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What is a VRLA battery?

-Dr Michael McDonagh

A Valve Regulated Lead Acid (VRLA) battery is simply a lead acid battery in which the electrolyte has been immobilised in order to recombine hydrogen and oxygen. It has a sealed construction with pressure release valves to prevent gases from escaping, It is this which gives it its name. Because the electrolyte is no longer in a fluid state, due to either being mixed with silica powder to form a gel or absorbed into a finely textured glass mat, the gases produced are not free to form bubbles and rise to the surface of the electrolyte. Instead they are trapped in the immobilised matrix and forced to travel to the opposite poles by the pressure gradient produced when on charge. In a free liquid this would be impossible.

In a VRLA battery the oxygen produced at the positive migrates to the negative where it is reduced to reform water.

Overcharge reaction at positive plate: H2O = 2H+ + 2e- + 1/2O2

Recombination at the positive plate: 1/2O2 + Pb + H2SO4 = PbSO4 + H2O

The first VRLA batteries (silica gel) were produced in the 1930s by Elektrotechnische Fabrik Sonneburg, then improved and commercialised by Sonnenscein in the late 1950s, again gel type. The AGM battery appeared in the early 1970s and was the brainchild of Gates Rubber Corporation. This was a spirally wound construction similar to a capacitor. It was in the 1980s that the familiar flat plate construction was introduced by Chloride and Tungstone in the UK. Modern VRLA batteries are generally divided into flat plate AGM and tubular plate GEL batteries, the tubular plate construction having the benefit of longer cycle life and better deep cycle resistance. The AGM benefits from higher discharge rates and faster recharge times.

Because the batteries retain and recombine the oxygen and hydrogen produced when charging, they do not need to be topped up with water over their guaranteed life. Added to this benefit of lack of maintenance is the ability to seal the battery, which prevents the release of combustible hydrogen. Other benefits which derive from the immobilisation of the electrolyte include non-spillage or leaking of corrosive acid which makes handling and transport safer. These attributes are clear advantages to businesses and consumers, making this technology far more user friendly than flooded lead acid batteries. In commercial operation it can mean cost reductions by removing gas extraction equipment and can make better use of available storage space due to its ability to operate on its side, giving more flexible installation options.

With these advantages, it is hardly surprising that the VRLA battery is regarded as the most versatile and user-friendly member of the lead acid family. This is reflected in the off-grid market applications in which this technology has significant market share. There are 25 such applications discussed in this blog.

The first market sectors to examine are those attached to the leisure industry, specifically: marine, camping, golf cart and golf buggies. In all of these applications the battery is required to be deep discharged. Although the requirements are similar, the size of the loads and operating pattern can be very diverse. A typical electric golf caddy would use a 12V 18Ah to 35ah capacity for use mostly in the summer, probably bi-weekly. In contrast, a golf buggy carrying passengers on a large commercial vehicle would normally require a 48V construction with a total capacity of up to 200 Ah. They would be run daily, generally discharging to 80% of their rated capacity. In this type of commercial operation the low maintenance costs of a VRLA battery would be an advantage. The cycle life is also an important factor: the longer it is the better the RTI. In many cases using 2V tubular gel cells with their better cycle life and deep discharge, damage resistance would make a lot of economic sense.

Camping and campervan use relies on batteries to power lighting and small appliances such as a TV or refrigerator. Use is irregular and intermittent but usually deep cycle in nature. The lack of maintenance, or gas production on charge and the spill-proof design and safe handling make VRLA batteries ideal in these situations. The batteries are invariably a monobloc design for 12V or 24V systems with capacities ranging from 85Ah to 200Ah. These batteries are not normally in daily use and in many cases are damaged by long periods of being stored in a discharged or semi-discharged state, particularly in winter or off-season periods.

The heading of marine applications covers uses from an electric barge, using the batteries as the primary source of power, to supporting ancillaries such as a navigation device and facilities such as a refrigerator, TV or cabin lighting. Where the primary source of power is for propulsion, the requirement is for a long, steady output with occasional peak bursts when accelerating or starting. For ancillary use, the power outputs are usually lower and more variable as equipment is switched on and off. In the latter case, batteries are normally recharged from the propulsion motor whilst it is in operation. There is also a relatively new marine requirement, that of trolling motors. These are electric motors and propeller units able to silently propel fishing boats in the water without scaring away nearby fish shoals.

The size, capacity and voltage requirements for marine applications varies widely and needs to be identified from the usage pattern and equipment being powered. Electric barges for example may operate at 110V, powering an electric motor and all of the on-board living facilities. Often this requires 2V tubular gel cells in a series parallel configuration to provide the voltage, high capacity and cycle life required for the electrical system. In the case of supplying ancillary equipment, a monobloc design is most commonly used. These would be 12V monoblocs with capacities ranging from 80Ah to 220Ah. Trolling motors however, usually take a 12V 35 ah battery.

The next category to consider is that of the uninterrupted power source (UPS) Battery. In this, the battery has to supply very short burst of high current to counteract a momentary drop in power or voltage. It is generally used for equipment such as radio transmitters or computers to prevent interruptions in transmissions or loss of programmes and data. In this category we can include: telephone back up power, telecoms towers, small comms, PC terminals, ICT, server rooms, data centres and industrial transformer-based networks. In these cases, the power requirement is satisfied by intermittent, rapid bursts of high current. Although frequent, these are shallow discharges and due to the constant recharge condition, the batteries are never deeply discharged. For non-industrial commercial use, the lack of gas and acid fumes enables batteries to be used in office and hi-tech environments where there are personnel and sensitive electronic equipment. The lower maintenance costs when compared with flooded batteries is another significant attraction.

The operating voltages can vary greatly from a supply for a single home PC to an industrial 3-phase AC supply of 440V. Whilst the majority of installations use 12V monoblocs in series/parallel configurations to meet the application voltage and autonomy requirements, larger industrial installations often go for a 2V tubular gel option. Capacities can vary from 25 Ah for small office or domestic installations up to 250 Ah for an industrial UPS.

For standby power operations there are different requirements to that of UPS. These can be intermittent or regular deep discharges with a full recharge to cover either a contingency such as emergency lighting, operation of portable test equipment used for example in environmental monitoring or portable communication devices like walkie talkies for military use. Security alarms and systems, office or telephone backup power would have similar usage profiles, that is intermittent deep discharge to 80% of capacity. The batteries used could be either monoblocs or 2V gel cells connected in series to make up the operating voltages of up to 48V. The capacity of the batteries is dependant on the size of the installations, typically ranging from 2v 20Ah for a domestic security alarm to 2v 1000Aah for a large industrial back-up power.

Applications where equipment is regularly cycled, would include uses such as temporary traffic signalling, mobile lighting, domestic and industrial areas with scheduled power outages, diesel hybrid or solar power installations. All of these will have defined times of operation and will normally be a daily occurrence. There is no ideal battery size or configuration as there are big variations of power draw and operating voltages. For larger installations often 2V tubular gel batteries are best solution due to their ability to withstand deep discharges and their superior cycle life. For smaller or less onerous operations, monobloc batteries are generally preferred, particularly where there are space restrictions. The size of a battery installation would depend on the operating voltage, the load and the run time of the equipment.

Not all market sectors fall neatly into one category. Many cell phone telecoms towers and other radio communication systems use battery backup either as a standby or a regular power source depending on the quality and availability of a local supply (if any). In either case there is most likely a deep cycle requirement for the battery. The most common battery type is the 12V monobloc because of its high power density, compact dimensions and ease of fitting. In some large high-power tower units, 2V VRLA cells mounted on their sides are the best solution. The lack of maintenance and gas production, makes the VRLA battery ideal for remote locations, particularly where batteries are installed in confined spaces.

The other major category which spans several markets is that of the traction battery, which is the primary source of motive power for a moving device. This applies to may categories of vehicle, from fork lift trucks to electric bicycles covering industrial and private use. In all cases it requires deeply discharging the battery. For fork-lift trucks the lack of maintenance costs is a major benefit. The downside however, is that the batteries take longer to recharge, are more expensive and cannot be as deeply discharged as their flooded counterparts. However, this has to be balanced against the costs of flooded cells, which includes fume extraction equipment. There is another significant benefit due the due to the lack of acid fumes and gas released on charging. In an enclosed food warehouse, even with air extraction equipment, some acid fumes from charging will remain within the flooded battery which escape during the operation of the fork lift truck and may contaminate stored food. The ideal battery for the forklift market which includes electric pallet trucks, is the 2V tubular cell, normally in 12V to 80V series configurations. The capacity can vary between 25 Ah for a pallet truck to 2v 1000Ah for a large high tonnage fork lift truck.

Non-industrial use covers markets such as mobility scooters and wheelchairs for medical conditions. There is a small sector which uses lead acid batteries for e-bikes, rickshaws and small EV applications which are regarded as non-leisure use. All of these applications require batteries of compact design with fairly high energy density. This is mostly provided by the monobloc range of batteries.

An uncommon use for VRLA batteries is that of engine starting. However, many modern vehicles are using the AGM versions for just that purpose. Because of its immobilised electrolyte it is less prone to failure from electrolyte stratification and sulphation than flooded designs. It is particularly suitable for stop-start vehicles where a flooded battery’s life can be as little as 6 months. Because it has highly compressed glass mat holding the active material of the plates in place, it is very suitable for off road and all terrain vehicles where shock and vibration can easily damage flooded battery plates. Because it costs more than a flooded battery, it is not a standard component in most mass-produced automobiles. However, it is a natural choice for the luxury vehicle market where its high cranking power, longer life and superior cold start capability are considered worth the small additional expense.

Submarines are entirely dependant upon battery power when submerged as there is no oxygen to supply a combustion engine. Sizes of submarine vary from small leisure and research craft to military underwater boats around 70 metres long and weighing more than 2,000 tonnes. Military submarines have traditionally used very large flooded tubular designs of around 2,000 Wh per cell with up to 175 cells connected in series. For obvious reasons, having batteries which do not produce explosive gas on board a sealed craft is highly desirable. In military use, batteries are normally charged from the diesel engines when the craft is on the surface and properly vented. However, if the job of battery maintenance in very cramped and difficult conditions is removed it is a major advantage. Because it is a deep discharge application and longer cycle life means less downtime replacing batteries, 2V tubular gel cells are an obvious and almost mandatory choice for this application. In contrast, small research and leisure craft may use monobloc batteries to save space.

Due to the diverse range of designs operating voltages and capacities can vary enormously.

Whatever the application or operating pattern or equipment loads that require VRLA batteries, there is one constant and reliable source of help and expertise: Microtex Energy Private Limited.

When 50 years of manufacturing and design experience is combined with in-house and internationally recognised experts the result is an unprecedented and unique customer service. The service goes far beyond advice and product availability: it provides continuous and readily accessible back up support for as long as is required by the customer, not the product guarantee. You can rely on Microtex to always put the customer first, and unlike other battery companies, keep them there.

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