The task of the car battery, traditionally located in the engine compartment, is well known: Without a battery, the vehicle cannot be started. In addition to the starter motor, spark plugs, glow plugs, vehicle lighting and electronic assistants also require electrical energy. But what is the structure of a car battery and how does it work??
The lead-acid battery: components and structure
Many car drivers are aware of the high weight of a car battery when buying a new one – starting at approx. 10.5 kg up to 30 kg are possible. Responsible for this are the lead plates in the battery cells.
Components and structure of a battery cell
: In a lead-acid battery, the positively charged plate (active mass) consists of lead oxide (PbO2) and is immersed in an electrolyte.
- Positive grid: the positive grid is made of a lead alloy and is used to hold the active mass and as a current collector.
The negatively charged plate (active mass) consists of pure lead (Pb) and is also immersed in an electrolyte.
- Negative grid: Like the positive grid, this grid is also made of a lead alloy and serves the same purpose.
Separate the electrodes of different charge by a separator pocket.
The electrolyte is a mixture of sulfuric acid and distilled water (H2SO4). This electrolyte can be liquid (as in classic wet batteries or advanced EFB technology), gel-like, or bound in a glass fleece (as in AGM technology for advanced start-stop applications).
Several positive electrodes result in a positive plate set and several negative electrodes result in a negative plate set. A negative and a positive set of plates together make a block of plates. A plate block is a battery cell.
A conventional starter battery consists of a series connection of 6 cells, each with a nominal voltage of about 2 V, and thus builds up a voltage of exactly 12.72 V when fully charged. The number of plates per cell results in the capacity and cold start capability of a battery.
Rule of thumb: The more plates there are in a cell, which together form a larger surface area, the more cold-cranking amperage (CCA) a battery can deliver. However, if the space of the cell is used to accommodate fewer but thicker plates, the cycle life increases. This means that the battery is designed for a higher charge throughput (the steady charging and discharging process).
The cells are housed in a case made of an acid-proof plastic (polypropylene). In classic SLI batteries, this is closed by a lid with a labyrinth system, which prevents the leakage of battery fluid and separates the liquid from the gas. Early batteries had screw plugs which allowed refilling with distilled water. Today’s batteries are completely maintenance-free – no water needs to be added and no water is allowed to be added. Although AGM batteries still have "disposable plugs", these must not be opened under any circumstances.
Function of a car battery: chemical energy becomes electrical energy
A car battery stores energy in chemical form and converts it into electrical energy. In this electro-chemical process, four materials react with each other:
Applying an external load starts the chemical reaction in the battery:
- The electrolyte, a mixture of sulfuric acid and distilled water (H2SO4), splits into positively charged hydrogen ions (H + ) and negatively charged sulfate ions (SO4 2- ).
- Simultaneously, electrons (2e – ) migrate from the negative to the positive electrode via the external load.
- To balance this flow of electrons, sulfate ions migrate from the electrolyte to the negative electrode, where they react with the lead (Pb) to form lead sulfate (PbSO4).
- Lead sulfate also forms in the positive electrode: the bond of oxygen (O2) in the lead oxide (PbO2) is broken by electron migration and the oxygen migrates into the electrolyte. The residual lead (Pb) binds with the sulfate (SO4) from the electrolyte.
- There, the oxygen binds with the hydrogen to form water (H2O). As the sulfuric acid is consumed by the formation of lead sulfate, the concentration of the electrolyte solution decreases. Once the concentration of sulfuric acid drops to a certain level, the battery must be recharged.
- When charging, the chemical processes run in reverse order. At the end, the original elements can be found again: The positive electrode consists of lead sulfate (PbSO4), the negative electrode consists of pure lead (Pb) and the electrolyte consists of diluted sulfuric acid (H2SO4).
Since this conversion process is lossy, each car battery can only survive a limited number of cycles. Their useful life is therefore limited.
Problems of the lead-acid battery: sulfation and acid stratification
If a battery is charged at too low a voltage or always runs at low voltage (below 80%), acid stratification, also called stratification, occurs.
In doing so, the acid in the electrolyte will stratify due to poor mixing of its. Different densities cause stratification of sulfuric acid at the bottom and water at the top of the battery. As a result, only the middle part of the electrolyte, i.e. only one third, can be used for the discharging and charging process.
One possible cause of acid stratification is predominantly short trips with simultaneous use of many electrical consumers. In such a case, the alternator does not have enough time to charge the battery sufficiently.
One consequence of acid stratification is sulfation. If this occurs in a battery, or if it is constantly not sufficiently charged, the lead sulfate (PbSO4) crystallizes on the electrodes, from which larger crystal formations develop over time. This process is called "sulfation". Crystallization prevents the transformation of lead sulfate back into the original components lead and respectively. Lead oxide, which results in inhibited charge pickup and reduced cold start lead.Sharp crystals can also damage the separators or cause cell short circuits.
To counteract these effects and prevent premature battery failure, a battery should never be exposed to a deficiency charge for a prolonged period of time. For this purpose, it is recommended that batteries be tested regularly and fully charged when necessary.
New battery technologies: AGM and lithium-ion
While conventional lead-acid batteries still have a high share, the market is changing rapidly: Innovative battery technologies for start-stop vehicles, such as AGM, use acid bonded in fleece for higher cycle stability and guarantee reliable performance in modern vehicles with increased energy demand. Another advantage of AGM: Acid stratification is no longer possible due to the bound acid.
A new generation of car batteries for micro-hybrid vehicles even operates at 48 V, using cells with lithium-ion technology.