How does a lead-acid battery work

At a Lead accumulator (short Lead acid battery, especially in the case of motor vehicles, see also starter battery) is a version of the accumulator in which the electrodes in the charged state consist of lead and lead dioxide and the electrolyte consists of dilute sulfuric acid.

Lead-acid batteries are considered reliable and inexpensive for a service life of a few years. Compared to other battery technologies, however, they are quite heavy and have only a low energy density.

However, they are also used, among other things, to store energy for electric vehicles. See also:Battery. The best-known application is probably the starter battery for motor vehicles.



The first attempts to develop a lead-based accumulator were made in 1801 by the physicist Johann Wilhelm Ritter, and later, in the middle of the 19th century, by the German doctor Josef Sinsteden. He put two large lead plates in a vessel with dilute sulfuric acid. Charging the battery produced lead dioxide (lead (IV) oxide) on one of the plates and lead on the other.

In 1859 Gaston Planté improved the arrangement of the lead plates, which is still used today.

The lead-acid battery became of industrial interest when Emile Alphonse Faure developed a process around 1880 in which the lead-acid battery reached a high capacity after just a few charging cycles (forming). Henri Tudor developed the first technically usable lead accumulator in 1886.


When charged, lead accumulators consist of lead (IV) oxide (PbO) at the positive pole2), on the negative made of finely divided, porous lead (lead sponge). 37 percent sulfuric acid (H.2SO4) is used. They are characterized by the short-term allowance of high currents, which are necessary for vehicle or starter batteries, for example.

When discharged, both poles are made of lead (II) sulfate (PbSO4).

The nominal voltage of a cell is 2 volts, but the voltage fluctuates between approx. 1.75 and 2.4 volts depending on the state of charge and the charge / discharge current.

The acid density is also a measure of the state of charge. It is approx. 1.28 g / cm³ with a fully charged battery and 1.10 g / cm³ with a discharged battery (source: Varta battery encyclopedia).

Lead accumulators should not be deeply discharged, as this leads to irreparable damage and can render the accumulator unusable. A suitable charge regulator should be used for charging in order to avoid harmful overcharging and to limit gassing.

A lead accumulator can also gas if it is contaminated by precious metals. Parts of the noble metal are deposited on the lead electrode and thus reduce the overvoltage of the hydrogen. Oxyhydrogen gas can develop, which can ignite dangerously due to sparks when the battery connections are disconnected or electrostatic charging, e.g. of the plastic housing by rubbing.


In the meantime, lead-acid batteries have a long service life of several years thanks to technical progress and regular maintenance. Nevertheless, the lead batteries age. This is primarily due to the internal corrosion (with only external K. see also: Pole fat) of the lead framework of the electrodes, the formation of fine short circuits and the sulphation of the lead. This sulfation causes the PbSO4-Combining crystals to form larger and larger clusters. This reduces the electrochemically active surface of the PbSO4. The PbSO dissolves through this smaller surface4 getting worse, it takes a long time to get a sufficiently high concentration of Pb2+ is present. In addition, the electrical conductivity of sulfate is lower than that of lead. The resulting increased internal resistance of the cell leads to a greater voltage drop under load.

See also:Accumulator, galvanic cell

Chemical processes

The energy density of the PB battery is 0.11 MJ / kg, half as much as that of the NiMH battery (0.22 MJ / kg).

The following chemical processes take place during discharge:

Positive pole:

Negative pole:

(When loading, the processes run in the opposite direction.)

The overall reaction:

To the right, the lead-acid battery is discharged with energy, and to the left, when energy is supplied, it is charged.

From the electrochemical series one can now calculate the potential difference, i.e. ultimately the electrical voltage that arises.

Self discharge:

Lead (IV) oxide is not stable in sulfuric acid solution.

The overvoltage of the hydrogen, which makes charging a lead-acid battery possible in the first place, slows this process down.

Purity requirements:

Certain impurities like Fe2+-, Co2+-, Cu2+, Cu+-, or Ag+-Ions reduce the overvoltage of the water and oxygen, so that the self-discharge is accelerated.

Sealed lead-acid batteries

Lead-acid batteries can also be manufactured in a sealed design. This is called in English VRLA (valve regulated lead acid, roughly translated: lead-acid battery with pressure relief valve).

Sealed lead-acid batteries are structured as follows:

  • The cells are welded shut, there is only a pressure relief valve.
  • The electrolyte is fixed, so no longer liquid.

The electrolyte can be set in two ways:

  • Gel battery: By adding silica to sulfuric acid, the electrolyte solidifies to form a gel.
  • Fleece battery: A glass fiber fabric is placed between the electrodes, which completely absorbs the electrolyte. This type is also known as AGM (Absorbent Glass Mat) or MF (maintenance free-) called battery.

The fixed electrolyte makes it possible to operate sealed lead-acid batteries in any position.

With gel accumulators there is practically no acid stratification (relevant loss of capacity due to segregation, with denser acid at the bottom, watery components at the top), in fleece accumulators it is at least reduced compared to closed accumulators.

The internal resistance of gel lead-acid batteries is higher than that of comparable non-sealed lead-acid batteries. They are therefore less suitable for delivering high currents, as are required when used as a starter battery. Fleece batteries can supply the same high currents as the open versions (e.g. SSB / Effekta 100AH ​​batteries can supply 800A for 5 seconds) and are therefore particularly suitable for electric vehicles (e.g. CityEL)

Since the cells are welded, it is not possible to open the battery, for example to refill water. This is also not necessary, since sealed lead-acid batteries emit significantly less gas than conventional lead-acid batteries. Gas channels form through the fixed electrolyte. The oxygen formed by the side reaction at the positive electrode can therefore migrate directly to the negative electrode and recombine there to form water.

If the sealed lead-acid battery is overcharged (i.e. if the voltage is too high), an excess of oxygen is generated that can no longer recombine. Hydrogen is generated to the same extent at the negative electrode. In this case the gases escape through the pressure relief valve and the battery can dry out over time. Since it is not possible to refill the electrolyte, sealed lead-acid batteries require an adapted charging process. It must be avoided that the battery is charged over a long period of time at an excessively high voltage, which is associated with strong gassing.

Another possibility is to add a bit of a catalytically active material on which the water and oxygen can react back to water.

In addition, when charging with excessive voltage with sealed lead batteries, there is a risk of Thermal runaway. The internal oxygen circuit heats the battery. An increase in the battery temperature leads to an increased charging current at a constant voltage. This leads to increased gas development and the oxygen cycle is strengthened. This self-reinforcing process can overheat and destroy the battery.


In general, a distinction is made between backup batteries and traction batteries. While buffer batteries support an existing energy supply, traction batteries are used as an independent energy source.

  • Application examples for backup batteries
    • Starter batteries (e.g. for vehicles)
    • Uninterruptible power supply (UPS) (emergency power supply, alarm systems)
    • Central power supply systems for emergency lighting
    • Solar batteries in photovoltaic systems (island systems)
  • Examples of traction batteries
    • Electric vehicles
      • electric forklifts
      • electric wheelchairs
    • Submarines


  • Karl-Joachim Euler: Sinsteden - Planté - Tudor. On the history of the lead accumulator. Comprehensive University of Kassel, Kassel 1980.
  • Heinz Wenzl: Battery technology / optimization of the application - operational management - system integration. Expert-Verlag, Renningen-Malmsheim 2002, ISBN 3-8169-1691-0
  • Andreas Jossen, Wolfgang Weydanz: Use modern accumulators correctly, Printyourbook 2006, ISBN 9783939359111
  • D.A.J. Rand, P.T. Moseley, J. Garche, C.D. Parker: Valve-regulated lead-acid batteries, Elsevier 2004, ISBN 0-444-50746-9

Category: Battery