EFB vs. AGM Battery (Which Is Best For Stop Start)

You’re likely here because we promised a full and frank breakdown of the EFB vs. AGM battery. And that’s exactly what you’ll get.

EFB stands for Enhanced Flooded Battery, and AGM stands for Absorbed Glass Mat. We’ll tell you about how they both work.

These two battery types are by far the best equipped lead-acid batteries for modern vehicles with start-stop technology (or is it stop-start…) and power-hungry electronics systems. We’ll tell you which is best for start-stop vehicles (clue: it depends on your vehicle).

That’s why we’re comparing them – most people will choose between these two battery types if they have a newer car, especially if it’s a bigger model, or has state-of-the-art car technology.

Sounds like we need to know which is best then? Read on and find out.

EFB vs. AGM Battery

Introducing EFB vs. AGM Batteries

Let’s take a look at what both EFB and AGM batteries offer, how they differ from traditional Lead-acid batteries, and how they differ from each other.

Why were EFB and AGM batteries developed?

Simply because traditional lead-acid batteries aren’t up to scratch any more .

Most lead-acid batteries for vehicles are the flooded (wet) type of lead-acid battery.

Reason 1 – Modern car tech left flooded batteries behind

Flooded batteries have served their purpose for many years. They were needed to provide a large burst of energy to start the vehicle, and had the secondary purpose of providing a little electricity for a few electronics, such as lights and the radio.

However, the modern vehicle has much larger electrical needs for on-board electronic components and accessories. The humble, old-fashioned flooded battery is not equipped for such a high and regular drain of charge.

Reason 2 – Standard Flooded batteries weaknesses…

Traditional flooded batteries have other weaknesses, too. They suffer terribly from both acid stratification and sulfation.

Acid stratification means when the electrolyte solution becomes more and more poorly mixed – the sulphuric acid falls to the bottom and the distilled water rises to the top. This is bad for the battery because it needs a proper mixture of acid and water to be able to deliver power. 

Stratification is caused when the battery is undercharged, or has been poorly charged with too low a voltage or too low charging Amps. This shortens the battery lifespan.

Sulfation means when lead sulfate gathers on the lead plates of the battery. Over time, this lead sulfate can form crystals and harden, preventing the battery from delivering as much power as it should be able to. 

If action is not taken to desulfate the battery, the crystals will grow until they become too hard and cannot be removed. Then the battery will no longer deliver power and is effectively dead.

Both weaknesses will worsen the battery performance – lessening the ability to accept charge and also the ability to start the vehicle – and will reduce the lifespan of the battery.

EFB Batteries

EFB (Enhanced Flooded Battery)

EFB Batteries are best for smaller or mid-range cars with start-stop technology

The acronym EFB stands for Enhanced Flooded Battery. They may also be termed AFB (Advanced Flooded Battery) or improved flooded battery, but EFB is the more widely-established name.

They are Flooded type

Enhanced Flooded Batteries are lead-acid batteries. And as you can guess from the name they are a flooded (wet) type of battery. Most cars on the road have flooded (wet) batteries and this has been the case for several decades now.

A flooded battery is so called because it consists of lead plates surrounded by a free-flowing liquid called an electrolyte. The electrolyte is a solution of sulphuric acid (H2SO4) and distilled water.

So a flooded battery means that the electrolyte is in liquid form and flows around the battery plates. In other battery types (such as AGM and gel) the electrolyte is not in liquid form.

The electrolyte is divided into 6 cells by the lead plates and plate separators. Each cell provides around 2V, making the total 12V. When the battery is fully charged, each cell contributes 2.13V, resulting in a fully charged 12.8V.

Why are EFB batteries better than standard Flooded batteries?

EFB batteries represents the next step in technology for flooded batteries. They have longer cycle life and can cope much better with deep discharge than traditional flooded batteries.


Because of the following technology:

      1) a polyester scrim

This is a fleece-like material made of polyester that effectively holds the electrolyte in place.

This causes a significant reduction in acid stratification.


 Because the unique design of the scrim makes it more difficult for the sulpuric acid/distilled water mixture to separate. And, as we mentioned earlier, the better mixed the electrolyte remains, the more power the battery can deliver. 

Not only that, since acid stratification degrades the battery, EFB batteries will also last longer.

      2) special Carbon additives in the lead plates

These additives are added during the plate manufacturing process, which have much higher DCA (Dynamic Charge Acceptance) than normal flooded batteries – basically, that means they can absorb more electrical charge in relation to the capacity of the battery.

Therefore, they can cope better when they’re in a partial charge state.

As a result of the additives, EFB batteries are more resistant to sulfation – so the battery lifespan is longer and it will maintain power for longer.

      3) Internal components are designed to improve the mixing of the electrolyte when the vehicle is moving.

As a result of the internal layout of the EFB battery, the vibration caused by normal vehicle movement is harnessed to mix the sulphuric acid and water more effectively. The inertial movements of the car allow constant circulation of the sulphuric acid.

      4) A special grid structure, with more electrodes and thinner plates can more effectively transfer electrons and with higher voltage.

This allows more charge to be transferred ie. more power can be delivered more reliably.

The result of these technology advances are the 2 key benefits of EFB batteries:

Improved charge acceptance and significantly better cyclic durability when they’re used in a reduced state of charge (as is essential for Start-stop technology).

Cyclic durability can otherwise be termed as the ability of the battery to cope with frequent and high discharge. Frequent and high discharge is exactly what is produced from using the stop-start function, at the same time as powering high-draw electronic accessories.

EFB batteries, therefore, are likely to provide at least double the engine starts compared to a normal flooded battery. And their cranking power is equally good.

EFB batteries cost around 25% more than normal flooded batteries and are heavier, but the increased cost is offset by the massively improved performance and cyclic durability. 

They have far better shallow cycling performance and are more resistant to the sulfation and acid stratification that are the main killers of flooded batteries. So EFB batteries will also last much longer than normal flooded batteries.

Some battery chargers have a special mode to recondition the battery, such as CTEK’s excellent RECOND Mode, but it’s still better to have a battery that is less prone to sulfation anyway. If you’re interested, here’s how best to remove sulfation.

As an estimate, EFB batteries will give you 80,000 starts, compared to around 30,000 starts from normal flooded batteries.

Who should choose Enhanced Flooded Batteries?

EFB batteries are for:

 1) smaller to mid-range cars with start-stop technology

– specifically cars that have basic automatic start-stop systems

   2) Cars that don’t have start-stop function, but have high-demand energy needs (ie. Lots of driving in urban environment, with highly regular stops and starts, acceleration and deceleration)

3) Cars that don’t have stop-start function but have high energy-consuming electronics equipment and devices to power.

If you want to see the top models, check out the Best EFB Battery UK.

AGM batteries

As we mentioned earlier, AGM stands for Absorbed Glass Mat. Like all lead-acid batteries, an AGM battery has lead plates and an electrolyte mixture of sulphuric acid and distilled water.

AGM (Absorbed Glass Mat) Battery

Unlike flooded batteries, the electrolyte is absorbed on a fibre glass mat separator, instead of free-flowing around the battery.

AGM batteries are maintenance-free and leak-proof. They were developed as a result of modern cars needing much more power than ever before, but flooded battery technology being ill-equipped to deal with the increased power demands.

Absorbed Glass Mat (AGM) Batteries compared to Flooded batteries

AGM batteries have significantly better power performance than flooded batteries, and also have significantly longer lifespan. 

They also have excellent cold start capabilities, far superior to flooded batteries. 

Lab results for some AGM batteries indicate they have as much as 5 times better cyclic durability, and up to 20% better starting power than flooded batteries.

AGM have more starting power & fuel efficiency than EFB (or any Flooded batteries)

Since the electrolyte in flooded batteries is free-flowing, in liquid form, a lot of space in the battery is needed for this liquid. AGM batteries, with the electrolyte absorbed on the glass mats, allow more efficient use of the battery’s volume. 

As a result, AGM batteries have far greater starting power than all flooded batteries (including EFB) as they have more reaction surface area, allowing greater energy density within the battery. 

The result is faster engine rotation when the engine is started; and this also results in optimised fuel efficiency.

Recombinant lid technology – makes them maintenance-free

As with flooded batteries, AGM batteries have 6 cells. The design of each cell valve, together with the fibre glass mat separators, mean that they don’t lose water. 

This is termed recombinant lid technology – the oxygen emitted from the positive lead plate and the hydrogen emitted by the negative lead plate combine back into water. And this water recycles back into the sulphuric acid/water mixture. 

So they don’t need the free-flowing electrolyte and don’t need maintenance (some older flooded batteries need to be topped up with distilled water).

No Acid Stratification

AGM batteries, then, don’t have ANY acid stratification. The electrolyte is not in liquid form, so the water and acid have no ability to separate. 

Acid stratification is one of the main reasons that battery degrade and ultimately die, so this is a key reason for AGM batteries much improved cyclic durability and lifespan.

Greater pack pressure resilience- which means longer lifespan and cyclic durability

The absorbed glass mat design allows for much greater pack pressure. All lead plates in a battery have an active paste material on their plates. One of the reasons that all batteries eventually fail is the shedding of this paste. 

AGM batteries’ ability to withstand greater pack pressure means they have significantly less active paste material shedding.

Why does that matter? It means yet another reason for the increased cyclic durability – they will last longer than flooded batteries.

Higher pack pressures resilience in AGM batteries also make them more resistant to the vibration that occurs when the vehicle is moving. Flooded batteries can’t cope as well with vibration – it causes them to degrade faster.

Cost and Weight of EFB vs. AGM batteries

AGM batteries are heavier than traditional flooded batteries, and a little heavier than EFB batteries as well. 

The cost of AGM batteries is at least 50% more than normal flooded batteries (and can be as much double the price). Therefore, they are often around 20-30% higher priced than EFB batteries.

The latest EFB batteries can match their performance (and stop-start functionality) for smaller to mid-range cars; therefore these cars will now more commonly use EFB batteries since they’re cheaper.

AGM batteries have exceptional cycle stability, so they’re much better equipped than any other battery type (including EFB) to cope with the engine being turned off and on again several times in quick succession; always able to restart without any problem. 

This cycle stability also allows AGM batteries to provide sufficient power for on-board electrical components and technology while the vehicle is stopped; and then still reliably start the vehicle. This strength 

EFB batteries are still have much better cyclic durability than standard flooded batteries, however.

If we were so bold as to TLDR the topic of the EFB vs. AGM battery, it’d be AGM>>EFB>>Flooded.

If you have an AGM battery, be aware that they need a battery charger with an AGM mode. The best battery chargers in the UK are by far the NOCO 10 model and CTEK MXS 10 model.

Who should choose AGM batteries?

AGM batteries are for:

1) mid-range to larger cars (eg. SUVs), higher-end cars, with start-stop technology.

2) Cars with regenerative braking technology, and/or passive boosting. These cars MUST use AGM.

3) Cars with high-end, sophisticated on-board electrical equipment and accessories.

It’s important to note that cars with an advanced stop-start capability and AGM battery must be replaced with an AGM battery, not any other kind of battery.

Warning to replace AGM batteries with another AGM

Why must AGM batteries be replaced with AGM batteries?

The type of cars with AGM batteries tend to be high-end, premium vehicles and they have a Battery Management System (BMS) that constantly monitors and communicates with the battery. 

If a different battery type was used, it may cause harm to the start-stop system, problems with the in-car electronics, and may result in the battery dying more quickly.

However, you can upgrade an EFB battery to an AGM battery. That’s because an AGM battery is equipped to cope with the same things as an EFB battery, and in fact, is better equipped (as this whole article demonstrates!).

In our best 096 battery article, we discuss the fact that even batteries with the same dimensions and same Capacity and CCA value cannot be considered equal if they are different battery types. AGM batteries outrank EFB batteries, which in turn, outrank traditional Flooded batteries.

Why would you upgrade from EFB to AGM?

When you decide on AGM vs. EFB batteries consider this:

If your car has sophisticated on-board electronics to power, an AGM battery is significantly better able to deal with this increased power demand than an EFB battery.

Even if an EFB battery has been able to cope with the electrical power supply requirements, an AGM battery is more likely to provide a high power supply (which electrical devices prefer) during the times when the vehicle is stopped. And is still able to cope with the start-stop function more reliably.

And for the following reasons:

  • AGM batteries have more starting power, especially when it’s cold.
  • Absorbed Glass Mat batteries have longer lifespan.
  • AGM batteries may provide more fuel saving – the most effective operation of the stop-start functionality can save fuel in the long-term.
  • The design features of AGM batteries also make them better equipped for high temperatures.

EFB or AGM: Made Your Choice?

If you’ve chosen between EFB and AGM, then you know which type of battery you need. Now, make sure you get the right battery Ah size (What Does Ah Mean On A Car Battery?). And make sure you get the best brand (best brands of car batteries in the UK).

How much do these car batteries cost?

Luckily, we’ve done a huge analysis exploring just that.

Here’s how much a new car battery costs in the UK (2022).

A Summary of the EFB vs. AGM Battery

Hopefully, by now it’s clear to you that both these batteries represent a major step up in quality compared to standard flooded batteries.

Both Enhanced Flooded Batteries and Absorbed Glass Mat batteries have more starting power, more cyclic durability, better ability to cope with high discharge, and better ability to cope with sulfation and acid stratification than normal flooded batteries.

However, this article’s purpose is to compare the EFB vs. AGM battery. And the fact is that in each of the categories mentioned above, AGM batteries are superior.

That said, EFB batteries are absolutely fine, and provide major improvements over standard flooded batteries, if you have a smaller car with start-stop technology, and don’t have regenerative braking, passive boost or other energy-intensive technologies.

AGM vs EFB battery (Best for Start Stop)