Deep Cycle Batteries
Batteries used in boats, recreational vehicles and electric golf carts are called deep cycle batteries. These differ from car batteries because they are made to provide continuous power rather than just a quick burst of starting power.
Deep cycle batteries, despite their name, wear out after many cycles of charging and discharging. They are the low end of your battery options and should be used only if you are on a very limited budget.
Industrial Strength Batteries
Also deep cycle, but much more resistant to deterioration over multiple charging/discharging cycles, industrial strength batteries include these types:
These are similar in function to older kinds of car batteries, but much more robust. They are called flooded because there are caps on the top of the battery that you open and pour in distilled water in order to keep the internal lead and acid components wet.
The downside of using flooded batteries is that they emit a chemical gas discharge when in use. This is a caustic and potentially explosive emission, so these batteries are not safe to use inside an enclosed area such as your home, attic or garage,
If installed in an outside shed or weather-enclosure, you need to also install vents so the gas can be dispersed outside the enclosure.
Gel batteries belong to a category of batteries called valve-regulated lead-acid (VRLA). Gel batteries aren’t really filled with gel at all. These are also lead-acid batteries, but are constructed in a way that you do not need to add water, and no hazardous fumes are vented during usage or storage. This makes gel batteries safe to use inside your home or in other enclosed areas.
These are a much better option than flooded batteries, but there is still one that is much better.
Absorbed Glass Mat (AGM)
Most experts, including me, agree that AGM batteries are the best for storing solar energy. A woven glass mat is placed between the plates inside the battery. This mat absorbs and holds the electrolyte making them both spill- and leak-proof.
They are as safe as Gel batteries, but they hold their charge longer, discharge slower, and survive well over extended charging and discharging cycles.
If you can afford them, AGM batteries are the preferred way to go.
Solar energy scientists are experimenting with other high-tech ways to store solar energy. But for now, batteries are your best option.