What’s an AGM battery – and is it the same as VRLA?

Strident AGM battery GP12-42

All these different terms are so confusing…

An AGM battery is a sealed, valve-regulated lead acid battery. Sometimes you might also see it described as VRLA (valve regulated lead acid) or SVR (sealed valve-regulated).

Aren’t all acid batteries the same?

No.  A standard wet battery, such as a car battery, contains liquid electrolyte.  If the battery is tipped or punctured this can spill and cause corrosion.  Wet batteries can only be installed upright and there are limitations on how they can be transported.  For example they can’t be carried on aircraft without special containers. You can’t send them through the post and some parcel carriers won’t take them. In addition they can’t be used near sensitive electronic equipment or in a closed environment as they may give off harmful fumes. If continually over-discharged, or left in a discharged condition for any length of time, wet batteries lose capacity and become permanently damaged. A wet battery will also need ‘topping up’ from time to time.

So, what’s different about an AGM battery?

Firstly, an AGM battery is constructed in a different way to a wet battery, which makes it much more suitable for mobility use.  The electrolyte is absorbed in mats of fibreglass which separate the plates in the battery, and the battery is sealed using special pressure valves.  This makes the battery far more tolerant of vibration. Because the acid is absorbed there’s no risk of spillage, so the battery can be operated in virtually any position.  AGM batteries can also be safely transported by air. (Note – some airlines may have restrictions)

In addition, the chemical reaction inside the battery works slightly differently.  An AGM battery uses a ‘recombinant’ reaction.  The oxygen and hydrogen produced in the battery are recombined to form water, which is retained in the battery. So it never needs ‘topping up’, unlike a wet battery.  This is why a sealed  battery should never be opened, as additional oxygen from the air would disrupt the reaction – and the warranty would be void.

An AGM battery contains less electrolyte than a traditional battery. This is called ‘acid starved’ as opposed to ‘flooded’.  The advantage of this is that it protects the plates during deep discharges, when most of the battery capacity is used – which is what happens during typical mobility use.

Is an AGM battery the same as a gel battery, then?

Not quite.  Both are recombinant batteries, and both are sealed valve regulated (SVR) – a.k.a. valve regulated lead acid (VRLA).  In a gel battery the electrolyte has the consistency of Vaseline.  An AGM battery uses glass fibre mats like sponges to trap the electrolyte.  So both gel and AGM batteries are non-spillable and have the same advantages over traditional batteries. Because AGM and gel batteries are both acid-starved they’re particularly suitable for deep discharge applications.

How do I choose between AGM and gel?

Both have their advantages, and in fact if you Google ‘which is better for mobility gel or AGM?’ you’ll find opinion is split! Bear in mind that not all batteries are created equal.  Batteries made with cheaper quality materials won’t perform as well, whether they’re AGM or gel. Strident AGM batteries are manufactured to a superior specification and carry a full 15 months’ no-quibble warranty.

Gel batteries are more acid-starved than AGM, so are better suited for super-deep discharge applications, for example bariatric powerchairs.  However, in colder environments the power in a gel battery will decline faster.  AGM batteries are better where you need high current and high power, particularly in very cold environments.  Conversely, gel batteries do better in higher operating temperatures.

AGM batteries have a higher power capacity than gel batteries of the same physical size.   Gel batteries generally cost more than AGM. Over against this, when cared for properly they can last longer. A lot depends on how the batteries are used and charged, and gel batteries are more prone to premature failure if they aren’t charged optimally.  For the average user top-quality AGM batteries, such as Strident, are a good all-rounder.

Gel batteries can be replaced by AGM, but if you’re currently using AGM batteries they can’t be replaced by gel.

Does it matter which charger I use?

Certainly!  Both types of battery need to be charged with the correct charger, and this is particularly critical for gel batteries.  Undercharging or overcharging will reduce a battery’s capacity and shorten its life.  Overcharging is particularly harmful as it dries out the electrolyte.  The quickest way to ruin your batteries is to use a car or motorcycle battery charger!

We always recommend battery chargers that use Intelligent Charge Technology to get the best performance and longest life from your batteries. This 3 stage charging process ensures your batteries are charged as quickly as possible then safely maintained at the correct level of charge until you need to use them.  Check out our range here.





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