1. How AGM vs Lead Acid Batteries Work

The AGM battery and the standard lead acid battery are technically the same when it comes to their base chemistry. They both use lead plates and an electrolyte mix of sulfuric acid and water and have a chemical reaction that produces hydrogen and oxygen as a byproduct.

However, this is when they start to diverge.

Here’s how:

A. Flooded Lead Acid Battery

The flooded lead acid battery (FLA battery) uses lead plates submerged in liquid electrolyte. The gases produced during its chemical reaction are vented into the atmosphere, causing some water loss. Because of this, the electrolyte levels need regular replenishment.

B. AGM Battery

The AGM battery uses fiberglass mats sandwiched between lead plates. It’s where the battery gets its name – Absorbent Glass Mat (AGM). The glass mat wicks the electrolyte solution, keeping it suspended in place, so it’s not free-flowing.

Because the AGM is a sealed battery, there’s minimal to no off-gassing.

Gases produced during the chemical reaction recombined with the electrolyte.

And if there is excess gas (such as when the battery is overcharged), a vent releases it to maintain internal pressure.  

2. Maintenance Needs

The AGM battery is maintenance-free and can be placed in more enclosed areas as there’s no off-gassing except for the occasional venting. It’s suited for use in vehicles with batteries in trunks and under seats or in locations where maintenance can be hard to do.

On the other hand, the flooded battery requires regular electrolyte servicing and needs to be in a well-ventilated area as it releases gases and steam.

3. Durability, Vibration And Shock Resistance

The AGM battery tends to be built harder than the flooded lead acid battery, as it originally served military and aviation use.

The sandwiched configuration of glass mat and battery plates in the AGM battery translates to components that don’t fall apart easily. This structure results in a battery that’s shock and vibration resistant — making them favorites in race cars and motorcycles.

Forceful movements and heavy vibrations can damage flooded battery plates, and they need to be mounted securely to minimize these effects.

4. Mounting Flexibility And Spillage

The glass mat technology in the AGM battery makes it spill-proof and position insensitive. You can mount it in many configurations (just don’t turn it upside down).

However, the flooded cell battery has a liquid electrolyte, so it must always be upright to prevent spills. Spilled electrolyte can cause corrosion if not cleaned up.

5. Internal Resistance and Power Output

The AGM battery’s internal resistance is among the lowest of the various lead acid batteries. While a new flooded lead acid battery can have an internal resistance of 10-15%, a new AGM battery can be as low as 2%.

Low internal resistance translates to increased battery voltage output.

It also means a reduced loss of heat as power circulates in the system.

AGM batteries also respond to loading better than flooded lead acid or gel batteries. They handle large power demands so well that they’re the go-to lead acid variety for start-stop vehicles.  

6. Charging Time

Low internal resistance also grants the AGM battery faster charging times. Not as fast as a lithium battery, but up to 5x more than a flooded lead acid battery, when using the same power source.

7. Depth Of Discharge

AGM batteries have an 80% depth of discharge (DoD), which is better than the 50% DoD offered by a flooded cell battery. This makes the AGM battery well-suited to deep cycle applications.

Even so, it’s not recommended to discharge either battery type below 50% of its capacity — unlike the lithium battery, which can be fully discharged.

Note: Depth of Discharge indicates how much battery capacity can be discharged safely without damaging it.

8. Temperature Tolerance

The AGM battery generally performs better in all temperatures and tends towards good Cold Cranking Amp (CCA) ratings.

The electrolyte that’s held in the glass mat doesn’t expand like a liquid while frozen. This makes AGM batteries resistant to cold weather damage. So while the battery likely won’t work in a frozen environment — it won’t crack, at least.

Flooded lead acid batteries, on the other hand, will freeze in the cold. The battery plates can crack, and the cases can expand and leak.

In extreme heat, the flooded lead acid battery will evaporate more electrolyte, risking the battery plates to atmospheric exposure (the lead plates need to stay submerged).

9. Sensitivity To Overcharging

Flooded lead acid batteries are much more tolerant to overcharging than AGM batteries.

The sealed aspect of AGM batteries makes them more prone to thermal runaway, which can be triggered by overcharging. Even if you discount thermal runaway, overcharging will shorten an AGM battery’s lifespan faster.

So, when charging an AGM battery, use a regulated battery charger to control the voltage and current going into the battery.

Note: Thermal runaway is when a battery generates too much heat than it can dissipate. The battery will dry out and melt, release toxic chemicals, and cause fires or explode in extreme cases. Nearby batteries will be affected and may result in a domino effect.

10. Lifespan And Self-Discharge

AGM batteries generally last longer than standard lead acid batteries. Because of their low self-discharge rate, AGM batteries also last longer than their flooded counterparts when not in use.

A well-maintained AGM can last up to 7 years, while flooded batteries typically last around 3-5 years. You’ll know your battery has issues if your car has trouble starting.

11. Corrosion And Sulfation

The flooded battery is more prone to corrosion than the AGM battery because it can vent acidic steam and is likelier to spill and leak electrolyte liquids.

However, both batteries will suffer sulfation if left in a state of discharge for too long. AGM batteries are a little more resistant, though, partially because they have a slower self-discharge rate.

If you see extensive corrosion on your battery terminals, it’s probably time to contact your mechanic for a replacement battery.

12. Cost-Effectiveness

The flooded battery is cost-effective and reliable as a starter battery for standard cars. AGM batteries can be up to 2-3 times more expensive than a conventional battery.

Now that we’ve seen how the AGM battery and flooded lead acid battery compare, let’s go through some FAQs.

8 Battery FAQs

Here are answers to some frequently asked questions on car batteries:

1). Q: What’s An AGM Battery?

      A: The Absorbent Glass Mat (AGM) battery was developed in the early 80s as an alternative to NiCd batteries. It’s a type of sealed lead acid battery (SLA battery) that’s maintenance-free and spill-proof.

It’s used in all types of applications — from marine to off-grid power systems and in advanced vehicles with significant power demands.

2). Q: What’s A Flooded Lead Acid Battery?

      A: The flooded lead acid battery (FLA battery) is the most common lead acid battery type and has been in use over a wide variety of applications for over 150 years.

It’s often referred to as a standard or conventional lead acid battery. You’ll also hear these conventional batteries called a wet cell battery — because of their liquid electrolyte.

If you drive a regular car, chances are, you have a wet cell battery under your hood.

3). Q: What are Sealed Lead Acid Batteries?

      A: The sealed lead acid battery (SLA battery) is a subset of lead acid batteries. It’s also known as a Valve Regulated Lead Acid battery (VRLA battery).

Unlike wet cell batteries, the sealed battery offers no access to its internal compartment. Instead, it uses a one-way valve to regulate pressure resulting from internal chemical reactions.

4). Q: Is A Gel Cell Battery An AGM Battery?

      A: No.

The AGM battery is often confused with the gel battery since both are sealed battery types.

However, where the AGM battery uses a glass mat to hold its electrolyte, the gel battery uses a  silica agent to suspend its electrolyte in gel form.

The gel cell battery is rarely used in cars as they don’t do well as a starter battery. It’s also very sensitive to overcharging, and the gel can be damaged if this happens.

5). Q: Is An EFB Battery An AGM Battery?

      A: No.

EFB is short for Enhanced Flooded Lead Acid Battery.

The EFB battery is a flooded battery designed as an improvement over conventional batteries that’s also cheaper than an AGM battery. It’s used in basic start-stop vehicles.

6). Q: What’s A Deep Cycle Battery?

      A: The starting battery delivers quick bursts of power to start a car engine.

The deep cycle battery is designed to deliver steady power over an extended period of time.

You’ll find the deep cycle battery in backup technology, as alternative energy storage, or in marine vehicles. In fact, it’s also known as a marine battery because that’s one of its most common uses.

All types of lead acid batteries — whether they are AGM, gel cell, or flooded — are used in deep cycle applications. All lithium batteries are “deep cycle,” too.

7). Q: Are There Lithium Car Starter Batteries?

      A: Yes. And no.

The lithium ion battery is typically used to power electric vehicles (EVs). It works differently than the conventional lead acid battery, and can’t be adequately recharged by the alternator fitted with a regular car engine.

However, there are lithium starter batteries — used for their lighter weight and compact size in motorsports. The lithium ion battery can also deliver constant power over any rate of discharge.

Will we see lithium starter batteries for regular cars anytime soon?

Probably not, as they’re pretty expensive.

8). Q: Can I Use AGM Or Lead Acid Batteries As A Battery Bank?

      A: Yes.

Both the AGM and flooded lead acid deep cycle batteries can act as a battery bank and charge up with a solar panel.

A flooded lead acid battery bank will be a cost-effective setup.

However, it’ll require regular maintenance and may take up more space because the batteries will need to sit upright.

An AGM battery bank would be easier to configure because they can lie on their sides.

They’re also maintenance-free. However, they’ll cost more than a flooded battery bank.