Like Byron, I use a BatteryMinder to keep my batteries fresh. It's a good product, and I highly recommend it because it will save your battery and save you money. That goes double (or triple, since that's the cost difference) for people who buy more expensive maintenance-free AGM batteries for their trailer. (AGM batteries are more durable in harsh environments, last longer, and charge more efficiently than traditional "flooded cell" batteries.)
OK, that's the opinion part of this post. If you want to learn more of the how-and-why science of why batteries die and how the BatteryMinder works, you can read on. Otherwise you can just bug out now.
Lead-acid batteries die when they become "sulphated." More accurately, they loose their ability to hold a charge as the lead plates in the battery become progressively more covered and clogged by hard lead sulphate crystals that form as a by-product of discharging the battery.
When lead-acid batteries discharge, the sulphuric acid in the battery water reacts with the lead in the plates inside the battery to create electrons, hydrogen ions, and lead sulphate. When the battery is re-charged the process is reversed. Over-charging the battery splits the water (H2O) in the battery into hydrogen and oxygen
gas in a process called electrolysis. (More precisely, hydrolysis.)
So, here's the thing: it may not seem intuitive, but lead sulphate comes in two very different forms, a soft, fluffy one and a hard crystal type. Whenever your battery discharges it create some of each form of lead sulphate, and the ratio varies depending on how well the battery is charged.
That lead sulphate comes in two forms may not seem intuitive, but you've seen this before: think about diamonds and the graphite in pencil leads. They're both exactly the same thing, elemental carbon, but diamonds are hard enough to cut glass where graphite crumbles onto the page when you drag a pencil across it.
The soft fluffy form of lead sulphate is your battery's friend, because it readily converts back into the elemental lead and sulphuric acid your battery depends on to hold a charge. The hard crystal lead sulphate is your battery's mortal enemy, because they it refuses to break up into lead and sulphuric acid when standard charging techniques are used and because it prevents the sulphuric acid and lead from coming into contact with one another so the battery can make electricity for you.
When your battery is fully charged and you pull power from it almost all of the lead sulphate it creates is deposited in the "soft" form, but as you pull the battery charge down, you form more and more of the hard lead sulphate crystals. When the battery discharges below about 60% of it's total capacity, the rate at which the crystal sulphate forms starts to increase rapidly, by the time the battery reaches the 15% mark, it is the dominant form. This is why "Cycling" a lead-acid battery is a good way to kill it.
There are two other times that lead sulphate crystals are more likely to form in your battery. One is when you "boil off" (or, more technically, convert your battery water into oxygen
and hydrogen gas through the process of hydrolysis) and lower the water level to the point where the lead plates become exposed. The other is when your battery just sits, and "self-discharges" while it's not being used.
Using a "float charger" when you store your trailer prevents lead-acid batteries from self-discharging and creating crystal sulphate. It also keeps them fully charged for when you're ready to use them. The down-side is float-charging can also boil off the battery water through hydrolysis and expose the lead plates. (Using a good-quality trickle charger, like the BatteryMinder, Battery Tender, and others slows the process of boiling your battery water off, so you don't have to check your water leves quite as often. Sealed, "maintenance-free" AGM batteries are less prone to boiling, which is one of the reasons they last longer.) So trickle or float chargers can be your friend, as long as you maintain your battery water levels with distilled water. (NEVER tap water, which contaminates the acid solution and can kill the battery.)
I said that standard charging techniques can't convert lead-sulphate crystal back into its more useful "soft" form, but non-traditional charging techniques can. By charging the battery with precisely-timed little pulses of energy, you can get the atoms and molecules to start moving about and break up.
The process is something like giving a child in a swing a series of little pushes. At first the swing only moves a little, but after a while the child is swinging so madly that the chains of the swing go slack at the top of the arc and give a little jerk before the swing heads back down. Using pulses of electrical
charge at the right frequency causes the lead sulphate crystals in a battery to start moving around in much the same way. When they reach the point where they experience that little jerk, they separate and decompose into the non-crystalline soft form, but this approach only works when the battery is fully charged (in "float" mode). When the battery isn't fully charged the energy goes into charging the battery instead of making the crystals vibrate.
The BatteryMinder's claim to fame is it is a float charger that creates these little pulses. Used in place of your trailer's hookup converter or battery charger when your trailer is stored and it can double or triple your battery's lifetime. Better yet, it keeps your trailer battery working like new and prevents it from "running down" early in the evening like most older batteries do.
Lead Sulphate crystals are not the only thing that kills lead-acid batteries. Another major culprit is "rust," or, more accurately, the formation of red lead oxide. Oxidation is a slow and inevitable process, but you can slow the process down by using an intelligent trickle charger that is less prone to boiling your battery water and creating the oxygen
gas that's required to form the lead oxide and less prone to boiling water off so the plates are exposed to the oxygen-rich air. No battery will last forever.
Side note: One of the reasons I like AGM (Absorbed Glass Mat) batteries is they are less prone to form either lead sulphate crystals or lead oxide, so they last longer. Their glass mat and lead plate sandwich structure also prevents vibration (from towing on rough roads) from damaging the plates and keeps the chemicals involved in the power-storage and release process closer together, making them easier to charge. (Meaning you don't loose as much power in the process of charging the battery.) And, finally, they are maintenance-free batteries that you never have to add water to.
My wife and I boondock a lot, and depend on two 50 watt solar panels to keep our battery charged, so a well-functioning battery is an important part of our camping experience. When we store our trailer, however, we keep it under cover and use a BatteryMinder to float-charge the battery. It works like a charm and keeps our expensive AGM battery in good shape. I highly recommend it.