The car battery: How does it work and what variants are there?

22. Oct 2020 | Industry + More

The car battery supplies and stores electrical energy for the starter and electrical consumers of a vehicle. But how does it work? And what variants are there?

Drivers usually only notice how important a car battery is when they are empty. Once that happens, nothing works. After all, the starter requires power to start the engine, which is why the car battery is also referred to as a starter battery. The engine cannot be started without power. An external assisted start is the only solution. For example, using a jumper cable and a "donor vehicle" to provide the electricity. Vehicle professionals can use the assisted start devices from the Reanimator family or the Flash battery chargers from Herth+Buss. A constant power supply is particularly important for diagnostic work in the workshop.

In addition to the starter, the car battery also supplies emergency lighting (light, warning light) and electrical consumers such as the car radio when the engine is not running.

How does the car battery work?

The most common version is the lead-acid battery. It stores electrical energy in chemical form and then passes it on as direct current. 12 V starter batteries have six series-connected cells. Each cell consists of a stack of alternately arranged positive and negative electrodes, each made up of a lead grid. These positive and negative electrodes made of lead compounds are immersed in an electrolyte of diluted sulphuric acid. An electrolyte is a material in which charged particles can move. The positive electrode is made of lead dioxide, the negative electrode is made of porous lead. Special partitions, the so-called separators, separate the electrodes from each other.

When power is being drawn, the battery is discharged. Both electrodes are converted into lead sulphate during this process. The generator/alternator recharges the battery during travel. This converts the lead sulphate back into the starting materials of lead dioxide and porous lead. As car batteries are repeatedly recharged, they are also referred to as accumulators.

Modern cars present major challenges for car batteries: An increasing number of electrical consumers need to be supplied with power. Diesel engines and high-capacity petrol engines also require a high neutral-start power with high starting currents. And start-stop systems in particular require state-of-the-art starter batteries.

Every starting process for the battery is associated with a high energy consumption. The energy required is high, particularly in modern vehicles with automatic start/stop systems, where the engine is switched off and restarted frequently. Continuous discharging and charging also increases the general load.

Different battery types

A distinction is primarily made between the following types of battery in passenger cars:

  • maintenance-free starter battery
  • completely maintenance-free starter battery
  • AGM starter battery
  • EFB starter battery

Maintenance-free starter battery:
With the maintenance-free starter battery, the electrolyte level during operation should only be reduced to the extent that it does not need to be refilled within a period of two years: Maintenance-free starter batteries have a filling plug for filling the battery acid and filling the acid level with distilled water. Maintenance-free starter batteries are lead-acid batteries.

Completely maintenance-free starter battery:
In contrast to a maintenance-free starter battery, the completely maintenance-free battery is actually maintenance-free. These batteries are sealed tightly, are stored in filled state and do not require the acid level to be topped up. Lead-acid batteries are also completely maintenance-free starter batteries.

EFB starter battery:
The EFB starter battery (EFB = Enhanced Flooded Battery) is a further development of the conventional lead-acid battery. It is used in vehicles with a start-stop system and has a polyvlies material on the positive plate, which enables a longer service life. Compared with conventional starter batteries, the EFB starter battery can handle a much higher number of charging cycles.

AGM starter battery:
AGM starter batteries (AGM = Absorbent Glass Mat) are used when a high charge throughput rate is required. Here, the electrolyte is bound in a micro-porous glass fibre mat, which is used to insulate positive and negative electrodes instead of conventional separators. The AGM battery is characterised by a high cold start current. With an AGM battery, the engine can be switched off several times at short intervals and restarted without any problems occurring when starting up again. This makes it suitable for use as a powerful battery for start-stop systems: Compared with conventional starter batteries, the EFB starter battery can handle a much higher number of charging cycles.

And what other types of batteries are there?

Gel battery:
In the case of a gel battery, the electrolyte is bonded in a solid multi-component gel by adding silicic acid. This gives the gel battery the advantage of greater insensitivity to vibrations and overall lower electrode wear. It is sensitive to temperature fluctuations, however. Due to its greater internal resistance, the gel battery is also unable to provide a high cold start current at short intervals. Overall, the gel battery is not suitable as a starter battery. It is primarily used as a service battery in commercial vehicles or mobile homes.

Li-ion-based starter batteries:
Previously, lithium-ion starter batteries have been primarily used in motorbikes or in motor sport. They are more compact and lighter. The cycle stability of the lithium-ion starter battery is higher than with a lead-acid battery. However, Li-ion starter batteries are much more expensive than lead-acid batteries.