Conventional batteries consist of a series connection of galvanic cells in order to provide a sufficiently large voltage. To increase both capacity and the maximum deliverable current, additional cells can also be connected in parallel. However, it is important that all cells supply exactly the same voltage; otherwise, equalization currents will flow between the different potentials, causing the battery to discharge more quickly.
Since the potential of a single element cannot be measured directly, a standard electrode has been defined. This consists of a graphite electrode surrounded by gaseous hydrogen, which is by definition assigned a potential of 0 V. The potentials of other elements are then given relative to it in the so-called electrochemical series. Selected elements that play an important role in electrochemistry and in batteries are listed below:
| Element | Oxidized | Reduced | Potential [V] |
|---|---|---|---|
| Gold | Au³⁺ | Au | +1.50 |
| Mercury | Hg²⁺ | Hg | +0.85 |
| Silver | Ag⁺ | Ag | +0.80 |
| Copper | Cu²⁺ | Cu | +0.34 |
| Hydrogen | H⁺ | H | 0.00 |
| Lead | Pb²⁺ | Pb | -0.13 |
| Nickel | Ni²⁺ | Ni | -0.23 |
| Zinc | Zn²⁺ | Zn | -0.76 |
| Magnesium | Mg²⁺ | Mg | -2.36 |
| Lithium | Li⁺ | Li | -3.04 |
Table: Extract of the electrochemical series for selected elements.
Batteries that must be disposed of after discharge are called primary cells. In Europe today, the most commonly used primary cell is the alkaline-manganese battery, whereas zinc-carbon batteries were more widespread in the past.
The alkaline-manganese cell consists essentially of a gel of zinc powder, which is oxidized during the reaction. The electrons migrate through a conducting rod to a metal plate. The zinc gel is surrounded by an electrolyte of potassium hydroxide (KOH), which transfers the negatively charged OH⁻ ions from the anode to the cathode. The cylindrical cathode consists of manganese dioxide and surrounds the entire cell. Both oxidation and reduction take place in several steps; the main reaction can be written as:
\[
\mathrm{Zn + 2MnO_2 + 2H_2O + 2OH^- \;\rightarrow\; [Zn(OH)_4]^{2-} + 2MnO(OH)}
\]
The electrochemical potential of manganese dioxide is +0.74 V, while zinc has a potential of -0.76 V. The difference between these two values yields a voltage of 1.5 V, corresponding to that of standard household batteries.
In addition to primary cells, there are also secondary cells. In this type of battery, the reaction can be reversed by applying an external voltage, meaning they can be recharged. They can therefore store energy and are commonly called accumulators (rechargeable batteries). Typical modern examples are lead-acid and lithium-ion batteries.
Lead-acid batteries are often used in vehicles to power electronics, as they can deliver high currents even at low temperatures. The cathode consists of a lead plate coated with lead dioxide, while the anode is made of pure lead. The overall reaction, which applies to both charging and discharging, is:
\[
\mathrm{Pb + PbO_2 + 2\,H_2SO_4 \;\rightleftharpoons\; 2\,PbSO_4 + 2\,H_2O}
\]
From the electrochemical series, the anode reaction yields -0.36 V and the cathode reaction +1.68 V. The total cell voltage is therefore about 2 V per cell. To achieve the 12 V typical of a car battery, six cells must be connected in series.