What's an AGM battery, what's an EFB battery - and more importantly, what's the difference?

Here's what they are, how they work and why you might need one.

What is an AGM battery?

In technical terms AGM stands for Absorbed Glass Mat. An AGM battery is different to regular batteries because the electrolyte liquid (battery acid) is absorbed into the glass mats like a sponge instead of the sulphuric acid being free floating as in a conventional battery.

Why would I need an AGM battery?

Many of the technologies and accessories now included in modern cars put extra demands on their batteries. AGM batteries are designed to cope with these higher energy drains and demands. For example, the Stop-Start technology featured in many modern vehicles requires a battery with a higher output and AGM batteries excel in this regard. Where Stop-Start is enabled, the vehicle’s engine cuts out instead of idling when the vehicle is stationary in order to save fuel. AGM batteries ensure that when your car has stopped, it will rapidly and seamlessly start again when you’re ready to move off.

AGM deep cycle batteries are also safe to install inside the cabin of a vehicle to run accessories, as they don’t release harmful gases. Some AGMs are dual purpose so they can also start a vehicle's engine

If your car currently has an AGM battery, then it’s important that you replace it with an AGM battery to ensure your car’s health is maintained.

AGMs have several advantages:

  • AGMs produce higher amp power output for the same size of calcium battery, meeting the increasing demands of car electrics and technologies.
  • AGMs are better suited to the modern car charging system.
  • Dual-purpose AGMs can function as both a starting battery and as an accessory battery, so they can support a multitude of new accessories and car technologies.  
  • Deep-cycle AGMs are safer to install in the passenger compartments of vehicles because they’re sealed and don’t produce harmful fumes or gases.
  • Because an AGM is fully sealed it is also maintenance free. 

What is an EFB battery?

An enhanced flooded battery (EFB) can also be used in cars that feature Stop-Start technology. It is considered an entry level battery for use in cars with this technology. EFBs are wet-filled and similar to standard flooded batteries, however, there are differences that give these batteries enhanced cycling abilities and improved charge acceptance. This means the battery recovers more quickly, as its application needs to handle heavy-duty, cyclic, Start-Stop applications.

Does my car need an EFB battery?

Your vehicle will only require an EFB battery if it currently has one. For example, vehicles with Stop-Start technology may use one of these batteries. If in doubt, check with us before ordering a new battery.

Both AGM and EFB batteries are designed for the frequent cycling required when a vehicle is equipped with Stop-Start technology. Vehicle owners new to the Stop-Start feature may be concerned about the impacts of it on the battery, however, batteries that are required for this technology to function correctly are built to tolerate constant stopping and starting. It is also why replacing your battery with the correct type is essential to maintaining its performance.

What is a calcium battery?

Calcium batteries are lead acid batteries that have small amounts of calcium added to the plates. Battery plates are made from lead alloy, however a pure lead grid structure is not strong enough to hold the battery material vertically, which is why other properties must be added to increase the mechanical strength of the lead. The two most common alloyed metals used are antimony and calcium.

What the added antimony does is increase the electrical resistance of the alloys and subsequently the grids produced from them. These grids require a lot of electrical conduction for optimum performance but when compared to calcium, they are roughly 3-10 per cent less conductive.

In positive grids containing antimony, the process of conductivity means some of the antimony is released from the corrosive product of the grid and transferred to the negative plate. This action during charging promotes the breakdown of water and generation of hydrogen. This gassing phenomenon causes water loss in other batteries, resulting in the need to periodically add water to maintain the battery’s condition. RAC Batteries are fully sealed and maintenance free.

Does my car need a calcium battery?

The benefits of replacing antimony in the plates include improved resistance to corrosion, no excessive gassing and reduced water usage. The use of calcium also cuts down on the loss of charge from the internal chemical reactions required in the lead-antimony self-discharge. This means they are less likely to be affected by extreme weather conditions and higher charge rates. Many modern cars now use them to cope with the higher power demand from engines and accessories.

Calcium batteries are regarded as low maintenance, as they practically do not evaporate the electrolyte and therefore will not need water added.

Despite the upsides, calcium batteries should still be charged with an appropriate battery charger if not used regularly. If you’re going to use it as a deep cycle battery, it’s best to hook up a charger designed for calcium batteries to get the most out of its lifespan.

Need extra battery advice?

Call our motoring advice helpline, available from Monday to Friday 9am to 5pm. 

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Last updated August 2021