Keeping your batteries full and healthy is important. Look after them and they will last a long time. Abuse them and they will start letting you down after a few months.
Keeping batteries healthy means charging them fully, regularly and in a controlled way.
There are many different types of solar and wind charge controllers and generally speaking they all use 3 or 4 different stages to charge a solar or leisure battery. These are :-
Bulk is when most of the energy is pushed into the batteries. Absorption is when the batteries are nearing full and less charge current is used. Float is the stage that keeps them topped up.
Equalization is when the charge controller creates bubbles in the batteries to mix the acid to prevent it from stratifying. Different batteries require different charging currents so always make sure the settings in the charge controller match the battery.
Overcharging and or undercharging a battery will eventually permanently damage it
Solar charge controllers regulate the power output to the batteries by letting some of the power flow back around the panels. Wind turbine charge controllers divert power to a dump load (a big coiled resistor) when the batteries are full. This is achieved using what's known as pulse wave modulation or PWM. The charge controller pulses on and off at a high frequency to regulate the amount of current that the batteries receive.
Used batteries are bad for the planet!
Don't buy cheap batteries and make sure you look after them. A good quality battery will last 5-10 years. It's a false economy to buy low cost batteries because you'll need to buy two of them. Transporting batteries uses a lot of fuel and throwing them away is costly and very bad for the planet!