Deep cycle batteries are the heart of any mobile power solution. They provide electrical storage for you to use any time you need it. A deep cycle battery consists of primarily lead plates and acid mixed together in a series of cells which create a galvanic effect. This mixture creates an electrochemical reaction which can produce an electric current. This is what we typically call a Lead-Acid Battery. These are the most common batteries used in RVs, and they come in 6 and 12 volt packages that vary in size and reserve capacity.
Other types of deep cycle batteries used in RVs and boats are Absorbed Glass Mat (AGM), and Gel Celled. These types are more expensive than regular lead-acid batteries because they are sealed, which means they cannot spill battery acid when tipped over. AGM and Gel batteries are ideal for situations where the batteries need to be mounted upside down or sideways.
Many RVers prefer the use of AGM batteries over Gel however due to their rugged sustainability in harsh conditions. A Gel Celled battery must be charged at a much slower rate than AGM or Lead-Acid which is not ideal for situations where you need to capture as much solar power as possible before the sun goes down. AGM batteries however are very tough, and can handle extreme temperatures and vibration which is why they are a good choice for mobile applications.
6 Volt golf cart batteries are also a very popular choice among RVers today. They have thicker lead plates compared to a deep cycle 12 volt battery which can sustain deeper power draw than a 12 volt deep cycle battery. The only stipulation is that you require two of them to tie in a series connection. That means that you will need a thick jumper wire tied from one of the positive posts on one battery, to the negative post on the other, leaving and open positive and negative terminal between them. This makes the two batteries work together to produce 12 volts. 6 volt deep cycle batteries are taller and heavier than 12 volt batteries, and they cost a bit extra, but the advantage is their ability to last a long time with a heavy draw. This is the best option if you are planning to use a high powered AC inverter.