four or five marine starting batteries is a big nut to risk every winter.
For long-term storage of a charged battery, a tender is what you want. A $20 750ma tender is not very good at charging weak batteries. So if you want to charge and store, then a bigger "smart" charger is the ticket. The better ones have some added features, but they all do basically the same thing. Some advertise 4 charging/tender stages, others as many as 8. They vary voltage and amperage during charging, but the tender stage is either a float or a pulse.
With a good tender, once the battery capacity reaches 95-100%, the trickle charge stops. As the storage voltage drops below 95%, the tender will apply a light pulsing charge to bring it back up above 95%, and then stop. It will go on doing this indefinitely.
If your charger has a tender function like that, then you're set.
What you don't want to do is leave a battery sit on a traditional 10 or 20A+ charger indefinitely, even the "automatic" ones. The trickle charge float is too high and doesn't really shut off, which can lead to electrolyte boil off. We have a big Rizk charger that works great to revive a flat battery, but you don't want to use one for storage.
The other thing you want to avoid is deep discharge of a starting battery. Most never fully recover from it. Letting the battery just sit unconnected in the cold all winter will lead to some self-discharge. Leaving it connected to a unused modern car is a sure way to discharge it. With some computer-laden cars, a dead battery creates more problems.
A decent tender for off-vehicle battery maintenance is about $20. A smart charger starts at about $30. Using a tender will also relieve the alternator if the car sits weeks between starts. If connected to a modern vehicle, absolutely use a TVSS spike/surge protector, just like you would with any other computer.
There are 6v chargers and tenders, and universal models that handle both 6 and 12v. The big two "smart" makers are Ctek and Schumacher, but there are many others.
