lead-acid, agm battery and co. – how the car battery works

Quietly it does its job and most drivers hardly take notice of it. Until it fails: the car battery. How does a car battery work and what are the terms like lead-acid battery, AGM battery, starter battery and consumer battery?? We bring light into the darkness and explain how a battery works and which types there are.

The car battery is a real powerhouse and plays an important role in the car. It is able to provide several hundred amperes of current at short notice and often has to supply consumers with a total power of more than 2 kW. Accumulator is basically the correct term for the car battery, because unlike a battery, it is rechargeable. We use both terms here.

Without the car battery it does not work

The battery has a very important, even protective function for the vehicle electrics and electronics. The alternator of the car generates an alternating voltage, but the battery and the on-board technology are designed for direct voltage. Therefore the alternator has a rectifier, which forms a direct voltage from the alternating voltage. Well almost, because this is not really a steady DC voltage, which is a problem for the sensitive on-board technology. The accumulator works against it by its capacitor ability and smoothes the voltage additionally. In this way the battery also minimizes EMC problems.

At the same time, the battery absorbs voltage peaks and troughs coming from the alternator, because its voltage fluctuates. The alternator is constantly ramped up and down depending on engine speed and current consumption. All things that electronics do not like at all.

Further only the battery makes dynamic peak loads possible, which the alternator cannot supply in the speed. Such loads occur when strong consumers are switched on, such as z.B. the rear window heater or the servo motor of the electric steering system.

You should not drive without a car battery.

The accumulator is the buffer between the alternator and the car’s electrical system. It ensures fault-free operation of the electronics and protects the sensitive components from destruction. This also includes the alternator charge controller. You can drive without an alternator, but not without a battery.

The classic car battery: the lead-acid battery

The typical car battery is a “wet” lead-acid battery. With increasing electronics, number of consumers, hybrid and stop-start technology, other battery types are also finding their way into cars. However, we are concerned with this classic type, as it is very widespread.

The lead-acid battery consists of a collection of pairs of plates, the electrodes. A certain number of these pairs are housed in separate chambers called cells. One plate forms the positive electrode and is made of lead oxide, the other is the negative electrode and is made of lead. The plates are in pockets that form a porous separator. The cells are connected in series. This means that the negative pole of the one outer cell is led out of the housing as a negative pole. The positive pole of this cell is then connected to the negative pole of the next cell, etc. The outer cell on the other side has the positive terminal on the outside. In a 12-volt car battery, there are six such cells, each of which generates 2-volt quiescent voltage.

To generate the energy, an electrolyte is still needed between the plates. As the name suggests, this is an acid. More precisely, a sulfuric acid-water mixture, hence the attribute “wet”. For other types, such as the AGM battery and gel battery, other material constructions are used.

To get the maximum power, there must be as large a plate surface as possible. Therefore, the electrodes are in fact not flat plates, but grids. The grids consist of a lead alloy, which are coated with porous lead oxide or lead paste, the electrically active mass. We will see later that this also has a decisive disadvantage.

Internal structure of a

Processes during current draw and charging

When current is drawn from the car battery, the lead oxide and lead convert into lead sulfate and the acid content in the electrolyte decreases. During charging, this process is reversed, the lead sulfate becomes lead oxide and lead again, the acidity increases. This sulfation, which is often mentioned as a problem, is basically not a problem, but the functional principle. It only becomes problematic when large sulfate crystals are formed during sulfation or when not all crystals can be converted again.

It would be nice if this process of sulphation and regeneration would run forever. In practice, however, this is unfortunately not the case. The battery ages and loses capacity. Incorrect handling of the battery, especially incorrect current draw, insufficient and incorrect charging, accelerate this aging process. More about this later.

Important characteristics of the accumulator

The battery is defined by some important parameters. The Voltage indicates the environment in which the battery can be used. Typically 12 volts. In the truck sector, two batteries are then connected in series to obtain the on-board voltage of 24 volts.

The Capacity indicates the amount of electricity that can be stored. The specification is in Ah (ampere-hours). An 80 Ah battery can theoretically supply 80 amps for 1 hour or 1 ampere for 80 hours. This capacity can be removed only theoretically, because the battery would be broken afterwards. capacity decreases with temperature.

The Nominal capacity According to the DIN 50342 standard for starter batteries, this indicates the charge that can be discharged within 20 hours until the final discharge voltage of 10.5 volts is reached at a constant discharge current and at 25° Celsius. The voltage of the battery must not drop below the final discharge voltage, because from then on it is considered to be deeply discharged and is then usually unusable. For 12 volt batteries, this voltage is 10.5 volts (1.75 volts per cell). In short, the nominal capacity describes the actual usable capacity of the battery.

13 volts resting voltage, a healthy battery

Another very important parameter is the Cold test current. This value expresses the battery’s ability to supply sufficient starting current in cold conditions. This is also regulated by DIN 50342. The voltage between the poles must be at least 7.5 volts (1.25 volts per cell) at -18° Celsius 10 seconds after the start of discharge with a given discharge current.

Why the battery ages

Loss of material

As described above, we need a large electrode surface area for the high currents needed to start the car. Therefore a porous paste is used instead of smooth surfaces. If current is now being drawn or charged, mechanical stresses are created at these grids, which are coated with paste. The reason for this is the constant formation and reconversion of the lead sulfate. It takes up more space than the original material and so the volume at the grids constantly increases and decreases. As a result, small particles repeatedly break away from the grid and fall to the bottom of the car battery. The material thickness and the electrically active surface decrease, the battery loses strength and capacity.

Vibrations have the same effect, they cause material to break out of the lattice and thus additionally force the aging process. To prevent the material falling to the ground from eventually causing a short circuit between two plates, the bottom of the car battery is a kind of tub. However, EFB/AFB, gel and AGM batteries also provide a remedy for these problems, as we will see below.

Aging due to improper discharging and charging

The processes described above take place in every lead-acid battery, even if it is ideally cared for and treated. Incorrect storage, discharging and charging also accelerates the aging process. The problem is the lead sulfate mentioned above. It forms in the form of crystals. These have poor conductivity and are not needed as an active mass to generate electricity. The larger such a crystal is, the worse it conducts and on a larger surface area. In addition, the larger they are, the more difficult or impossible it is to convert them back to their original material, even if some manufacturers of chargers promise this. Capacity is permanently lost.

Now when do these large crystals form? One cause is the wrong current drain. High currents make fine crystals grow, which are easy to convert back. Low discharge currents, on the other hand, produce large crystals. Continuous withdrawal of low currents, therefore, shortens the life of a starter battery. This happens especially when it is used as a consumer battery for small devices over long periods of time.

The second cause of large crystals is time. Many off-road and touring vehicles are not driven on a daily basis. Mere standing around in a non-fully charged state causes the small crystals to coalesce into large ones. The emptier the battery is, the more this growth takes place.

How crystal formation is reduced

In principle, you can avoid the formation of large crystals or at least reduce it to a minimum by keeping the battery fully charged as often as possible and using it for its intended purpose. A starter battery is designed for starting a vehicle: it is best to supply high current for a short time. Only small crystals are formed and these are degraded as completely as possible during charging.

A full charge of the car battery is usually never achieved

It is difficult to really fully charge the car battery, especially in winter or during frequent short trips. The alternator in a vehicle without smart charging technology is designed to get the battery just about fully charged in the summer if enough driving is done. One of the reasons is that too much charging voltage leads to overheating and gassing. In the process, too much voltage creates dangerous, because highly explosive hydrogen. And because the gassing voltage is lower in summer than in winter, the charge controller is set to stay just below the gassing voltage in summer. In winter, on the other hand, a higher charging voltage would be needed to fully charge than in summer.

In addition, there are many additional consumers in winter, such as rear window heating, fans or seat heating, which further reduce the current required for charging. The result: it becomes even more difficult to fill the battery. Modern vehicles with temperature-controlled charging technology have an advantage here, since the charging voltage is brought to the tolerable maximum depending on the temperature.

But even in summer, a one hundred percent charge is rarely achieved. Often the distance driven is too short, so that sulfate crystals always remain. Unfortunately, the small crystals always recede first, which additionally promotes the growth of larger crystals.

What to do to keep your car battery fit, we have described here: Winterization and battery maintenance for little-used vehicles.

Deep discharge

If you draw too much current from the battery, this is called deep discharge. In a 12-volt car battery, this state is reached at a resting voltage of about 10.5 volts between the poles. The quiescent voltage is the voltage you can measure at the poles when no loads are connected. This is 1.75 volts per cell. Even a single deep discharge can damage the battery to such an extent that it is unusable, as it dissolves a great deal of material from the grid.

In such a case, fast charging is often resorted to. However, this causes further damage to the battery because the charging current is much too high in order to save time. A normal charge is done with a maximum of 10 percent of the nominal capacity. A deeply discharged battery can only be coaxed back into operation with a long charge and low charge currents, if at all.

Car battery - Slow charging protects the battery

Maintenance-free batteries

A battery loses water during operation, maintenance-free or not. It escapes as a gas that is produced during charging. If the battery is overcharged, it loses a lot of water. Another reason for water loss are high temperatures. Then the voltage at which gassing starts falls below the operating voltage. A slight gassing starts and the water content drops. To prevent dangerous overpressure from occurring and the gas from escaping, small holes in the housing are.

With the loss of water, the acid content increases and it can happen that the electrodes are partially no longer surrounded by the electrolyte. The surface dries out and its capacity is permanently lost. Ideally, the lost water can be replaced. In the past, this was also common practice, but the attribute “maintenance-free” is more effective in advertising. If you have a car battery in which water can still be added, you should only use distilled water.

The fact that the loss of liquid accelerates the aging, the manufacturers of maintenance-free batteries know of course and steer against it. They use labyrinth lids to allow the water to condense and recirculate. This minimizes water loss.

In principle, a battery whose fluid level can be monitored and topped up has an advantage in terms of service life. In practice, however, it has been shown that even the maintenance-free batteries can still live long enough.

EFB/AFB, Gel and AGM batteries

As you have seen, many aging problems are associated with the formation of large crystals and the loss of material in the grid. EFB/AFB (Enhanced/Advanced Flooded Battery), Gel and AGM (Absorbent Glass Mat) batteries are also based on the lead-acid principle, but address this problem. EFB- resp. AFB batteries try to reduce the loss of grid material with a more robust, special grid and a polyester fabric, keeping the material in place so it does not sink to the bottom.

In gel and AGM batteries, the electrolyte is bound in solid material. The gel battery uses silica to bind the sulfuric acid, the AGM battery uses fiberglass mats. This allows these car batteries to operate independently of their position, which is especially interesting for off-road vehicles. For a normal lead-acid battery, turning it upside down can mean death if the material that has sunk to the bottom spreads between the plates and causes a short circuit. In addition, the acid would leak.

The AGM and gel battery type is also almost immune to another aging factor: acid stratification. Among other effects related to the distribution of current on the grid, concentrated sulfuric acid sinks to the bottom in the battery because it is considerably heavier than water. This again promotes crystal growth in the entire grid area. Above because of the lack of acid concentration and the resulting inhibited charging, below because of high acid concentration and the resulting corrosion. The glass fiber fleece almost completely eliminates this uneven distribution, and the gel even eliminates it completely.

VARTA AGM system - structure of the VARTA AGM accumulator

Starter and consumer battery

Cars are delivered with a starter battery. Normally this is a “wet” lead-acid battery. Its lead grids are thin and very densely packed, creating a large surface area. This type of battery is therefore very good at delivering high currents for a short time. Just as is required for engine starting, where several hundred amps flow.

Consumer batteries, also called utility batteries, have thicker plates and are often designed as AGM, EFB/AFB or gel batteries. Both makes them more cycle resistant. A cycle is defined as a discharge (maximum 50 percent) and charge (100 percent) process. Cycle-resistant batteries tolerate deeper discharges and lower currents better. The possible deeper discharge of cycle solid batteries is the reason why they need a different charging method than a starter battery.

This is especially true for gel batteries, since their charging voltage is just below the gassing voltage. If the battery is overcharged, water can settle on the gel layer, which permanently reduces the performance. If outgassing occurs, the electrolyte is also lost via the safety valve, which means the battery is ruined. Therefore a high quality charger is recommended here.

Gel batteries also have disadvantages in current compared to EFB/AFB and AGM batteries because the gel inhibits the movement of charge carriers. In return, it can withstand a few deep discharges and is leak-proof in normal operation.

Charging the battery

At this point, it is clear that the longer a car battery is fully charged, the longer it will last. But also the charging itself, must be done correctly. Everyone has certainly heard of “charging procedures” and “charging curves”. Or that a lead-acid battery must be charged differently than, for example, an AGM battery. Although all based on the same principle, this is true.

Basically, charging should always be done in a well-ventilated place. Gases are produced during the charging process and must be removed. If the charge is too strong due to a defect or a bad charger, a dangerously large amount of hydrogen (oxyhydrogen) is produced, which ignites extremely easily and then leads to an explosion. In the car, the battery is therefore installed in an open, well-ventilated place or there are vents that must not be closed either.

Charging process and procedure

All charging processes have one thing in common, the fuller the battery, the higher its voltage and the lower the charging current. Once the cells are charged and the electrochemical conversion of the material is complete, only little or no current may be supplied. Otherwise, the energy can no longer be absorbed by the cells and they heat up. This will eventually lead to outgassing again.

There are several different charging methods for the individual battery types. The usual method for lead-acid batteries is the IU method, also called CCCV (Constant Current Constant Voltage). First charging with a constant current (I) in the amount of 5 to 10 percent of the specified capacity of the battery. If the current would not be limited, it would be much higher and this should be avoided. The voltage is lower than the charge end voltage specified for the battery type. During charging, the voltage of the battery increases. If the final charge voltage of 14.4 volts specified for this type of battery is reached (2.4 volts per cell), the charger switches to a constant voltage of 13.8 volts. As charging continues, the charging current is further reduced as the charge increases. If the current then drops below a certain value, you have to disconnect the charger.

Car battery - Typical course of the IU charge curve

Alternator and charge controller

In normal vehicle operation at least the starter battery is charged by alternator. Vehicles that are not classic cars and have a normal lead-acid battery have a simple charge controller. It works according to the described IU charging method, which prevents overcharging and thus enables a long service life. As described above, it is designed to be able to fully charge the battery during sufficient driving time in summer. For hybrid vehicles and those with stop-start function, the battery management is much more complex. Other battery types are used there and the charging control and monitoring is much more sophisticated.

Old, electromechanical charge controller (1971)


Chargers are used either at home or with a second battery in the vehicle. In both cases, the aim is to really fully charge the car battery and give it as long a life as possible.

If you are looking for a charger with which you can permanently keep a battery fit, you should choose one with an IUoU charging method and selectable charging curves for the different battery types. This charging curve is also the recommended one for fleece and gel batteries. It starts with IU charging in order to switch to a charge retention voltage when full charge is reached and to limit the charging current to a maximum of 1 percent of the rated capacity. In very good devices, the battery temperature is also monitored.


To ensure that you get the most out of your car battery for as long as possible, you should heed a few tips.

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