Which type of lead-acid battery is best?

Type of lead&#;acid battery

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A 12V VRLA battery, typically used in small uninterruptible power supplies and emergency lamps

A valve regulated lead&#;acid (VRLA) battery, commonly known as a sealed lead&#;acid (SLA) battery,[1] is a type of lead&#;acid battery characterized by a limited amount of electrolyte ("starved" electrolyte) absorbed in a plate separator or formed into a gel; proportioning of the negative and positive plates so that oxygen recombination is facilitated within the cell; and the presence of a relief valve that retains the battery contents independent of the position of the cells.[2]

There are two primary types of VRLA batteries, absorbent glass mat (AGM) and gel cell (gel battery).[3] Gel cells add silica dust to the electrolyte, forming a thick putty-like gel. AGM (absorbent glass mat) batteries feature fiberglass mesh between the battery plates which serves to contain the electrolyte and separate the plates. Both types of VRLA batteries offer advantages and disadvantages compared to flooded vented lead&#;acid (VLA) batteries or each other.[4]

Due to their construction, the gel cell and AGM types of VRLA can be mounted in any orientation, and do not require constant maintenance. The term "maintenance free" is a misnomer as VRLA batteries still require cleaning and regular functional testing. They are widely used in large portable electrical devices, off-grid power systems and similar roles, where large amounts of storage are needed at a lower cost than other low-maintenance technologies like lithium ion.

History

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The first lead&#;acid gel battery was invented by Elektrotechnische Fabrik Sonneberg in .[5] The modern gel or VRLA battery was invented by Otto Jache of Sonnenschein in .[6][7]

The first AGM cell was the Cyclon, patented by Gates Rubber Corporation in and now produced by EnerSys.[8]

The Cyclon was a spiral wound cell with thin lead foil electrodes. A number of manufacturers adopted the technology to implement it in cells with conventional flat plates. In the mid s, two UK companies, Chloride Group and Tungstone Products, simultaneously introduced "ten year life" AGM batteries in capacities up to 400 Ah, stimulated by a British Telecom specification for backup batteries to support new digital exchanges.

In the same period, Gates acquired another UK company, Varley, specializing in aircraft and military batteries. Varley adapted the Cyclon lead foil technology to produce flat plate batteries with exceptional high rate output. These gained approval for a variety of aircraft including the BAE 125 and 146 business jets, the Harrier jump jet and its derivative the AV-8B, and some F16 variants as the first alternatives to then standard nickel&#;cadmium (Ni-Cd) batteries.[6]

Basic principle

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Cutaway view of a automotive battery

Lead&#;acid cells consist of two plates of lead, which serve as electrodes, suspended in an electrolyte consisting of diluted sulfuric acid. VRLA cells have the same chemistry, except the electrolyte is immobilized. In AGM this is accomplished with a fiberglass mat; in gel batteries or "gel cells", the electrolyte is in the form of a paste like gel created by adding silica and other gelling agents to the electrolyte.[9]

When a cell discharges, the lead and diluted acid undergo a chemical reaction that produces lead sulfate and water. When a cell is subsequently charged, the lead sulfate and water are turned back into lead and acid. In all lead&#;acid battery designs, charging current must be adjusted to match the ability of the battery to absorb the energy. If the charging current is too great, electrolysis will occur, decomposing water into hydrogen and oxygen, in addition to the intended conversion of lead sulfate and water into lead dioxide, lead, and sulfuric acid (the reverse of the discharge process). If these gases are allowed to escape, as in a conventional flooded cell, the battery will need to have water (or electrolyte) added from time to time. In contrast, VRLA batteries retain generated gases within the battery as long as the pressure remains within safe levels. Under normal operating conditions the gases can then recombine within the battery itself, sometimes with the help of a catalyst, and no additional electrolyte is needed.[10][11] However, if the pressure exceeds safety limits, safety valves open to allow the excess gases to escape, and in doing so regulate the pressure back to safe levels (hence "valve regulated" in "VRLA").[12]

Construction

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Each cell in a VRLA battery has a pressure relief valve which will activate when the battery starts building pressure of hydrogen gas, generally a result of being recharged.[12]

The cell covers typically have gas diffusers built into them that allow safe dispersal of any excess hydrogen that may be formed during overcharge. They are not permanently sealed, but are designated to be maintenance free. They can be oriented in any manner, unlike normal lead&#;acid batteries, which must be kept upright to avoid acid spills and to keep the plates' orientation vertical. Cells may be operated with the plates horizontal (pancake style), which may improve cycle life.[13]

Absorbent glass mat (AGM)

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AGM batteries differ from flooded lead&#;acid batteries in that the electrolyte is held in the glass mats, as opposed to freely flooding the plates. Very thin glass fibers are woven into a mat to increase the surface area enough to hold a sufficient amount of electrolyte on the cells for their lifetime. The fibers that compose the fine glass mat do not absorb and are not affected by the acidic electrolyte. These mats are wrung out 2&#;5% after being soaked in acids just prior to finish manufacturing.

The plates in an AGM battery may be of any shape. Some are flat, whereas others are bent or rolled. Both deep cycle and starting type of AGM batteries, are built into a rectangular case according to Battery Council International (BCI) battery code specifications.

AGM batteries are more resistant to self discharging than conventional batteries within a wide range of temperatures.[14]

As with lead&#;acid batteries, in order to maximize the life of an AGM battery, it is important to follow the manufacturer's charging specifications. The use of a voltage regulated charger is recommended.[15] There is a direct correlation between the depth of discharge (DOD) and the cycle life of the battery,[16] with differences between 500 and cycles depending on DOD.

Gel battery

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Broken gel battery with white gobbets of the gelated electrolyte on the plates

Originally a kind of gel battery was produced in the early s for portable valve (tube) radio LT supply (2, 4 or 6 V) by adding silica to the sulfuric acid.[17] By this time the glass case was being replaced by celluloid and later in s other plastics. Earlier "wet" cells in glass jars used special valves to allow tilt from vertical to one horizontal direction in to or .[18] The gel cells were less likely to leak when the portable set was handled roughly.

A modern gel battery is a VRLA battery with a gelated electrolyte; the sulfuric acid is mixed with fumed silica, which makes the resulting mass gel like and immobile. Unlike a flooded wet cell lead&#;acid battery, these batteries do not need to be kept upright. Gel batteries reduce the electrolyte evaporation, spillage (and subsequent corrosion problems) common to the wet cell battery, and boast greater resistance to shock and vibration. Chemically they are almost the same as wet (non sealed) batteries except that the antimony in the lead plates is replaced by calcium, and gas recombination can take place.

Applications

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Many modern motorcycles and all-terrain vehicles (ATVs) on the market use AGM batteries to reduce likelihood of acid spilling during cornering, vibration, or after accidents, and for packaging reasons. The lighter, smaller battery can be installed at an odd angle if needed for the design of the motorcycle. Due to the higher manufacturing costs compared with flooded lead&#;acid batteries, AGM batteries are currently used on luxury vehicles. As vehicles become heavier and equipped with more electronic devices such as navigation and stability control, AGM batteries are being employed to lower vehicle weight and provide better electrical reliability compared with flooded lead&#;acid batteries.

5 series BMWs from March incorporate AGM batteries in conjunction with devices for recovering brake energy using regenerative braking and computer control to ensure the alternator charges the battery when the car is decelerating. Vehicles used in auto racing may use AGM batteries due to their vibration resistance. AGM batteries are also commonly used in classic vehicles since they are much less likely to leak electrolyte, which could damage hard to replace body panels.

Deep-cycle AGMs are also commonly used in off-grid solar power and wind power installations as an energy storage bank and in large-scale amateur robotics, such as the FIRST and IGVC competitions.

AGM batteries are routinely chosen for remote sensors such as ice monitoring stations in the Arctic. AGM batteries, due to their lack of free electrolyte, will not crack and leak in these cold environments.

VRLA batteries are used extensively in power wheelchairs and mobility scooters, as the extremely low gas and acid output makes them much safer for indoor use. VRLA batteries are also used in the uninterruptible power supply (UPS) as a backup when the electrical power goes off.

VRLA batteries are also the standard power source in sailplanes, due to their ability to withstand a variety of flight attitudes and a relatively large ambient temperature range with no adverse effects. However, charging regimes must be adapted with varying temperature.[19]

VRLA batteries are used in the US Nuclear Submarine fleet, due to their power density, elimination of gassing, reduced maintenance, and enhanced safety.[20]

AGM and gel-cell batteries are also used for recreational marine purposes, with AGM being more commonly available. AGM deep-cycle marine batteries are offered by a number of suppliers. They typically are favored for their low maintenance and spill-proof quality, although generally considered a less cost effective solution relative to traditional flooded cells.

In telecommunications applications, VRLA batteries that comply with criteria in Telcordia Technologies requirements document GR-, Valve-Regulated Lead&#;Acid (VRLA) Battery String Certification Levels Based on Requirements for Safety and Performance, are recommended for deployment in the Outside Plant (OSP) at locations such as Controlled Environmental Vaults (CEVs), Electronic Equipment Enclosures (EEEs), and huts, and in uncontrolled structures such as cabinets. Relative to VRLA in telecommunications, the use of VRLA Ohmic Measurement Type Equipment (OMTE) and OMTE-like measurement equipment is a fairly new process to evaluate telecommunications battery plants.[21] The proper use of ohmic test equipment allows battery testing without the need to remove batteries from service to perform costly and time-consuming discharge tests.

Comparison with flooded lead&#;acid cells

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VRLA gel and AGM batteries offer several advantages compared with VRLA flooded lead&#;acid and conventional lead&#;acid batteries. The battery can be mounted in any position, since the valves only operate on over-pressure faults. Since the battery system is designed to be recombinant and eliminate the emission of gases on overcharge, room ventilation requirements are reduced, and no acid fume is emitted during normal operation. Flooded cell gas emissions are of little consequence in all but the smallest confined areas, and pose very little threat to a domestic user, so a wet cell battery designed for longevity gives lower costs per kWh. In a gel battery, the volume of free electrolyte that could be released on damage to the case or venting is very small. There is no need (or ability) to check the level of electrolyte or to top up water lost due to electrolysis, thus reducing inspection and maintenance requirements.[22] Wet-cell batteries can be maintained by a self-watering system or by topping up every three months. The requirement to add distilled water is normally caused by overcharging. A well-regulated system should not require top-up more often than every three months.

All lead-acid batteries - charging requirements

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An underlying disadvantage with all lead&#;acid (LA) batteries is the requirement for a relatively long recharge cycle time arising from an inherent three-stage charging process: bulk charge, absorption charge, and (maintenance) float charge stages. All lead&#;acid batteries, irrespective of type, are quick to bulk charge to about 70% of capacity during which the battery will accept a large current input, determined at a voltage setpoint, within a few hours (with a charge source capable of supplying the design C-rate bulk stage current for a given Ah battery).

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However, they then require a longer time spent in the current-tapering off intermediate absorption charge stage after the initial bulk charge, when the LA battery charge acceptance rate gradually reduces, and the battery will not accept a higher C-rate. When the absorption stage voltage setpoint is reached (and charge current has tapered off), the charger switches to a float voltage setpoint at a very low C-rate to maintain the battery's fully charged state indefinitely (the float stage offsets the battery's normal self-discharge over time).

If the charger fails to supply a sufficient absorption stage charge duration and C-rate (it 'plateaus' or times out, a common fault of cheap solar chargers), and a suitable float charge profile, the battery's capacity and longevity will be dramatically reduced.

To ensure maximum life, a lead&#;acid battery should be fully recharged as soon after a discharge cycle as possible to prevent sulfation, and kept at a full charge level by a float source when stored or idle (or stored dry new from the factory, an uncommon practice today).

When working a discharge cycle, a lead-acid battery should be kept at a depth-of-discharge (DOD) of less than 50%, ideally no more than 20-40% DOD; a true[23] LA deep-cycle battery can be taken to a lower DOD (even an occasional 80%), but these greater DOD cycles always impose a longevity price.

Lead&#;acid battery lifetime cycles will vary with the care given, with best care they may achieve 500 to cycles. With less careful use, a lifetime as few as 100 cycles might be expected (all dependent upon the use environment too).

Charging sealed batteries

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Because of calcium added to its plates to reduce water loss, a sealed AGM or gel battery recharges more quickly than a flooded lead&#;acid battery of either VRLA or conventional design.[24][25] Compared to flooded batteries, VRLA batteries are more vulnerable to thermal runaway during abusive charging. The electrolyte cannot be tested by hydrometer to diagnose improper charging that can reduce battery life.[25]

Comparison summary

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AGM automobile batteries are typically about twice the price of flooded-cell batteries in a given BCI size group; gel batteries as much as five times the price.

AGM and gel VRLA batteries:

See also

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References

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Further reading

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Books and papers

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• Valve-Regulated Lead&#;Acid Batteries. Edited by Patrick T. Moseley, Jurgen Garche, C.D. Parker, D.A.J. Rand. p202
• Vinal, G.W. ( Jan 01) Storage batteries. A general treatise on the physics and chemistry of secondary batteries and their engineering applications. Energy Citations Database (ECD) : Document #
• John McGavack. The Absorption of Sulfur Dioxide by the Gel of Silicic Acid. Eschenbach Print. Company, .

Patents

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There are many variations of Lead Acid batteries. Each designed for its own particular application with specific discharge and charge characteristics. These battery types are specifically designed for a set designated end application.
It is important to pick the right lead acid battery for your particular application. Failure to do so can reduce performance and in some instances irreversible damage to the battery, resulting in a drastic reduction of its overall life span.

Generally speaking Lead Acid batteries are broken down into two main categories; Flooded (or wet) Cells and Maintenance Free Sealed Lead Acid Batteries (SLA).

Flooded Lead Acid Batteries

Flooded Lead Acid batteries are the most commonly found lead acid battery type and are widely used in the automotive industry. They provide the most cost effective solution, as the least cost per amp hour, of any lead acid battery type.

The modern wet cell comes in two styles; serviceable and maintenance free. Normal flooded batteries require extra care and regular maintenance in the form of watering, equalising charges and keeping the terminals clean. Flooded cells need to be mounted the right way up and can be susceptible to spillage.

Transporting Flooded Lead Acid Batteries brings with it its own challenges. Classified as a 'dangerous good', flooded lead acid batteries require very specific transportation methods and can only be shipped with accredited dangerous good certified shipping and courier companies.

Sealed Lead Acid Batteries

Commonly known as Valve Regulated Lead Acid (VRLA) or Sealed Lead Acid (SLA). SLA batteries are available in a few different formats. Their principal manufacturing process, including number of plates and plate thickness determines its designated end user application. SLA batteries tend not to sulphate or degrade as easily as wet cells and are regarded the safest lead acid battery to use.

Two main versions of Sealed Lead Acid Batteries (SLA) are commonly found. AGM (Absorbed Glass Matt) and Gel Cell (gelified electrolyte).

AGM Sealed Lead Acid Battery

AGM batteries offer the best price point in the Valve Regulated Sealed lead acid variety. AGM Sealed Lead Acid Batteries utilise an Absorbed Glass Matt (AGM) process which is superior to traditional flooded technology. Fine, highly porous, micro-fiber glass separators absorb the electrolyte, increasing efficiency by lowering internal resistance, which in turn boosts capacity. Lower internal resistance also means that the battery can be recharged much faster than conventional flooded or wet lead acid batteries. AGM batteries provide much larger capacity in a smaller case size and are able to be mounted on their side and shipped using standard shipping processes.
AGM batteries are found in many applications and are commonly used in; UPS, alarm and telecommunications industries, golf carts and trundlers, mobility vehicles, performance automotive and much more. As always, it is important to ensure you are selecting the right AGM battery for your application. Although the voltage, capacity, dimensions and ratings may be very similar across a range, each AGM battery has a specific application that they should be used in.

Gel Sealed Lead Acid Types

A common misconception is that all "sealed" lead acid batteries are GEL. Gel VRLA batteries contain a gelified electrolyte which differs to their AGM counterparts. Sulfuric acid is mixed with silica fume, which makes the resulting mass gel-like and immobile. This creates a completely maintenance free, non spill able lead acid battery product. Unlike a flooded or wet-cell lead acid battery, GEL cell batteries do not need to be kept upright and can be shipped using standard shipping process'.

GEL's inherit design reduces electrolyte evaporation, spillage and subsequent corrosion issues that are very common in flooded or wet-cell batteries. GEL batteries boast greater resistance to extreme temperatures, shock, and vibration. They are capable of withstanding over discharging, which typically causes irreversible damage to Flooded and some AGM batteries. They are ideal in applications where a constant current is required such as golf carts, mobility, power bank and RV power bank applications.
GEL are generally much more expensive than their AGM and Flooded counterparts. They have a very low discharge rate (1% per month), but they require specific charging practices and need to be charged with a GEL specific battery charger.

Deep Cycle Lead Acid Batteries

Deep Cycle Sealed Lead Acid batteries, as the name suggests, are specifically designed for deep cycling applications. They contain fewer plates than their cranking or starting counterparts. These plates are also much thicker. This reduces the total surface area, resulting in a battery that provides lower max current, but is capable of a much deeper state of charge.

Deep cycle batteries are typically discharged to 50% of their capacity and recharged again. This is known as the depth of discharge (DoD). This level of cycling is generally used in applications where the battery is providing constant current for long periods of time. Such as golf carts, mobility scooters, power banks, RV power banks, Solar energy systems etc.

The basic formula that we follow when recommending a deep cycle battery is to suggest one with a residual capacity approximately three times the estimated daily use.  It is recommended to bring Deep Cycle batteries back up to full charge every few months to maintain their levels true capacity. Failure to do so will reduce the batteries life and over time it will provide lower and lower capacity. Deep Cycle Lead Acid Batteries are usually catagorised by their amp hour rating (AHr). Amp Hour is a measure of the batteries capacity.

Deep Cycle batteries are available in both AGM or GEL variety.

Cranking or Engine Starting Lead Acid Batteries

Engine Start batteries have a larger number of thinner plates. The total current output is affected by the total surface area. With thinner plates per battery, the end result is an increased surface area that will provide a much higher current potential.

Generally Cranking batteries differ to their deep cycle counterparts as they have been specifically designed to produce a large burst of current in a short time frame. This is particularly useful in engine starting applications. Cranking batteries are generally catagorised based on their ccA (Cold Cranking Amps) rating. Cold Cranking Amps is a measure of the total current a fully charged battery at -18 degrees celsius can provide for 30 seconds, without dropping below 1.2v per cell (7.2v for a 12v battery). Typically this is 1% of the battery's capacity. The larger the ccA rating, the larger the engine the battery can turn over.

Cranking batteries are not designed to be deep cycled or discharged. They are designed to turn over an engine and sit on float charge which is provided by the vehicles alternator. Discharging a cranking battery will start causing irreversible damage to the battery plates. This will ultimately reduce its performance, total life span and in some cases cause complete failure.

Best of Both Worlds

Known by a few different names, but most commonly as Hybrid. These batteries are designed for both engine starting and deep cycling. This is particularly useful in marine and RV applications where a large cracking current is required to start an engine, as well as cyclic ability to power on board devices and appliances.

These batteries can be found in both GEL and AGM variety.

Standby Lead Acid Batteries

Standby Sealed Lead Acid batteries are the most basic variety of the Sealed Lead Acid range. As the name suggests, they have been designed only for standby applications where they operate on a float (very low) load, maintaining Uninterrupted Power Supplies (UPS), Alarm Systems, Telecommunications and Network Systems.
Standby batteries are generally of AGM variety.

Marine Specific Batteries

A marine battery can be cranking, deep cycle or a combination of both. The critical factor that allows this battery to be used in marine application is its construction process.
Marine conditions place a lot of stress and excessive vibration on a battery. Using a standard deep cycle and/or cranking battery for this application will produce normal results in the short term. However, in the long term, harsh marine conditions, excessive vibration and wear will damage the delicate lead acid battery, ultimately resulting in a battery that has lasted considerably less than its manufacturer rated lifespan. Batteries that are designed for marine will be specifically labeled as Marine Grade. Ask your retailer if the battery you are buying is designed to be used in marine conditions.

Marine batteries can be flooded, Gel or AGM.

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