AGM Batteries

A VRLA battery (valve-regulated lead–acid battery), more commonly known as a sealed battery or maintenance free battery, is a type of lead-acid rechargeable battery. Due to their construction, they do not require ventilation, can be mounted in any orientation, and do not require constant maintenance. The reduced venting is an advantage since they can be used in confined or poorly ventilated spaces. They are widely used in large portable electrical devices, systems and similar roles, where large amounts of storage are needed at a lower cost than other low-maintenance technologies like Li-Ion.

There are two primary types of VRLA batteries, gel cells and AGM. Gel cells add silica dust to the electrolyte, forming a thick putty-like gel. These are sometimes referred to as “silicone batteries”. AGM, short for “Absorbed Glass Mat”, batteries feature fiberglass mesh between the battery plates which serves to contain the electrolyte. Both designs offer advantages and disadvantages compared to conventional batteries, as well as each other.

Basic concept

Lead-acid cells consist of two plates of lead suspended in an electrolyte consisting of an aqueous sulphuric acid solution with a low concentration (typically 29–32%, by weight). A lead-acid battery consists of multiple cells wired in series in a single container. The battery produces power by reducing (or oxidizing) the lead plates and turning them into lead-sulphur-oxide while simultaneously increasing the pH of the electrolyte. When the battery is charged, this reaction is reversed; the plates turn back into pure lead, and the acid is re-created. This cycle is not perfect – hydrogen gas often escapes the electrolyte before it can re-combine into water.

To prevent the flammable gas from building up to dangerous levels, venting is required, both in the battery and anything it is placed within. These vents generally mean that the electrolyte can spill out of the battery if it is tipped over, which presents a hazard during shipping and makes them unsuitable for many portable applications. Furthermore, the constant loss of hydrogen leads to a reduction of water in the electrolyte, which must be replaced by opening the battery and “topping off” the water. Many modern batteries include a small float behind a window to allow visual inspection of the electrolyte level, and pop-off caps for refilling it.

VRLA batteries attempt to avoid all of these problems by immobilizing the electrolyte. In doing so, the hydrogen is trapped near the plates, and is available for re-combining when the battery is re-charged. This dramatically reduces the water loss during repeated charge/discharge cycles, and generally makes them “maintenance free”. It also reduces the amount of hydrogen build-up to the point that no venting is required, and the batteries can be completely sealed, which in turn allows them to be used in any orientation without fear of spilling.

During rapid recharging, the electrolyte may boil and pressurize the case, or gas build-up may be too rapid for recombination. These effects necessitate the “valve regulation”. This is normally in the form of a one-way pop-off valve that only opens in the case of pressure build-up. Even if the valve pops, the immobilizing agent prevents rapid, or any, acid leakage. Most designs are so stable and contain so little electrolyte that they won’t leak acid even if they are cut open.

The main downside to the VRLA design is that the immobilizing agent also immobilizes the chemical reactions creating power. For this reason, VRLAs have lower peak power ratings than conventional designs. This makes them less useful for roles like car starting batteries where usage patterns are brief high-current pulses (during starting) followed by long slow recharging cycles. VRLAs are mostly found in roles where the charge/recharge cycles are slower, such as power storage applications.


VRLA batteries have a pressure relief valve which will activate when the battery starts building pressure of hydrogen gas, generally a result of being recharged at excessive voltage, typically greater than 2.30 volts per cell. Valve activation allows some of the gas or electrolyte to escape, thus decreasing the overall capacity of the battery. Rectangular cells may have valves set to operate as low as 1 or 2 psi; round spiral cells, with metal external containers, can have valves set as high as 40 psi.

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.

VRLA cells may be made of flat plates similar to a conventional flooded lead–acid battery, or may be made in a spiral roll form to make cylindrical cells.

At high overcharge currents, electrolysis of water occurs, expelling hydrogen and oxygen gas through the battery’s valves. Care must be taken to prevent short circuits and rapid charging. Constant-voltage charging is the usual, most efficient and fastest charging method for VRLA batteries, although other methods can be used. VRLA batteries may be continually “float” charged at around 2.35 volts per cell at 25 °C. Some designs can be fast charged (1 hour) at high rates. Sustained charging at 2.7 V per cell will damage the cells. Constant-current overcharging at high rates (rates faster than restoring the rated capacity in three hours) will exceed the capacity of the cell to recombine hydrogen and oxygen.


The first AGM cell was the Cyclon, patented by Gates Rubber Corporation in 1972. The cyclon is a spirally wound cell with thin lead foil electrodes. A number of manufacturers seized on the technology to implement it in cells with conventional flat plates. The first manufacturer to achieve a significant market position was arguably Yuasa of Japan. Their low capacity lightweight batteries achieved rapid penetration in the alarm and emergency lighting markets by about 1980, and also some acceptance for UPS and PABX support.

In the mid-1980s two UK companies, Chloride and Tungstone, simultaneously introduced 10 year life AGM batteries in capacities up to 400 Ah, stimulated by a British Telecom specification for batteries for support of new digital exchanges. In the same period, Gates acquired another agm-batteryUK company, Varley, specialising 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 and its derivative the AV8B, and some F16 variants as the first alternatives to the normal NiCd batteries.

Moves to higher capacity AGM batteries were led by GNB’s Absolyte range extending to 3900 Ah. VRLA/AGM technology is now widespread in both stationary and automotive batteries

AGM (Absorbed glass mat)

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 fiber are woven into a mat to increase surface area enough to hold sufficient electrolyte on the cells for their lifetime. The fibers that compose the fine glass mat do not absorb nor are they affected by the acidic electrolyte. These mats are wrung out 2–5% after being soaked in acids, prior to manufacture completion and sealing.

The plates in an AGM battery may be any shape. Some are flat, others are bent or rolled. AGM batteries, both deep cycle and starting, are built in a rectangular case to B.C.I. battery code specifications.

Gel battery

Originally a kind of Gel Cell was produced in the early 1930s for portable Valve (tube) radio LT supply (2, 4 or 6V) by adding silica to the sulfuric acid. By this time the glass case was being replaced by celluloid and later in 1930s other plastics. Earlier “wet” cells in glass jars used special valves to allow tilt from vertical to one horizontal direction (Suitcase Portables 1927 to 1931 or 1932). The Gel cells were less likely to leak when the portable set was handled roughly.

A modern gel battery (also known as a “gel cell”) is a VRLA battery with a gelified 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 extreme temperatures 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.

The modern gel formulation and large scale production was from Otto Jache’s and Heinz Schroeder’s <US Patent 4,414,302> assigned to the German Co. Accumulatorenfabrik Sonnenschein GmbH. With gel electrolyte the separator was no longer such a critical, hard-to-make component, and cycle life was increased, in some cases dramatically. With gel electrolyte, shedding of active material from the plates was reduced.

More importantly, real gas recombination was used to make batteries that were not “watered” and could be called maintenance-free. The one-way valves were set at 2 psi, and this was high enough for full recombination to take place. At the end of charge when oxygen was evolved from overcharge on the positive plate it traveled through the shrink cracks in the gel directly to the negative plate made from high surface area pure lead and “burned” up as fast as it was made. This oxygen gas and the hydrogen adsorbed on the surface of the sponge lead metal negative plate combined to make water that was retained in the cell.

This sealed, non-spill feature made it possible to make very small VRLA batteries (1 –12 Amp hr. range) that fit into the growing portable electronics market. A large market for inexpensive smaller sealed Pb/Acid batteries was generated quickly. Portable TV, light for news cameras, children’s toy riding cars, emergency lighting, and the growing market for UPS systems for computer back-up, to name a few, were powered with small sealed VRLA batteries.