Types of batteries Lead-Acid, Flooded(wet), Gelled & AGM's
I am sharing a very well written article that explains different battery types.
Practically all batteries used in alternative energy solar power backup systems are Lead-Acid type batteries. Even after over a century of use, they still offer the best price to power ratio. ALL of the batteries commonly used in deep cycle applications are Lead-Acid. This includes the standard flooded (wet) batteries, gelled, and AGM (Absorbed Glass Mat). They all use the same chemistry, although the actual construction of the plates etc varies.
The lifespan of a deep cycle battery will vary considerably with how it is used, how it is maintained and charged, temperature, and other factors.
These are some typical expectations for batteries if used in deep cycle service.
Type of battery: Expected battery life in "deep-cycle" mode
Starting: 3-12 months
Marine: 1-6 years
Golf cart: 2-7 years
Gelled deep cycle: 2-5 years
AGM deep cycle: 4-7 years
Starting batteries are commonly used to start and run engines. Engine starters need a very large starting current for a very short time. Starting batteries have a large number of thinplates for maximum surface area. The plates are composed of Lead Sponge, similar in appearance to a very find foam sponge. This gives a very large surface area, but if deep cycled, this sponge will quickly be consumed and fall to the bottom of the cells. Automotive batteries will generally fail after 30 - 150 deep cycles if deep cycled, while they may last for thousands of cycles in normal starting used (2-5% discharge).
Deep cycle batteries are designed to be discharged down as much as 80% time after time, and have much thicker plates. The major difference between a true deep cycle battery and others is that the plates are SOLID Lead plates not sponge. This gives less surface area, thus less instant power like starting batteries need. Although these an be cycled down to 20% charge, the best lifespan vs cost method is to keep the average cycle at about 50% discharge.
Golf cart batteries are quite popular for small systems and RV's. The problem is that golf cart¯ refers to a size of battery (commonly called GC-2, or T-105), not the type or construction so the quality and construction of a golf car battery can vary considerably - ranging from the cheap off brand with thin plates up the true deep cycle brands, such as Crown, Deka, Trojan, etc. In general, you get what you pay for.
Marine batteries are usually a "hybrid"¯, and fall between the starting and deep-cycle batteries. In the hybrid, the plates may be composed of Lead sponge, but it is coarser and heavier than that used in starting batteries. It is often hard to tell what you are getting in a "marine" battery, but most are a hybrid.
Gelled deep cycle batteries, or "Gel Cell"¯ contain acid that has been "gelled"¯ by the addition of Silica Gel, turning the acid into a solid mass that looks like gooey Jell-O. The advantage of these batteries is that it is impossible to spill acid even if they are broken. However, there are several disadvantages (must be charged at a slower rate and lower voltage to prevent permanent damage, In hot climates, water loss can be enough over 2-4 years to cause premature battery failure).
AGM,or Absorbed Glass Mat deep cycle batteries have all the advantages (and then some) of gelled, with none of the disadvantages, and they can take much more abuse. Since all the acid is contained in the glass mats, they cannot spill, even if broken. This also means that since they are non-hazardous, the shipping costs are lower. In addition, since there is no liquid to freeze and expand, they are practically immune from freezing damage. Nearly all AGM batteries are combinant what that means is that the Oxygen and Hydrogen recombine INSIDE the battery. AGM's will cost 2 to 3 times as much as flooded batteries of the same capacity. In many installations, where the batteries are set in an area where you don't have to worry about fumes or leakage, a standard or industrial deep cycle is a better economic choice.
Many consider 8D's to be a combination of deep and starting. Course-there are dedicated starting batteries (with many thin plates for concentrated jolt for starting) and there are deep cycle batteries (fewer thicker plates that retain amps but are slower to release). You can use the 8D for deep cycle-but if it is a designed starting battery, will just have a shorter life. Which ever battery you use, should not allow the battery to go below 12v without a load on it.