The 2 worst things you can do to a battery is allow it to sit dead flat for long periods or keep it fully charged for long periods and the former is far more destructive than the latter.
Not to sound snotty but that sounds contradictory. The first statement I agree with, I just don't understand the second half. I would think eventually any kind of battery would loose energy over time whether something was drawing power from it (e.g. radio memory) or not so how could it stay fully charged?
I will agree that if a typically (non deep cycle) type battery goes dead and stays dead for a certain length of time (don't know how long that is), it's toast. As I found out on my Chevy van.
I was able to save the battery on my Trabant. Somehow the key got turned to the on position and the battery went flat. But I was lucky that it took a charge and has been OK ever since. That was over a year ago. I must have caught it in time.
I invested in some of those $20 trickle chargers from Harbor Freight and they have paid for themselves. I keep them connected to all my collector cars even if I don't plan on driving them for a couple of weeks. I even pulled the batteries off my riding mowers and have them hooked up to trickle chargers.
Here is a good reason not to use one of those "old type" battery chargers. The ones with an amp meter and needle.
This is the battery on my Fiat Spider. The battery is in the trunk with a cover over it. I had been using this old Craftsman battery charger my grandfather bought. Before I invested in trickle chargers, I'd just connect the battery charger every couple of months to my car(s). You would think when the needle falls to zero, the battery charger would kick off but they don't. If you leave them connected after the needle hits zero, they will continue to charge. Then there is this sulfur yellow residue all over the place.
Since using the trickle chargers, I have not had this problem.
Here is something I've yet to figure out. Same subject, only different type of battery.
Why is it if you leave AA, AAA, C or D batteries in a device (like a radio), they eventually leak? However if you remove the batteries, they typically will not leak.
So let's say you have two identical pocket radios. You buy a 10 pack of AA batteries, place four in one radio and four in another. If you leave the batteries in one radio, about a year later, the batteries will leak. But if you remove the batteries from the other radio when not in use and place them in a zip lock baggie, they will not leak. Even if all eight batteries go dead simultaneously.
I've yet to figure this out. Also I can't recall ever seeing a 9 volt battery leak acid.