How do Lead Acid batteries work?
There are so many sites devoted to lead acid batteries, if I wrote one more explanation I'd just be rehashing what others have said well enough. Basically, lead acid batteries all use a chemical reaction to produce electricity. This reaction moves sulfate from the sulfuric acid containing electrolyte to the plates. Fully discharged, the electrolyte is plain old water. The reaction is mostly reversible, so pumping electricity back into the battery moves the sulfate back into the electrolyte, increasing the concentration of sulfuric acid. The most common battery failure mode is getting the sulfate locked up in the plates so the battery will not fully recharge. Sulfation is easy, just leave your batteries discharged for a few days.
Wikipedia has a good article about lead-acid batteries.
Lead-acid batteries are the workhorses of recreational marine electrical systems. These batteries are surrounded with so much myth and misinformation that it is difficult to determine how to get the best performance and life out of them. After six years in business as a marine electrician, and having lived with batteries on my own boat for over two years, I think I understand them well enough to set out some true and helpful principles. I do not believe that having a deep understanding of the electro-chemical processes that underlie batteries is essential to using them most effectively. It is necessary to understand the "best practices" that have been developed to use batteries effectively and employ these principles rigorously. As the discussion progresses, I will describe best practices as well as ones that are not so good. Understanding what does not work well and why is key to understanding the entire subject. Otherwise you simply have a set of rules and no idea of when your practice is straying outside them. No set of rules can cover every conceivable situation. Without an understanding of the principles behind the rules, eventually you will reach a point where the rules do not tell you how to act.
All lead-acid batteries are much more alike than they are different. Whether deep cycle or starting; wet, gel or AGM; the essential characteristics of these batteries are determined by the electro-chemical system they all employ. The plates are comprised of lead, lead oxide and lead sulfate and the electrolyte is a mixture of water and sulfuric acid. Insulators separate the positive and negative plates and an assembly of alternating positive and negative plates are isolated and contained in a cell, the basic indivisible unit of the battery. The chemistry dictates that each cell will have a voltage slightly over two volts when it is in the charged state. Multiple cells (or a battery of cells) are packaged together in the same outer case to make what we recognize as a "battery." To raise the voltage to more practical levels, identical cells in the battery are connected in series. Six cells generate 12.65 volts when fully charged at room temperature. Because the cells are identical, an assumption is made that the cells will all discharge and charge at the same rate and behave like one large cell that happens to have a potential of 12.65 volts. Batteries may be connected in parallel or on series to make larger battery banks which provide more voltage and/or more current. The "identical cell" assumption is simply extended by these operations. Many of the problems with battery performance over time follows from the fact the individual batteries and each of their cells become different as they are used. Some cells become more deeply discharged than others and so forth. Best practices can minimize this issue, but it never disappears.
The chemical reaction in the cell of a battery can be thought of in fairly simple terms. Sulfate, a chemical consisting of sulfur and oxygen atoms, is the key. Sulfate does not exist in the battery by itself. Sulfate is either part of the plates where it forms lead sulfate or it is in water, where it forms sulfuric acid. When the battery is charged, most of the sulfate it contains is in the battery's liquid or "electrolyte." As electrical energy is removed and the battery discharges, sulfate is transferred to the battery plates. This reaction is reversible and sulfate moves back and forth between the electrolyte and the plates. Adding electrical energy drives the sulfate from the plates to the electrolyte. When the battery is completely discharged, all of the sulfate is in the plates and the liquid in the battery is simply water. When this electro-chemical reaction becomes partly irreversible, sulfate is locked up in the plates, the battery looks smaller in electrical terms and the battery is described as "sulfated." A sulfated battery still produces 12.65 volts, but it will not produce an electrical current, amps, for as long a time. So it looks smaller.
We will assume from this point that our batteries are all composed of six cells and have a fully charged voltage of 12.65 volts. The size of a battery is then described in terms of how much current it will produce over how much time, ampere-hours. It may surprise you to learn that the number of ampere-hours in a battery depends on how fast it is discharged. Batteries are commercially described by the number of ampere hours they will deliver over a twenty hour period, or C-20. A 100 ampere-hour battery will deliver five amps into a load over 20 hours. The voltage produced by the battery declines as it is discharged. The endpoint of the C-20 test is considered to be when the loaded battery voltage is reduced to 10.5 volts. If the same 100 ampere hour battery is discharged at 25 amperes, it will reach the test endpoint in something like 2.5 hours. That is only 62 ampere hours. If the same battery is discharged at less than the C-20 rate it will deliver more than 100 ampere hours. So the answer to the question "how long will my batteries last" is not a simple one. Various mathematical models have been constructed to describe the relationship between discharge current and ampere hours, or total available energy. A relatively simple model created by a fellow named Peukert in the 19th century is still quite adequate to describe the lead acid batteries we are using more than a hundred years later. For our purposes here it is not necessary to understand Peukert's Equation. You should just understand that the capacity of a battery or battery bank will vary widely depending on how it is loaded. Take the energy out faster and you get less of it.