{{Short description|Type of lead-acid battery}} [[File:12V VRLA Gel Battery.jpg|alt=AGM Gel Battery|thumb|A 12V VRLA battery, with gel technology inside for deep-cycle application]] A '''valve regulated lead‐acid''' ('''VRLA''') '''battery''', commonly known as a '''sealed lead-acid''' ('''SLA''') '''battery''',<ref>{{cite book |first=Thomas K. |last=Eismin |title=Aircraft Electricity and Electronics |edition=Sixth |page=48 |publisher=McGraw Hill Professional |date=2013 |isbn=978-0071799157 }}</ref> 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 [[Electrochemical cell|cell]], and the presence of a relief valve that retains the battery contents independent of the position of the cells.<ref>{{cite book |first1=David B. |last1=Linden |first2=Thomas |last2=Reddy |title=Handbook of Batteries Third Edition |publisher=McGraw-Hill |date=2002 |isbn=0-07-135978-8 |chapter=24 }}</ref>
There are two primary types of VRLA batteries: '''absorbent glass mat''' ('''AGM''') and '''gel cell''' ('''gel battery''').<ref>{{cite web |url=https://www.dnrme.qld.gov.au/business/mining/safety-and-health/alerts-and-bulletins/mines-safety/exploding-lead-acid-batteries |title=Exploding Lead Acid Batteries, Mines Safety Bulletin No. 150 |publisher=Queensland Government |location=Australia |date=2015-10-27 |access-date=2020-02-17 |archive-date=2020-02-17 |archive-url=https://web.archive.org/web/20200217053929/https://www.dnrme.qld.gov.au/business/mining/safety-and-health/alerts-and-bulletins/mines-safety/exploding-lead-acid-batteries |url-status=dead }}</ref> Gel cells add silica dust to the electrolyte, forming a thick putty-like gel; AGM batteries include [[fiberglass mesh]] between the battery plates, which contains the electrolyte and separates the plates. Both types of VRLA batteries offer advantages and disadvantages compared to flooded vented lead-acid (VLA) batteries or each other.<ref>{{cite web |date=2018 |title=Selecting the Proper Lead–Acid Technology |url=https://assets.ctfassets.net/nh2mdhlonj7m/4KJkHyvsw88ntK3kRncKb/874a2f00a9004a5454376f55668369ca/Trojan_AGMvsFloodedvsGel_121718.pdf |url-status=live |archive-url=https://web.archive.org/web/20230929115003/https://assets.ctfassets.net/nh2mdhlonj7m/4KJkHyvsw88ntK3kRncKb/874a2f00a9004a5454376f55668369ca/Trojan_AGMvsFloodedvsGel_121718.pdf |archive-date=2023-09-29 |access-date=2023-09-29 |publisher=Trojan Battery Company, California, USA}}</ref>
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-the-grid|off-grid power]] systems (including [[uninterruptible power supply|uninterruptible]] power systems), motor vehicles (as traction batteries for light [[electric vehicle]]s such as [[golf cart]]s and as [[Automotive battery|starter or auxiliary batteries]] for heavier vehicles) and similar roles, where large amounts of storage are needed at a lower cost than other low-maintenance technologies like [[Lithium-ion battery|lithium ion]].
==History== The first lead-acid '''gel''' battery was invented by Elektrotechnische Fabrik Sonneberg in 1934.<ref>{{cite web|url=http://www.netaworld.org/sites/default/files/public/neta-journals/NWSU06-OakesFeature.pdf|title=A Brief History of Batteries and Stored Energy|website=Netaworld.org|access-date=19 February 2019|archive-date=20 February 2019|archive-url=https://web.archive.org/web/20190220181444/https://www.netaworld.org/sites/default/files/public/neta-journals/NWSU06-OakesFeature.pdf|url-status=dead}}</ref> The modern gel, or VRLA, battery was invented by Otto Jache of [[Exide#Sonnenschein|Sonnenschein]] in 1957.<ref name="Desmond 2016">{{cite book |first=Kevin |last=Desmond |chapter=Jache, Otto |title=Innovators in Battery Technology: Profiles of 95 Influential Electrochemists |publisher=McFarland |date=2016 |isbn=978-1476622781 }}</ref><ref>{{cite web|url=http://www.sonnenschein.org/PDF%20files/GelHandbookPart1.pdf|title=Handbook for Gel-VRLA-Batteries : Part 1 : Basic Principles, Design, Features|website=Sonnenschein.org|access-date=19 February 2019}}</ref>
The first AGM cell was the Cyclon, patented by [[Gates Corporation|Gates Rubber Corporation]] in 1972 and now produced by [[EnerSys]].<ref name="agm">{{cite journal | author = Devitt | year = 1997 | title = An account of the development of the first valve-regulated lead/acid cell | journal = Journal of Power Sources | volume =64 | issue =1–2 | pages =153–156 | bibcode =1997JPS....64..153D | doi = 10.1016/S0378-7753(96)02516-5 |first=John}}</ref>
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-1980s, 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 [[British Aerospace 125|BAE 125]] and [[British Aerospace 146|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 battery|nickel–cadmium (Ni-Cd) batteries]].<ref name="Desmond 2016" />
==Basic principle== {{Main|Lead-acid battery}} [[File:Cutaway view of a 1953 automotive lead-acid battery.jpg|thumb|Cutaway view of a 1953 [[automotive battery]].]]
Lead-acid cells consist of two plates of lead, which serve as [[electrode]]s, suspended in an [[electrolyte]] consisting of diluted [[sulfuric acid]]. VRLA cells have the same chemistry except the electrolyte is immobilized. In AGMs, 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.<ref>{{cite book|last1=Wagner|first1=R.|editor1-last=Moseley|editor1-first=Patrick T|display-editors=etal|title=Valve-Regulated Lead–Acid Batteries|isbn=9780444507464|page=446|chapter=13.3 Gel batteries|date=2004-03-09|publisher=Elsevier Science}}</ref>
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.<ref>Robert Nelson, "The Basic Chemistry of Gas Recombination in Lead–Acid Batteries", JOM 53 (1), (2001).</ref><ref>{{cite web|url=http://www.tms.org/pubs/journals/jom/0101/nelson-0101.html|title=The Basic Chemistry of Gas Recombination in Lead–Acid Batteries|website=TMS.org}}</ref> 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").<ref name="DRB01"> Ronald Dell, David Anthony James Rand, Robert Bailey, Jr., ''Understanding Batteries'', Royal Society of Chemistry, 2001, {{ISBN|0854046054}}, pp. 101, 120–122.</ref>
==Construction== Each cell in a VRLA battery has a pressure relief valve that will activate when the battery starts building pressure of hydrogen gas, generally a result of being recharged.<ref name="DRB01" />
The cell covers typically have gas diffusers built into them, which allow safe dispersal of any excess hydrogen that may be formed during [[Overcharging (battery)|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.<ref>{{Cite book|last1=Vaccaro|first1=F.J.|last2=Rhoades|first2=J.|last3=Le|first3=B.|last4=Malley|first4=R.|title=INTELEC – Twentieth International Telecommunications Energy Conference (Cat. No. 98CH36263) |chapter=VRLA battery capacity cycling: Influences of physical design, materials, and methods to evaluate their effect |date=October 1998|pages=166–172|doi=10.1109/INTLEC.1998.793494|isbn=0-7803-5069-3|s2cid=108814630}}</ref>
==Absorbent glass mat (AGM){{anchor|AGM battery}}== 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. Thin [[fiberglass|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.<ref>{{Cite web |url=http://www.yuasabatteries.com/pdfs/TechManual_2014.pdf |title=Technical Manual: Powersports Batteries |website=YuasaBatteries.com |publisher=[[GS Yuasa]] |access-date=2019-12-25 |archive-url=https://web.archive.org/web/20170712134609/http://www.yuasabatteries.com/pdfs/TechManual_2014.pdf |archive-date=2017-07-12 |url-status=dead }}</ref>
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.<ref>{{cite web|url=http://support.rollsbattery.com/support/solutions/articles/4345-agm-charging|title=AGM Charging : Technical Support Desk|website=Support.rollsbattery.com|access-date=19 February 2019}}</ref> There is a direct correlation between the [[depth of discharge]] (DOD) and the cycle life of the battery,<ref name="agm discharge characteristics">{{cite web|url=http://support.rollsbattery.com/support/solutions/articles/4346-agm-discharge-characteristics|title=AGM Discharge Characteristics : Modified on: Mon, 6 Oct, 2014|website=Support.rollsbattery.com|access-date=19 February 2019}}</ref>{{failed verification|date=May 2026|reason=The relationship depicted in the source appears to be inverse, with cycle life increasing with a lower DOD, and not direct.}} with cycle life between 500 and 1300 cycles, depending on DOD.
==Gel battery== <!-- [[Gel battery]] and [[Gel cell]] redirect here. --> {{More citations needed section|date=December 2019}} [[File:Broken lead gel battery.jpg|thumb|upright=1.2|Broken gel battery with white gobbets of the gelated electrolyte on the plates.]]
Originally a kind of '''gel battery''' was produced in the early 1930s for portable valve (tube) radio LT supply (2, 4, or 6 V) by adding silica to the sulfuric acid.<ref>{{cite web | url = http://www.radiomuseum.org/r/chloride_exide_gel_cel_accumulator_jsk2.html | title = Exide Gel-Cel Accumulator JSK2 Power-S Chloride Electrical | last = Watterson | first = Michael | access-date = 2015-03-01 | date = 2014-06-28| website = RadioMuseum.org }}</ref> By this time, the glass case was being replaced by celluloid, and later, in the 1930s, other plastics. Earlier "wet" cells in glass jars used special valves to allow tilt from vertical to one horizontal direction, in 1927 to 1931 or 1932.<ref>{{cite web | url=http://www.radiomuseum.org/r/mcmichael_super_range_portable_fou.html | title=Super Range Portable four A (without tuning dial) | last1=Walchhofer |first1=Hans Martin |last2=Watterson |first2=Michael | access-date=2021-04-07 | date=2013-11-27 | website=RadioMuseum.org }}</ref> The gel cells were less likely to [[Battery leakage|leak]] when the portable set was handled roughly.
A modern gel battery is a VRLA battery with a [[gel]]ated [[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.
=== Comparison: AGM vs. Gel === While both Absorbent Glass Mat (AGM) and Gel batteries are categorized as Valve-Regulated Lead-Acid (VRLA) batteries and are commonly used in vehicles and backup power systems, they differ in several key performance aspects. The comparison below highlights major differences based on characteristics such as charge speed, vibration resistance, and deep cycle capability.<ref>{{Cite web |title=AGM vs Gel Battery – The Only Comparison You'll Ever Need |url=https://powseabattery.com/agm-vs-gel/ |website=Powsea Battery |date=2025-03-10 |access-date=2025-04-10 }}</ref>
{| class="wikitable" |+ Key Differences Between AGM and Gel Batteries ! Feature !! AGM Battery !! Gel Battery |- | Charging Speed || Fast || Slow |- | Discharge Rate || High || Low |- | Durability in Vibration || Excellent || Good |- | Performance in Extreme Heat || Good || Excellent |- | Deep Cycle Lifespan || Moderate || Long |- | Cost || Lower || Higher |}
==Applications== Many modern motorcycles and [[all-terrain vehicle]]s (ATVs) use AGM batteries to reduce the 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 used on luxury vehicles. As vehicles become heavier and equipped with more electronic devices such as navigation and [[Electronic stability control|stability control]], AGM batteries are being employed to lower vehicle weight and provide better electrical reliability compared with flooded lead-acid batteries.
[[BMW 5-series|5 series]] [[BMW]]s from March 2007 incorporate AGM batteries in conjunction with devices for recovering brake energy using [[regenerative braking]] and computer control to ensure that 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-the-grid|off-grid]] [[solar power]] and [[wind power]] installations as an [[energy storage]] bank and in large-scale [[Robot competition|amateur robotics]], such as the [[For Inspiration and Recognition of Science and Technology|FIRST]] and [[IGVC]] competitions.
AGM batteries are routinely chosen for remote sensors such as [[Measurement of sea ice|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 [[uninterruptible power supplies]] 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. However, charging regimes must be adapted with varying temperatures.<ref>Linden, Reddy (ed), Handbook of batteries, third ed, 2002</ref>
VRLA batteries are used in the US Nuclear Submarine fleet, due to their power density, elimination of gassing, reduced maintenance, and enhanced safety.<ref>{{cite web|url=http://www.businesswire.com/news/home/20050908005625/en/Exide-Earns-First-Ever-Production-Contract-Awarded-U.S. |title=Exide Earns First-Ever Production Contract Awarded by U.S. Navy for Valve-Regulated Submarine Batteries; Shift to Advanced Product Prompts Closure of Kankakee, Illinois, Battery Plant |publisher=[[Business Wire]] |year=2005|access-date=7 September 2016}}</ref>
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 qualities, although they are 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 [https://telecom-info.njdepot.ericsson.net/site-cgi/ido/docs.cgi?ID=SEARCH&DOCUMENT=GR-4228 GR-4228], ''Valve-Regulated Lead-Acid (VRLA) Battery String Certification Levels Based on Requirements for Safety and Performance,'' are recommended for deployment in the Outside Plant at locations such as Controlled Environmental Vaults, Electronic Equipment Enclosures, 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.<ref>[https://telecom-info.njdepot.ericsson.net/site-cgi/ido/docs.cgi?ID=SEARCH&DOCUMENT=GR-3169 GR-3169-CORE], Generic Requirements for Valve-Regulated Lead–Acid (VRLA) Battery Ohmic Measurement Type Equipment (OMTE).</ref> 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== 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 fumes are 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.<ref>{{cite book|first1=Donald G.|last1=Fink|first2=H. Wayne|last2=Beaty|title=Standard Handbook for Electrical Engineers|edition=Eleventh|publisher=McGraw-Hill|location=New York|date=1978|isbn=0-07-020974-X|pages=11–116}}</ref> 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 === An underlying disadvantage with all lead-acid (LA) batteries is the requirement for a relatively long [[Rechargeable battery|recharge]] cycle time arising from an inherent [[IUoU battery charging|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 [[Electric current|current]] for a given Ah battery).
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 substantially 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<ref>{{Cite web|url=https://marinehowto.com/what-is-a-deep-cycle-battery/|title=What is a Deep Cycle Battery?|first=Rod|last=Collins|date=April 7, 2015}}</ref> 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, and with the best care, they may achieve 500 to 1000 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 === 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.<ref>{{cite book | last =Barre | first =Harold | title =Managing 12 Volts: How to Upgrade, Operate and Troubleshoot 12 Volt Electrical Systems | publisher = Summer Breeze Publishing| year = 1997 | pages = 44 | isbn = 978-0-9647386-1-4 }}(stating sealed battery plates are hardened with calcium to reduce water loss which "raises the batteries' internal resistance and prevents rapid charging.")</ref><ref name = "Sterling FAQ">{{cite web|url=http://www.sterling-power.com/support-faq-2.htm|title=FAQ: What is the Best Battery System to Use for an Auxiliary Charging System|last=Sterling|first=Charles|date=2009|access-date=2 February 2012|archive-url=https://web.archive.org/web/20120316225934/http://www.sterling-power.com/support-faq-2.htm|archive-date=16 March 2012|url-status=dead|df=dmy-all}}</ref> 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.<ref name="Sterling FAQ" />
=== Comparison summary === AGM automobile batteries are typically about twice the price of flooded-cell batteries in a given BCI size group; gel batteries are as much as five times the price.
AGM and gel VRLA batteries: * Have a shorter recharge time than flooded lead-acid batteries;<ref name="Calder">{{cite book |last=Calder |first=Nigel | title=Boatowner's Mechanical and Electrical Manual |edition=2nd |year=1996 |page=11 |publisher=International Marine |isbn=978-0-07-009618-9 |url-access=registration |url=https://archive.org/details/boatownersmechan00cald_0/page/11 }}</ref> * Cannot tolerate overcharging (overcharging leads to premature failure);<ref name="Calder" /> * Have a shorter useful life compared to properly maintained [[Wet cell#Wet cell|wet-cell batteries]];<ref name="Calder" /> * Discharge significantly less hydrogen gas;<ref name="Calder" /> * Are by nature safer for the environment and safer to use; * Can be used or positioned in any orientation.
==See also== * [[List of battery types]] * {{annotated link|Sand battery (electrochemical)|Sand battery}} * {{annotated link|Peukert's law}}
==References== {{Reflist}}
==Further reading==
===Books and papers=== * Valve-Regulated Lead–Acid Batteries. Edited by Patrick T. Moseley, Jurgen Garche, C.D. Parker, D.A.J. Rand. [https://books.google.com/books?id=5Rwryml3YMEC&pg=PA202 p. 202] * Vinal, G.W. (1955 Jan 01) Storage batteries. A general treatise on the physics and chemistry of secondary batteries and their engineering applications. Energy Citations Database (ECD): [http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=7308501 Document #7308501] * John McGavack. [https://books.google.com/books?id=0z3PAAAAMAAJ The Absorption of Sulfur Dioxide by the Gel of Silicic Acid]. Eschenbach Printing Company, 1920
===Patents=== * {{US patent|417392}} – Treatment Of Porous Pots For Electric Batteries. [[Erhard Ludwig Mayer]] and [[Henry Liepmann]] * {{US patent|3271199}} – Solid Acid Storage Battery Electrolyte. [[Alexander Koenig]] et al. * {{US patent|4134192}} – Composite battery plate grid * {{US patent|4238557}} – Lead acid battery plate with starch coated glass fibers * {{US patent|4414302}} – Method of making a lead storage battery and lead storage battery made according to this method. Otto Jache and Heinz Schroeder
==External links== {{commons category}} * [https://batteryworld.varta-automotive.com/en-be/why-special-battery-start-stop Why do I need a special battery for the automatic start-stop system?], published by Varta * [https://lifelinebatteries.com/blog/pros-cons-of-agm-batteries/ Pros and cons of AGM batteries], published by Lifeline
{{Galvanic cells}}
[[Category:Lead–acid batteries]] [[Category:Rechargeable batteries]] [[Category:Sulfuric acid]]