How a deep cycle battery works
A battery has three basic components: the anode (+), the cathode (-), and the electrolyte.

The chemical reaction that takes place within the battery creates a build up of electrons in the anode; whereas the electrons in the cathode are reduced by the electrochemical reaction. This creates a difference between the anode and the cathode. The electrons try to rearrange themselves to rectify this imbalance. So the electrons try to go towards the cathode but the electrolyte tries to prevent this from happening.

There are different types of batteries such as flooded batteries, gel batteries and AGM batteries (Absorbed Glass Mat). All are made differently. Where the flooded battery is the most common battery, like the standard lead acid battery in your car. The gel batteries, as the name suggests, have a gel-like substance in them and the AGM batteries consist of acid suspended in a glass mat separator.

Deep cycle battery ratings
There are two ways to rate a battery: volts and amps. Amp hours (Ah) are the rated capacity available in chemical energy inside a battery which is converted into electrical energy. It also refers to the amount of energy that the battery can store, or conversely, it can be seen as the discharge rate, which measures the time it takes to discharge a battery before it needs recharging. The capacity of the battery is reduced if the battery is discharged at a shorter period, for instance over 1 hour. The amp hour capacity will be reduced by about 50% and so will the amount of cycles.

Where the battery is discharged at a constant rate of current over a number of hours, this is referred to as the "C" rating. For example, many small batteries are rated at the C20 rate, this means that they will deliver their amp hour capacity if discharged over 20 hours. The types of batteries in large stand alone power systems are rated at the C100 rate which means that they are designed to be discharged over 100 hours or 4 days. This will give you a life span typically of about 15 years.

Discharge cycles
Selecting a battery can be confusing. While all will claim to be particularly well suited to a purpose, all batteries are not created equal, even within their own type such as AGM, Gel or Sealed Lead Acid. As a deep cycle battery can be quite an investment, you'll want one that will last the distance. One of the ways to determine this is the cycle rating; that is how many times it can be discharged and recharged. One of the best benchmarks for this is the IEC 896-2, based on a 100% discharge. While discharging a battery 100% is not recommended as it will significantly decrease the life of any deep cycle battery, the IEC 896-2 provides a good baseline for drawing comparisons between brand X and Y or even different battery lines from the same manufacturer.
• AGM Batteries
• Sealed Lead Acid Batteries
• Flooded Lead Acid Batteries
• Gel Batteries
• Battery Maintenance
• Battery explosions and other hazards

AGM Batteries
Absorbent glass mat (AGM) is a class of lead-acid battery in which the electrolyte is absorbed into a fiberglass mat. The plates in an AGM battery may be flat like wet cell lead-acid battery, or they may be wound in a tight spiral. The internal resistance of AGM batteries is lower than traditional cells, they can handle higher temperatures, and self discharge more slowly

These batteries have a valve which will activate when the battery is recharged at high voltage. Valve activation allows some of the active material to escape thus decreasing the overall capacity of the battery. The lids /vents typically have gas diffusers built into them that allow safe dispersal of any excess hydrogen that may be produced during charging.
They are maintenance free; and they can often be oriented in any manner, unlike normal lead-acid batteries which must be kept upright to avoid acid spills and to ensure the plates are sitting in the electrolyte.
• AGM batteries are totally sealed and are easy and safe to transport
• They NEVER topping up with water
• They can be safely mounted inside a boat, car, caravan, motorhome etc
• AGM batteries only need to be vented to atmosphere, they do not need to be in a sealed box vented to the outside like wet batteries, and can be mounted on their sides or ends if needed.
• Due to their very low internal resistance AGM batteries will fully charge at a lower voltage, and accept a much larger charge current, so when charging from a standard car/truck alternator these batteries will all but fully charge, and they will charge quickly, in about 3 hours!
• AGM batteries can be discharged deeper than conventional deep cycle batteries without major damage. • AGM batteries only self discharge at the rate of up to 3% per month, and even after 12 months sitting idle can be recharged and put back into full service without any major damage. (A standard deep cycle battery if treated the same way will have destroyed itself, it will no longer hold a good charge)
• AGM batteries were originally developed for the military, they are very robust.

Sealed Lead Acid Batteries
Sealed lead acid batteries are also known as valve regulated lead acid (VRLA) batteries, recombinant batteries and often called maintenance-free lead-acid batteries. Examples of VRLA batteries are:
• Absorbed glass mat (or AGM)
• Gel cells

The term term "seal lead acid battery" is misleading: a sealed battery would be a safety hazard due to overpressure risks the battery is overcharged. There is always a safety valve present, hence the name valve-regulated. Even the term "valve regulated" does not really describe this technology. These are really "recombinant" batteries. This means that oxygen evolved at the positive will mostly recombine with the hydrogen ready to evolve on the negative and thus prevent water loss. The valve is only a safety feature in case dangerous amounts of hydrogen are produced.

Flooded Lead Acid Batteries
Flooded lead acid batteries, or wet cells, are the oldest type of rechargeable battery still in use. This type of battery contains a liquid in an unsealed container. This means that the battery must be kept upright and in a well ventilated area to ensure safe dispersal of the hydrogen gas produced by these batteries during overcharging. The lead-acid battery is also very heavy for the amount of electrical energy it can supply.

Flooded lead acid batteries require period monitoring and topping up with distilled water. Despite these disadvantages, flooded batteries have high surge current, easier to troubleshoot and are relatively inexpensive.

Gel Batteries
A gel battery (also known as a "gel cell") is a sealed, valve regulated lead-acid battery and has a gel electrolyte. Unlike flooded lead-acid (wet cell) batteries, these batteries do not need to be kept upright. Gel cells virtually eliminate evaporation of the electrolyte, spillage (and subsequent corrosion issues) common to the flooded lead acid battery, and boast greater resistance to extreme temperatures, shock, and vibration. As a result, they are often used in automobiles, boats, aircraft, and other motorized vehicles.

Battery Maintenance
Deep cycle batteries such as those used in solar power systems have much thicker lead plates to make them last longer. Deep cycle batteries should never really be discharged to below 20% of their full capacity, because internal resistance causes heat and damage when they are recharged. Renewable energy systems usually use a low-charge or low voltage warning light or a low-voltage cut-off switch to prevent the type of damage that will shorten the battery's life.

Battery shelf life can be extended by storing them at a lower temperatures, because the chemical reactions in the batteries are slower. However, in order to reach their maximum voltage, batteries must be returned to room temperature. Therefore, most battery manufacturers do not recommend refrigerating batteries.

Battery safety, explosions and other hazards
A battery explosion is caused by the misuse or malfunction of a battery, such as attempting to recharge a non-rechargeable battery or short circuiting a battery. Explosions are most likely to occur when a short circuit generates very large currents. In addition, deep cycle batteries can release hydrogen when they are overcharged (due to electrolysis of the water in the electrolyte). Normally the gas dissipates quickly. However, this gas can be ignited by a nearby spark (for example, when removing the jumper cables).

Attempting to charge a battery beyond its electrical capacity, ie overcharging, can also lead to a battery explosion, leakage, or irreversible damage to the battery. It may also cause damage to the components in which the overcharged battery is used.

When a battery is recharged at an excessive rate, an explosive gas mixture of hydrogen and oxygen may be produced faster than it can escape from within the walls of the battery, leading to pressure build-up and the possibility of the battery case bursting. In extreme cases, the battery acid may spray violently from the casing of the battery and cause injury.

Battery explosions can also occur in maintenance free lead-acid batteries if the valves fail or are blocked. The pressure rises within the cells until a short-circuit ignites the hydrogen-oxygen mixture. Such explosions can cause severe injury. The problem can be detected in most batteries if the sides appear swollen, or if the battery feels hot to touch.