​Why Do We Need Special Batteries for the Automatic Start-stop System?
June 22 2022

Every starting process is associated with a large amount of energy from a car battery. The battery must be in a very good condition to reliably supply a high starting current. Especially with modern vehicles with automatic start-stop systems, the engine is switched off several times during the journey. During these phases the battery continues to supply all of the electrical consumers with energy. A conventional starter battery (SLI) is not designed for this charge output (i.e. continuous discharging and charging).

Only a battery with start-stop technology can deal with these demands and withstand the strain.


The generator charges the starter battery while driving

In a car which is not equipped with an automatic start-stop system, the starting process is usually a one-off event. While driving, the generator supplies the electrical consumers such as the entertainment system and navigation device, so that if the generator is working, there is no discharge of the starter battery during the journey.




The start-stop battery has a double load

The case is different for cars with start-stop technology. During a journey through the town, the battery is subjected to a series of start and stop periods. If the car is stationary at traffic lights with the engine stopped, electrical consumers such as lights, windshield wipers, radio or displays still need power. The load is increased by continuous discharge and charging.

In particular, short journeys present a challenge for the battery. Especially with old batteries and in winter, the charge acceptance of the battery reduces, so that adequate recharging during short journeys is not always ensured. This can result in gradual discharge of the battery, so that at some point, the battery no longer has enough energy to start the engine. Due to the special technology, the internal resistance of AGM batteries remains considerably lower than that of conventional batteries over their entire life, so that sufficient charge acceptance is also ensured over a long period and short journeys with many start-stop phases can be handled better by AGM batteries.


What is an AGM Battery?

An AGM battery is a car battery designed for two jobs: delivering powerful bursts of starting amps and running electronics for a long time.

And here’s the big deal: They tend to last longer than a regular flooded battery.

The acronym stands for “absorbed glass mat” and that’s one of many improvements made to Planté’s original train light battery. Glass mats, cushioning the ultra-thin lead plates, will squish like a sponge. In turn, manufacturers can squeeze more glass mats and lead into one battery. More lead equals more power. Plus, that squish factor means the battery’s insides are packed tightly.

AGM batteries also have valves regulating the amount of hydrogen and oxygen gas allowed to escape during charging. They fall under a broader category of valve-regulated, lead-acid (VRLA) batteries, typically used for storing a lot of power for a long time or for long-running power uses.

Developed in the late 1970s, AGM batteries mostly served as backup power for telephone boxes and early computer rooms. Their use expanded over the decades to include motorcycles, military, aircraft, submarines and power banks for offices.

Now, they’re showing up in everyday cars and trucks.


Three advantages of AGM batteries

1. Excellent cold-start characteristics

AGM batteries have a large cold start current. They powerfully start the starter motor and reduce its running time. Thanks to the cycle stability of AGM batteries the engine can be switched off and started again several times at short intervals, without the risk of difficulties when restarting.


2. Balanced state of charge

In modern vehicles, the AGM battery is not only charged by the generator, but is also fed by the braking energy recovery system. Normal starter batteries must be kept permanently fully charged and can therefore not absorb the energy which is generated by recuperation. AGM batteries can be operated in the partial charge range and therefore provide sufficient “extra” capacity to store the energy which is generated on braking.



3. High reserve capacity

The high reserve capacity ensures the power supply for all types of electrical components. The cycle stability of AGM technology ensures the power supply to components while also providing a high starting power. Even if the battery is not fully charged, because, for example, electronic assistants and consumers which ensure comfort are supplied during a stop, or the generator has been temporarily decoupled to reduce the load on the engine.


What’s the Difference Between an AGM and a Regular Car Battery?

AGM car batteries have unbeatable advantages over standard, flooded batteries:

● More starts per battery

● Faster recharging

● More durable construction

● Safer to handle

● Special valves protecting the battery's lifespan

Over the course of their lifespan, AGM batteries can start an engine more than 60,000 times. That’s more than three times the starts you’ll get out of a conventional battery.

And AGMs recharge faster than typical batteries. Starting your engine depletes your battery only a small amount before the alternator takes over. When it does, the alternator recharges the battery — and keeps all the electrical components running in the car.

Because of their absorbed mats, AGMs withstand shaking and vibration better than typical batteries. They’re also listed as spill-proof, meaning the regulations are more relaxed about transporting them by air or by road.

This might sound like marketing hype. Instead, it’s just science.

Here’s how AGMs work.


How AGM Batteries Work?

The superpowers of an AGM battery come from two novel additions to Planté’s invention and a host of small design changes that fundamentally expand what car batteries can do.

First, a valve prevents evaporated water from leaving the battery case.

This might not sound like much more than the inverse of the one-way valves on coffee bean bags.

But this little trick is the secret to an AGM’s long life. Here’s how.

Its fundamental chemistry is still based on lead, sulfuric acid and water. When you draw power, the acid molecules move to the lead plates, leaving water and lead sulfate. You are removing the sulfuric acid from the solution to enable a chemical reaction between the paste on the plates. This process is reversed when you charge the battery.

However, there’s always a chance some water loss can happen when electricity splits H2O into hydrogen and oxygen gases. Losing those water molecules means the electrolyte stays more acidic than usual — cutting into the potential strength of the chemical reaction on the plates, and ultimately shortening the life span.

The AGM’s valve stops those gases from leaving.

Except if you’re overcharging the battery. When you use the wrong charger for an AGM, the current must pass through anything it can. That means breaking up more water molecules and building up too much gas inside. That’s when the safety mechanism kicks in, releasing some gas to reduce the pressure built up inside the battery.

Second: fiberglass mesh mats. They are the GM in AGM (absorbed glass-mat) batteries.

Ultra-thin glass fibers soak up all the electrolyte (water and sulfuric acid) into thin pillows cushioning the lead plates. Instead of the free-flowing liquid inside of a regular car battery, the AGM carries its charge in soaked sponges coating the lead plates. The glass mats’ complete coverage makes it easier to summon more power from an AGM battery — and make it easier to recharge.

In power, speed, long life and durability, the AGM battery has standard batteries beat.


Interesting facts about start-stop technology

Start-stop systems require a battery with modern technology, because normal starter batteries are not designed to meet the higher demands of these vehicles.

The architecture of the electrical system in modern vehicles requires a compatible battery technology in order to function properly and reliably. For this reason, in many vehicles, new batteries must be “registered” – the Battery Management System (BMS) must know what type of battery is installed in the vehicle in order to exploit its full potential. If an incorrect battery is installed in the vehicle or is not correctly registered, this can result in premature battery deterioration and another breakdown.


For this reason, only EFB or AGM batteries should be installed in vehicles with automatic stop-start systems. If an AGM battery is already installed in the vehicle, it must always be replaced with another AGM battery.