While there are many battery chemistries today, and new types becoming commercially viable over time, we deal with the lead acid types, flooded, AGM, and true Gel, as they are widely used in the applications we specialize in. Lead acid battery technology has been used commercially for over a century. Some archeological finds of the appropriate materials in a man made configuration suggest the principle has been known and used much longer than that. Their construction is of lead alloy plates, and an electrolyte of sulphuric acid and water. A battery is made up of a number of cells, and the lead acid chemistry dictates a fully charged voltage of about 2.12 volts per cell. Thus, a nominal 6 volt battery has three cells with a full charge voltage of 6.3 to 6.4 volts, and a 12 volt battery has six cells, and a full charge voltage of 12.7 volts. High quality, high performance lead acid batteries may may exhibit higher cell voltage.
The cell has two plate types, one of lead and one of lead dioxide, both in contact with the sulfuric acid electrolyte as either a liquid, absorbed in a mat, or a gel. The lead dioxide (PbO2) plate reacts with the sulfuric acid (H2SO4) electrolyte resulting in hydrogen ions and oxygen ions (which make water) and lead sulfate (PbSO4) on the plate. The lead plate reacts with the electrolyte (sulfuric acid) and leaves lead sulfate (PbSO4), and a free electron. Discharge of the battery (allowing electrons to leave the battery) results in the build up of lead sulfate on the plates and water dilution of the acid. More on sulfation and its problems later. The specific gravity of the electrolyte as measured with a hydrometer in flooded batteries, indicates its relative charge (strength), or level of dilution (discharge). The reversibility of this reaction gives us the usefulness of a lead acid battery. The sealed versions contain the water, hydrogen, etc. under normal use, for recombination, and eliminate the maintenance of checking water levels, and corrosion around the terminals.
Charging the battery is reversing the process above, and involves subjecting the battery to voltages higher than its existing voltage. The higher the voltage, the faster the charge rate, subject to some limitations. There is a gassing point to consider, and true gel batteries have a lower peak charge voltage, because bubbles can occur in the gel which don't dissipate, and result in battery damage. More on this in the charging tutorial.
The electrolyte may be absorbed into a mat type material so there is no free electrolyte (AGM battery), or may be in a gel format which also stabilizes it (true Gel battery). Current lead-acid batteries are basically distinguished as deep cycle/storage (rated in amp hours), or automotive SLI type (Starting/Lighting/Ignition), rated in cranking amps. There are also combination types, rated for both duties, but these usually have a lower cranking amp rating than a starting battery of the same group size.
SLI batteries are designed to release a high burst of amps for a short time (a starting sequence), and then be relatively quickly recharged from the equipment's charging system (alternator). Typically, a starting sequence discharges less than 3% of the battery capacity. SLI batteries are not designed for repeated deep discharge, and their life is considerably reduced when subjected to this. There are wet (flooded) and totally sealed, maintenance free batteries (AGM - absorbed glass mat) in this class. These generally have a high plate count, and the plates are relatively thin. They are rated in CA, cranking amps (at 32 degrees F), and CCA, cold cranking amps (at 0 degrees F).
Deep cycle batteries are designed with thicker plates, to have a constant discharge rate, and to be deeply discharged and subsequently accept recharging. They are called RV, marine, deep cycle, storage, and sometimes golf cart batteries, as these are the typical markets they apply to, as well as others. There is no benefit to deeply discharging deep cycle batteries as a maintenance procedure, and they have no memory effect. They are typically rated in amp hours (ah), but may have a CA and CCA rating, if they are dual purpose, or occasionally used for starting purposes.
Deep cycle lead acid batteries are available in two configurations - wet and sealed. A wet cell battery has a higher tolerance to overcharging, however, it will release hydrogen gas when charging that must be properly vented, and the water levels must be checked frequently. Sealed lead acid batteries can be of AGM (Absorbed Glass Mat) or Gel construction, and both are sometimes called VRLA (valve regulated lead acid) batteries. Frequently the term "Gel" is used to refer to any truly sealed, maintenance free battery, and this practice causes confusion to battery consumers, as the AGM and true Gel have some different characteristics, particularly in the charging requirements of the true Gel. Both types are maintenance free, have no liquid to spill and gassing is minimal. Other names for the sealed types are starved electrolyte, maintenance-free, dry cell, and spill proof. Most of these are Department of Transportation (DOT) approved for air transport, and classified as non-hazardous.
The Gel is the least affected by temperature extremes, storage at low state of charge and has a lower internal discharge rate, but has peak charge voltage requirements that are measurably lower than a flooded or AGM battery. An AGM battery will handle overcharging slightly better than the Gel Cell. Included in the AGM category are the Optima™ and the Odyssey™, as well as several other high performance sealed batteries. The smaller batteries you find in house alarm systems, computer UPS (uninterruptible power supply) boxes, etc., that say "sealed lead acid", "spill proof", or "maintenance free", are almost always AGM type batteries. If it doesn't say "gel" on it, or have a "G" in the part number, it's not a gel.
We mentioned the Optima™ and the Odyssey™ high performance batteries. There are others such as the Rock Racing™ as well. These batteries use premium materials and construction techniques and achieve excellent results, which the price tends to reflect. The Odyssey units exhibit extremely high burst amps for the first 5 seconds, a critical feature in starting high displacement or high compression engines. They also can be totally discharged and recharged many times (rated at 400 cycles at 80% depth of discharge). For dual purpose, starting and deep cycle, these are hard to beat. We keep an Odyssey PC1500 charged and ready in the shop for emergency jumps or other situations, and testing. Enough said.
Battery capacity is a measure of the energy the battery can store and deliver to a load. It is determined by how much current a battery can deliver over an industry standard period of time. The unit of measure is called "ampere hour" (ah). The battery industry standard is a 20 hour rate, i.e. how many amperes of current the battery can deliver over 20 hours at 80 degrees F until the voltage drops to 10.5 volts for a 12 V battery and 21 volts for a 24 V battery. For example, a 100 ah battery will deliver 5 amps for 20 hours. Occasionally a company or marketer will use a 10 hour rate or some other rate, so be sure which rate you are given when comparing brands and group sizes.
Battery capacity is also expressed as Reserve Capacity (RC) in minutes. Reserve capacity is the time in minutes a battery can deliver 25 amps at 80 degrees F until the voltage drops to 10.5 volts for a 12 V battery and 21 volts for a 24 V battery. A relationship between amp hours (ah) and reserve capacity (RC) can be approximated with this formula: ah = RC times 0.6
As discharge rate is increased above the industry standard 20 hour rate, the usable capacity decreases, due to the "Peukert Effect". The decrease is not linear, and is shown in the chart below.
This must be taken into consideration when sizing a battery for a particular application. If it is a high current draw, battery capacity must be increased over the simple calculated amp hour requirement.
Battery life is shortened the more deeply it is discharged in each cycle. Increasing a battery bank capacity over minimum requirements will increase the life of the bank. True Gel batteries tend to have a higher number of cycles than AGMs when cycled deeply, hence their frequent use in golf carts and wheelchairs/scooters when sealed batteries are used, and deeply discharged daily.
Lead acid batteries lose capacity in low temperatures. At 32 degrees F, a battery will deliver about 75% of its rated capacity at 80 degrees F. This needs to be considered when sizing a battery bank of required capacity for colder environments. A heated or insulated compartment is advisable for very cold climates. High temperature keeps battery chemistry more active, and measurably decreases battery life. A battery that may last 5 years in a 60 F to 80 F environment, may last only 2 years in a desert environment.
Batteries are subject to an internal discharge, also called self-discharge. This rate is determined by the battery type, and the metallurgy of the lead used in its construction. Wet cells, with the cavities inside for electrolyte, use a lead-antimony alloy to increase mechanical strength. The antimony also increases the internal discharge rate to between 8% and 40% per month. For this reason, wet cells should not be left unmaintained or uncharged for long periods. The lead used in Gel and AGM battery construction does not require high mechanical strength since it is stabilized by the gel or mat material. Usually calcium is alloyed with the lead to reduce gassing and the internal discharge rate, which is only 2% to 10% per month for the AGM and Gel batteries.
Any battery discharge, including internal discharge, produces sulphation on the battery plates as part of the chemical cycle, and given enough time, this sulphation hardens, causing diminished battery capacity at best, or total loss of function. Routine charging after use, or use of a "floating" charger for long periods of storage (boat batteries, ATVs, etc.) prevents this diminished capacity and maximizes battery life. A large portion (approaching 50%) of lead acid batteries have diminished capacity or become unusable due to sulphation, and never reach their rated lifespan. There are electronic devices (chargers and stand alone devices) for dealing with sulphation, but the best practice is avoiding the situation in the first place with proper battery management, including use of quality 'smart' chargers.
From the discussion above, it can be seen that there are several issues pertaining to battery life. Recharging in a timely fashion after use, avoiding total discharge if possible, routine maintenance charging or use of a "float" charger on batteries in storage or out of season (jetski, snowmobile, ATV, etc.) are all things which contribute to good battery life. Avoiding extreme temperatures, especially heat, when possible, and checking water levels in flooded batteries are essential as well. There are some applications which are more likely to reach the end of the cycle life of a battery, and have diminishing capacity as a result. Wheelchairs and scooters used daily and heavily fall into this category.
I have added a page dealing with battery prices and the reasons they are rising and probably will continue to do so for a while. This situation contributes to the reasons to seek maximum battery life.
When two or more batteries are connected in series (positive to negative in a string), their voltages add up but their AH capacity remains the same. So, two 12 V, 100 ah batteries connected in series result in a 24 V, 100 ah pack. The negative of one battery connects to the positive of a second battery, and the remaining terminals are the system connections.
When two or more batteries are connected in parallel (positive to positive, negative to negative), their AH capacity (amperage) adds up but their voltage remains the same. So, two 12 V, 100 ah batteries connected in parallel result in a 12 V, 200 ah pack.