Sealed Lead Acid (SLA) – Absorbent Glass Mat (AGM) vs Gel

In the commercial battery world two technologies are mixed up more than any other – Absorbent Glass Matt and Gel. Even some leading battery websites hold misinformation on what they are and what they are best suited for. Both are referred to as Sealed Lead Acid batteries but they have different constructions designed for different uses.

A bit of background

Both AGM and Gel are based on the lead acid concept discovered in 1859. The plates are made from lead and the electrolyte is acidic (see What is a lead acid battery for more detail on the structure of lead acid units).

When lead acid was introduced commercially, it was revolutionary. This was the first battery that could be recharged. Although lead was expensive, its basic design was suited for mass production, which kept costs down at an affordable level for many applications.

Using thick plates meant that it could be used for deep cycle applications (long slow discharging to power lighting, fridges, etc.), while a thinner plate made it ideal for applications that required fast, strong current surges (particularly engine starting).

This basic design invented over 150 years ago is still in use today but it was not without its faults:

• If the battery case was cracked corrosive acid could leak out.
• It could not be installed at an angle because this would reduce the amount of acid in contact with the plates and increase the chance of leaks occuring.
• Lead is a soft metal, so the plates were prone to buckle, which sheds their active material paste or causes plates to touch and short out the entire unit.
• In hot environments, the electrolyte could evaporate, meaning the cells needed to be topped off from time to time.

Maintenance free lead acid

In the mid 1900s a maintenance free version was introduced on a wide scale which sealed the battery and stopped electrolyte evaporating or leaking, but it did not resolve the issues of damage caused by leaks from a cracked case, shorting out by buckling lead plates or poor performance and shortened life when installed at an angle.

Being a ‘sealed’ battery it is sometimes referred to as sealed lead acid (SLA), a source of confusion we will come to in a moment.

Absorbent Glass Mat (AGM)

The AGM concept, discovered in the 1970s, uses glass fibers woven into a mat which is 95% saturated in acid and then placed between the plates. This instantly resolved a number of issues in the older design (which became known as flooded lead acid).

• If the case cracked, nothing would leak out as the acid would remain in the mats.
• The unit could be mounted at any angle, even on its side and the plates would still maintain full contact with the electrolyte.
• The mat also became part of the structure, holding the lead plates in place –  limiting any buckling or, if buckling did occur, stopping the plates from touching each other.

The only downside was that the absorbent glass mat cost more to produce, therefore AGM did not universally replace flooded lead acid. In applications where cost was more important than the ability to mount a unit at an angle, flooded lead acid remained the chemistry of choice. Flooded lead acid is still in use today, particularly as an automotive starter battery and as a low cost deep cycle unit in the leisure industry.

AGM also became known as a type of Sealed Lead Acid (SLA) battery.

Gel

AGM had given greater flexibility to lead acid, but it hadn’t perfected all the flaws. In applications with vibration or jarring the mat could rub against the plates and actually cause damage rather than prevent it.

In the 1980s, the breakthrough came with the commercialization of a technique pioneered in the 1930s – using an electrolyte that looks very much like a silicon gel. The gel physically adds structure internally keeping the plates and their active material in place. As it ‘glues’ onto the plates, they become one and the same piece, so they move together when the unit is vibrated or jolted.

Gel batteries cost more to produce than AGM, so they did not replace the earlier technology. Not everyone needed (or was prepared to pay a premium for) the vibration resistant design.

Gel batteries, like AGM and maintenance free flooded lead acid, are referred to as sealed lead acid.

Which is better – AGM or Gel?

Theoretically, Gel is the superior technology, but the extra cost is only justified if the battery needs to withstand vibration or jarring.

Gel has proved popular in the field of power sports – quad bikes, jet skis, off road biking, ATVs (All Terrain Vehicles), etc. These are areas where heavy vibration and jarring are common place, so the solid nature of gel is an investment compared to AGM or flooded lead which would both need to be replaced more often.

Aside from power sports, AGM is a better choice due to its lower cost, making it far more popular for standard backup applications such as emergency lighting, alarm systems, etc.

Why lead acid technologies are often confused

As we’ve seen, we have three key types of lead acid battery – flooded, AGM and Gel. But we also have them described variously as ‘maintenance free’, valve-regulated’ and ‘sealed lead acid (SLA)’.

Depending on who you talk to, depends on which type of battery can be described which way.

Maintenance Free

Although all lead acid batteries need maintenance, sealed units need far less. A flooded lead acid battery that has been sealed, AGM and Gel are all often referred to as ‘maintenance free’.

Valve-regulated lead-acid

Sealed lead acid batteries are not truly sealed. If the battery were to overheat, say due to excessive charging, gases could build up and cause the unit to explode. As such they have pressure valves which allow gases to vent at a certain point.

It was these vents that lead some people to refer to sealed lead acid more accurately as ‘valve-regulated lead-acid’ instead. In practice they mean one and the same thing.

However, a ‘maintenance free’ flooded lead acid battery also works on the same principle and so some people will refer to these as valve-regulated lead-acid as well.

Sealed Lead Acid (SLA)

Again, closed flooded lead acid batteries are technically sealed lead acid by definition. This said, most people in the industry reserve the term ‘SLA’ for AGM or Gel, but do not assume this is universally true. Always check what the manufacturer or seller actually means by ‘Sealed Lead Acid’ by verifying how the electrolyte is stored:

• As a liquid? This is flooded lead acid and cannot be mounted at angles or used in applications with excessive vibrations.
• In a glass mat? This is absorbent glass mat and cannot be used in applications with excessive vibrations.
• As a silicon gel? This is gel … and can be used anywhere.

Summary

When looking for the right battery, focus on the type of battery – flooded, AGM or Gel – rather than the category – Maintenance Free, valve-regulated lead-acid or sealed lead acid.

The lines between the categories are blurred, so just because a battery is marked as SLA, do not assume it is either AGM or Gel. Any reputable unit should also state whether it is flooded, AGM or Gel.