If you don't take care of your wet cell deep cycle batteries, they will degrade and be useless when needed. The two things that kill more batteries than anything else is overcharging or leaving them in a discharged state. Overcharging gel cell batteries is really bad for them. The following that I am talking about applies to wet cell batteries.
If you do not have a need for the deep cycle batteries at this time and wish to have some to put away, buy them without the battery acid in them and have the acid to pour into them when they are ready to be used. This way they will be fresh when needed.
To keep your batteries in top shape, they must be kept properly charged. The thing that kills batteries is Sulphation that builds up on the battery plates. These are crystals that build up on the battery plates that stop the chemical action that produces the electricity. This happens to batteries that are properly maintained over time, but builds up far faster when batteries are overcharged or left in a discharged state. There are chargers that remove the Sulphation from the battery plates as they charge.
You must keep the acid level up to the fill line at all times. Never add more acid if your batteries are low on fluid, only add distilled water. The acid stays in the batteries, the water is what goes away.
As to the battery chargers. I have two different models of the same brand. Both are made by BatteryMINDer. One is a 1.3 amp trickle charger and will charge up to 6 batteries of like style. The other charges at up to 6 amps for faster charging. The lower priced one is $50, the other $100.
Both of these chargers have a Desulphation feature that rapidly removes Sulphation from the battery plates. When used in regular charging mode, they charge and remove Sulphation at the same time. When the batteries are fully charged, the chargers drop into a maintenance mode that keeps them fully charged without overcharging. You want to keep the chargers hooked up to the batteries at all times. Your batteries will last at a minimum of twice as long and up to six times longer than batteries that are poorly maintained.
There are a couple of other brands of chargers that remove the Sulphation on the market, but I have not used them.
If you have a weak battery, as long as it is not shorted out, the Desulphation process can make them usable again in many cases. It depends on how far gone they are.
The other thing that kills batteries is running them down below 10.5 volts or 50% before recharging them. The more your batteries are run down at each use before recharging, the fewer recharging cycles you will get from them.
In trying to save a weak battery, first charge the battery. If the battery is fully charged and does not have more than 10.5 volts, it is shorted and is junk. If the voltage is at least 11 volts, try Desulphation to see if the battery can be brought back to a usable state. It will never perform as good as new battery, but may be usable. Approx. 75% of batteries are replaced because they will no longer hold a charge is because of Sulphation. The sooner a bad battery is Desulphated, the better chance you have of saving it. Be sure to wear protective glasses and clothing when working on batteries as the acid can blind you and cause burns on your skin. The gasses given off from batteries are also explosive. Do not smoke, have an open flame or sparks anywhere near your battery when the caps are off. Work on them outside if possible.
Use and charge your batteries correctly and they will last a long time.