SMF 12v 33AH

VRLA Batteries – Explained

VRLA batteries – explained

-Dr Mike

One of the major drawbacks with charging a flooded lead acid battery is the breakdown and loss of water through hydrogen and oxygen release. This has more than one negative implication for the safe use and operation of all types of lead acid batteries. The release of potentially explosive mixtures of hydrogen and oxygen in confined spaces and the constant topping up of batteries or cells with distilled water are costly problems for the battery user. In order to remove these problems, the Valve Regulated Lead Acid (VRLA) battery has been developed. In this blog we examine the operating principle and the most common applications for this technology.

VRLA Battery Gas Recombination Explained
VRLA Battery “Gas Recombination” Explained


An SMF Valve Regulated Lead Acid (VRLA) battery differs from a vented lead acid battery in that it is sealed using a pressure relief valve which prevents the gases produced on charging from escaping. Keeping the gases in facilitates their recombination to re-form water which is lost on charging due to electrolysis. In order to ensure that the oxygen and hydrogen are located at the plate surface to enable the recombination reactions, the electrolyte is immobilised using either special glass mat separators or a gel forming silica added to the electrolyte.

There are some applications where these non-gassing and maintenance free properties are particularly important: remote unmanned installations, enclosed areas with proximity to sensitive equipment, personnel and food, or where maintenance costs are expensive. VRLA batteries are most common in the telecom, solar and power industries and may be used for standby, back-up or reserve power.

The telecom industry is a prime example of a remote application. The majority of towers built globally are located in areas difficult to reach or very far from habitation. The ability to function without maintenance is a key requirement which is perfectly fulfilled by 2v VRLA batteries. Often used to power transmitters for several hours daily and be recharged within hours they are a cost-effective option to standby diesel generators.

Solar power is the most popular renewable energy source. To make full use of an expensive installation, it is best to have a lead acid battery storage system to provide energy at night. Because batteries are often stored indoors it is critically important to ensure there are no potentially explosive gas emissions. VRLA batteries are a safe, reliable and low-cost solution in this application

Power supply industries need to have guaranteed power in case of generator breakdowns, sudden demand surges or when the required output exceeds supply capability. Batteries, which can provide high energy outputs in milliseconds, are the ideal way of dealing with peak demands or frequency drops. The maintenance free VRLA battery, in these cases, can be used to supply standby power, UPS or frequency control outputs, to supplement any power supply from any source.

The term maintenance free means that water additions are not required over a VRLA battery’s guaranteed lifetime. Most VRLA batteries will still need some maintenance such as cell balancing, connection tightening or even cleaning in some environments. As mentioned at the beginning, in VRLA battery designs, the reason the gases produced on charging are able to be recombined at the plates is that they cannot form bubbles and float up to the surface of the electrolyte (Fig.1). The purpose for immobilising the acid in this way is to prevent gas produced on charging to escape from the electrolyte. Instead, gas diffuses from the plates through the acid and collects in small fissures or acid gaps in the immobilised matrix. These gaps are deliberately created by ensuring that the medium is not saturated, i.e. starved of acid. The gaps, now full of gas, create a concentration gradient which drives diffusion of oxygen and hydrogen to the electrodes where they can combine to re-form water.

Sealed recombinant lead acid batteries have the acronyms: SLA and VRLA, meaning “Sealed Lead Acid” battery and “Valve Regulated Lead Acid” battery respectively. AGM is also often used to describe a recombinant lead acid battery and is also an acronym, referring specifically to the method of immobilising the acid ie. Absorbed Glass Mat. Whilst all AGM lead acid batteries are SLA or VRLA not all VRLA are AGM. There are two ways to immobilise the sulphuric acid in lead acid batteries: 

  • use of fine fibreglass mat to absorb the liquid acid (AGM) and two;
  • addition of fine silica particles which react with the electrolyte turning it into a gel (GEL). These two constructions AGM and GEL form all of the VRLA battery designs typically found in the vast majority of global markets.

Microtex manufacture a wide range of VRLA batteries of both AGM and GEL types for diverse applications. Why have two types of VRLA? The answer is the battery construction, a tubular plate for heavy duty deep cycle applications cannot use a glass mat separator so relies on a GEL electrolyte. The TGel range by Microtex offers the highest standard possible of tubular plate cells and batteries for a wide range of applications. The AGM is best suited for flat plate constructions in used mostly in float charge types of applications where grid growth is a life limiting factor. The Microtex AGM VRLA range covers most applications such as Telecom and UPS installations. Microtex have an unequalled advantage with their TGel and Safe energy designs as many of the constructional materials are manufactured in-house to provide batteries uniquely manufactured for their purpose. Added to this are decades of in global, in-service history with hundreds of satisfied loyal customers.

Sealed lead acid batteries provide safe, clean and maintenance free energy storage and delivery in most modern applications. They are a relatively cheap way of enabling users to tap into renewable energy resources in remote applications and are still the most cost-effective option for unmanned telecoms and UPS stations. In all the diverse applications with their individual requirements, be it deep cycle or float charge, Microtex offers bespoke designs of VRLA batteries to suit all of these applications. With correct sizing and charging controls, Microtex VRLA batteries in either GEL or AGM constructions can deliver extended lifetimes to provide the best combination of TCO and recyclability on the planet.


Dr Michael McDonagh CTO – Microtex

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