AGM vs Lead Acid Batteries: 12 Differences Plus Some FAQs
Your starter battery is probably one of two rechargeable battery types — it’s either a flooded lead acid or an AGM battery.
But how do these two batteries differ?
In this article, we’ll compare the AGM vs lead acid battery and see how they stack against each other. We’ll then expand into some FAQs for additional details on these car batteries.
AGM vs Lead Acid Batteries: 12 Key Differences
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 more 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 nic 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.
