A battery, can be any device that stores energy for later use. The word battery, is limited to an electrochemical device that converts chemical energy into electricity, by use of a galvanic cell. A galvanic cell is a fairly simple device consisting of two electrodes (an anode and a cathode) and an electrolyte solution. Batteries consist of one or more galvanic cells.

A battery is an electrical storage device. Batteries do not make electricity, they store it. As chemicals in the battery change, electrical energy is stored or released. In rechargeable batteries this process can be repeated many times. Batteries are not 100% efficient - some energy is lost as heat and chemical reactions when charging and discharging. If you use 1000 watts from a battery, it might take 1050 watts or more to fully recharge it. Slower charging and discharging rates are more efficient. A battery rated at 180 amp-hours over 6 hours might be rated at 220 AH at the 20-hour rate, and 260 AH at the 48-hour rate. Typical efficiency in a lead-acid battery is 85-95%, in alkaline and NiCad battery it is about 65%.

Sulfation is the formation or deposit of lead sulfate on the surface like a salt and in the pores of the active material of the batteries' lead plates. If the sulfation becomes excessive and forms large crystals on the plates, the battery will not operate efficiently and may not work at all. Common causes of battery sulfation are standing a long time in a discharged condition, operating at excessive temperatures, and prolonged under charging.

Batteries are divided in two ways, by application (what they are used for) and construction (how they are built). The major applications are automotive, stand by, cycle use. Cycle use includes solar electric (PV), deep cycle as motive power. The major construction types are flooded (wet), gelled, and AGM (Absorbed Glass Mat). AGM batteries are also sometimes called 'starved electrolyte' or 'dry', because the fiberglass mat is around 95% saturated with Sulfuric acid and there is no excess liquid. Flooded may be standard, with removable caps, or the AGM & Gelled is so-called 'maintenance free' (that means they are designed to no need topping during service period /warranty). AGM & gelled are sealed and a few are 'valve regulated', which means that a tiny valve keeps a slight positive pressure. They are sealed valve regulated (commonly referred to as 'VRLA' - Valve Regulated Lead-Acid). Most valve regulated are under some pressure (1 to 4 psi) .

The lifespan of a battery will vary considerably with how it is used, such as the working status, how to discharge and how you recharge it. Normally, the service life may varies from 1year to 10 years. It related to how it is maintained and charged, the temperature, and other factors.

The positive terminal of the first battery is connected to the negative terminal of the second battery, the positive terminal of the second is connected to the negative of the third, etc. The voltage of the assembled battery is the sum of the battery voltages of the individual batteries. So the batteries are connected: + to - to + to - to + to -, etc. The capacity of the battery is unchanged.

The positive terminal of the first battery is connected to the positive terminal of the second battery, the positive terminal of the second is connected to the positive of the third, etc. and The negative terminal of the first battery is connected to the negative terminal of the second battery, the negative terminal of the second is connected to the negative of the third, etc. So the batteries are connected: + to + to + and - to - to -. In this configuration, the capacity is the sum of the capacities of the individual batteries and voltage is unchanged. For example, if you take 5 6V 10AH batteries and connect the batteries in series, you would end up with a battery array that is 30 Volts and 10AH. If you connect the batteries in parallel, you would end up with a battery array that is 6 Volts and 50AH. By the way, this is how ordinary auto batteries are made. 6 2volt cells are put in series to give 12v battery and the 6 cells are just enclosed in one case. Many ni-cad batteries are done the same way.

Starting batteries (sometimes called SLI, for Starting, Lighting, Ignition) 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 thin plates for maximum surface area. The plates are composed of a Lead 'sponge', similar in appearance to a very fine 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 use （2-5% discharge）.

Deep cycle batteries are designed to be discharged deeply as much as 50-100% time after time, It used as mains motive power cycling use such as solar electric (PV), electrical vehicles ,tractions or big energy storage system. So it have much thicker plates(heavy duty design) than a standard automotive battery.

The Depth of Discharge (DOD) is a measure of how deeply a battery is discharged. When a battery is 100% state of charge, then the DOD is 0%. Conversely, when a battery is 100% empty, the DOD is 100%. The deeper batteries are discharged on average, the shorter their so-called cycle life. For example, 12V100Ah(nominal capacity is 100Ah) battery discharged 40Ah,then the DOD=40/100=40% . Deep cycle always designed to be discharged more than 50% DOD.

Marine batteries are considered a 'hybrid' battery which actually fall between the starting and deep-cycle batteries. Marine batteries are usually rated using 'MCA' or Marine cranking amps which is rated 32 degrees F, while CCA is at zero degree F. (For more information on CCA, CA & MCA, please see below)