Are lead-acid batteries highly toxic

The Starter battery (for motor vehicles: Car battery(see also lead battery) is an accumulator and supplies the electrical current for the starter of an internal combustion engine, for example a motor vehicle, power generator or the gas turbine of an aircraft.
A battery that is used as an energy source to drive an electric vehicle, on the other hand, is more likely to be referred to as a traction battery. In the case of hybrid drives, either a single battery or several batteries can be used.


Starting an internal combustion engine using the electric starter motor briefly requires high currents of several hundred up to 1000 amperes. The starter battery must be able to deliver this even in winter at low temperatures. In addition, the electrical voltage must not drop too much during the starting process. Therefore, starter batteries have a low internal electrical resistance. The ADAC offers information on types and purchase advice under [1].


Starter batteries are series connections of lead accumulator cells, each with a nominal voltage of 2.12 volts. In order to achieve a nominal voltage of 6 volts or 12 volts, 3 or 6 such cells must be connected in series to form a battery. Batteries with 24 volts are only necessary for trucks and can be implemented if necessary by connecting two (identical) 12-volt batteries in series.
Lead starter batteries can be divided into liquid, fleece and gel batteries.

Problems, treatment

Corrosive and toxic

The electrodes are made of lead or lead compounds and are therefore poisonous, the sulfuric acid they contain is highly corrosive. Extreme caution is therefore required when handling batteries. A burst battery, e.g. B. in an accident, may only be touched with appropriate protective measures, preferably by specialists. The electrolyte (sulfuric acid) must never penetrate the soil. Disposing of an undamaged battery is only permitted through the dealer or the workshop. Anyone who has come into contact with acid or chemicals from a battery should consult a doctor.

Loading, unloading

The constant charging and discharging during operation ensures a constant chemical change in the pressed-in substances lead, lead dioxide or lead sulphate. This inevitably leads to a gradual loosening of the pressure. Something similar happens due to vibrations when driving. But when the fabrics become loose, they expand and eventually fall down. They collect at the bottom of the battery. There are troughs there in which the 'sludge' collects, but at some point these are full and the sediment touches the cells. This causes an increasing short circuit in the cell. The battery is then also referred to as "slipping together". The increasing trickling of the grids can be equated with an increasing loss of capacity. Shortly before slipping together, the capacity is usually no longer sufficient to start, at least not in winter when it's cold.

Temperature dependence, "winter fatigue"

The deeper the starter battery has cooled down, the lower its capacity. Various systems are available on the market to prevent cooling or to allow heating. Before the onset of winter, the starter battery should be checked to see whether the capacity is still sufficient for starting at low temperatures. If the check is not carried out, the end of the service life of a battery is usually reached in winter, because the loss of capacity due to the cold is considerable and old, weak batteries, whose capacity was still sufficient for the start in summer, either lack the strength to withstand a longer cold start or are simply too weak in the case of severe minus degrees. At minus 20 degrees only about half of the normal capacity is available. At the same time, however, the cold makes the engine oil particularly tough and the starting process requires significantly more power. At extreme temperatures, the solution is sometimes chosen to remove the battery and take it with you into heated rooms overnight or if the battery is not used for a longer period of time. This was done regularly during the winter war, and the engine oil also often had to be heated up.


Another problem is overcharging the battery. A poorly set controller or an unregulated or overly powerful charger can lead to overcharging. When charging, all of the lead sulphate is initially converted back into lead and lead dioxide, but since the charging current continues to flow, the lead in the grid is now also attacked. In the process, the grid becomes larger and the strength of the pressed-in materials decreases.

Sludge formation, grid corrosion

The pressed substances fall out and form a sediment (Lead sludge), which leads to a continual decrease in capacity and ultimately to a short circuit in one or more cells. This phenomenon is called colloquially Silting up, more correct than Lattice corrosion of cells and occurs more frequently than is generally assumed. Maintenance-free batteries in particular rely on intact regulators that adjust the charging current and voltage to the state of charge. A cell short-circuit can also occur suddenly if a battery that is already muddy is transported abruptly or not evenly, or is moved far out of its normal position or is placed at an angle. That would explain the theory that only batteries that are not used should last longer.

Charging voltage, the "gas"

The charging voltage should be in the range of 13.8 to 14.4 volts at a temperature of 15 to 25 ° C. The charging current should ideally be a tenth of the value of the battery capacity (e.g. 4A at 40 Ah) and not exceed a third of the value of the capacity for fast charging. If the charging voltage is above 2.4 volts per cell (with 12-volt batteries this is a total of max. 14.4 volts), grid corrosion begins, which is noticeable in the form of "gases". That is also the reason why the battery should at least not be charged with high currents until it is fully charged. A quick charger can charge a discharged lead battery very quickly, but only up to approx. 70%, then you should continue charging with low charging currents in order to avoid grid corrosion.

Risk of explosion

Overcharging leads to "gassing" of the starter battery. Gassing is the electrolytic decomposition of the water contained in the dilute sulfuric acid. This creates oxygen and hydrogen, which together form oxyhydrogen.

Fluid level, cleanliness

Even with a maintenance-free battery, the fluid level should be checked regularly or checked by a specialist. The liquid should be about 10 mm above the top edge of the plate. If you carry out this check yourself, you will quickly notice that the plates gas slightly, especially immediately after a journey. This is a sign that water is being broken down and thus lost. If the liquid level drops below the edge of the plates, the capacity of the battery drops and the dry zone is damaged that cannot be reversed. The solution to the problem is apparently simple: the charging voltage would only have to be reduced, then the battery would not be charged to the point of gas. However, reducing the charging voltage by even a tenth of a volt leads to a battery that is not fully charged, which has glaring disadvantages on the other hand. So it is advisable to check the cells for their fluid level. If necessary, it is topped up with distilled water in each cell. The cells may only be closed with the original cell closure. You have to work very cleanly in order to avoid any contamination of the electrolyte.

Too little charge

Far more common than these errors is insufficient battery charge. Even when not in use, the battery continuously discharges itself to a small extent. If it is installed in a vehicle, almost the entire electrical system of the vehicle is live and there are always small consumers somewhere (clock, alarm system) or leakage currents that also lead to discharge .

Stand damage

If the vehicle is not used for a long time, the battery discharges more and more. Lead sulphate is formed on both plates. At first it appears like the raw materials in powder form, but in reality they are tiny crystals. These have a large surface area that allows a quick response when loading. However, they have the unpleasant quality that they grow together. If the battery is left idle at low voltage for a long time, large and hard crystals will form. On the one hand, these have a comparatively small surface area, which is synonymous with lower capacity, and, on the other hand, they can hardly be destroyed by charging. That means a huge loss of capacity. In this case one speaks of “coarse crystalline sulfation”. It ultimately leads to a total failure of the battery. It is important to ensure that the battery is always well charged. Especially with seasonally used vehicles such as two-wheelers, mobile homes, motor boats, snowmobiles, etc., the problems can be foreseen with certainty after a long period of non-use.


There are various devices on the market that are intended to prevent coarse crystalline sulfation. Usually a capacitor with a large capacity is charged repeatedly, which suddenly emits a strong current surge when discharged. Because this happens several times a minute, a charging current - albeit a very small one - should prevent the crystals from growing together. The natural resonance of the sulfate crystals can also be used to break down or destroy them. A good functional description of such battery desulfators, activators or pulsers can be found in the link: [2].

If the vehicle is to be idle for a longer period of time, it is always advisable to disconnect the negative pole of the battery and, if possible, connect a trickle charger. This is a charger with a very low charging current (approx. 50 to 100 mA), with a voltage limited to 14.4 volts if possible. This current compensates for the self-discharge without causing damage. Such devices are also offered solar-powered. The battery does not even have to be disconnected.

Other batteries, aircraft use

Compared to other battery technologies, the lead battery is very heavy with the same storage capacity. In aircraft (petrol engine or turbine drive), nickel-cadmium batteries (decreasing), increasingly nickel-metal-hydride batteries, silver-zinc batteries and recently also lithium-ion batteries are used as starter batteries.

Automotive engineering

The English technical term for starter batteries in motor vehicles is SLI battery For Start, light, ignition / Start, lights, ignition. When the internal combustion engine is running, the battery is recharged by a generator, the so-called alternator.

In addition, it supplies the electrical consumers in the vehicle when the alternator does not run or runs too slowly - a task that is becoming more and more important with a growing number of convenience functions in the car.

nominal voltage

The actual voltage of the on-board network of motor vehicles is above the nominal voltage of the battery while driving, since it is supposed to be charged while driving. The end-of-charge voltage is temperature-dependent. For 12-volt batteries, it should be 14.4 volts (reason and context, see above). Nevertheless, the nominal voltage of the battery is usually given as the voltage of the vehicle electrical system. Cars are usually 12 volts, trucks are 24 volts, older cars (e.g. older VW Beetles) and some motorcycles also have 6 volts.


The indication of the capacity Q takes place in the unit of measurement ampere hours (Ah) for here e.g. 20 hours of discharge time T at 27 ° C (K20). A fully charged starter battery with a specified nominal capacity Q = 36 Ah can then have an average current of I. = Deliver 1.8 amps. With the formula Q = I.·T For a given capacity and a given time, the mean current follows - with a slightly decreasing voltage also decreasing I. = Q/T, so here:

If a discharge current is known, the maximum possible time results from:



With higher amperages, lower temperatures or advanced aging of the starter battery, the actual capacity is lower than the nominal capacity.

During a discharge with a constant current strength, the speed at which the voltage of the starter battery falls changes. The mean value of the voltage during the discharge time, which would enable the energy or work to be calculated in the unit of measurement watt-hour (Wh), is not given.

Examples of the capacity of starter batteries

  • 50cc scooter: 6 Ah (12 V)
  • Motorcycle: 12 Ah (12 V)
  • Small car: 36 Ah (12 V)
  • Car (compact class): 50 Ah (12 V)
  • Car (upper class): 100 Ah (12 V)
  • Truck: 100 Ah and more (12 V, 24 V)

The required capacity depends on both the cubic capacity and the type of engine. Due to their higher compression, diesel engines generally require a higher starter current than comparable large gasoline or gas engines. The presence of heavy electrical consumers also influences the required capacity, since the starter battery serves as a buffer when the alternator speed is low and consumption is high. Some vehicle manufacturers therefore deliver vehicles with air conditioning as standard with a more powerful starter battery.

Maintenance, care and testing

  • Even with maintenance-free batteries, the fluid level (commonly called “the electrolyte”) should be checked at least every autumn. Top up with distilled water if necessary.
  • Check of the controller by a specialist workshop for charging voltage and charging current. The charging voltage must be at least 13.80 volts and should not exceed 14.4 volts.
  • Battery chargers should work with approx. 14.4 volts in the upper charging range and the charging current for unregulated chargers should not exceed a tenth of the capacity of the battery. When the battery is deeply discharged, rapid charging with high currents is possible up to approx. 70% of the full charge, but here too the voltage must not exceed 14.4 volts.
  • The battery should be checked after charging. Various systems are available on the market. The acid density can be determined using a hydrometer. It is important to note the density of this battery when fully charged.

The most common density of 1.28 g / cm³ when fully charged is assumed here as an example: When fully discharged, the density has dropped to 1.10 g / cm³, at 1.23 g / cm³ it is only half charged. If you work with such a spindle, you get a good overview of the state of charge, but you have to open the cells and suck in a sample of the electrolyte. This is only recommended if you have sufficient experience.

Another possibility is to measure the battery voltage. No dismantling is required for this. Most devices are simply connected to the starter battery. It should be noted that this measurement can only be carried out when the battery has settled down, i.e. when the battery has settled down. H. about 2 hours after the last charge / ride / discharge. The calmed battery shows a voltage of 12.65 volts when fully charged. The voltage should not drop below 12.53 volts, which is approx. 85% of the full charge. At 12.25 volts the battery is half charged, at 11.9 volts it is almost empty. Should it continue to be discharged, it can only achieve part of its original capacity even if it is subsequently fully charged. Capacity check: voltage says nothing about capacity. An old battery, when fully charged, has the nominal voltage, but does not reach the high instantaneous current when starting (400 to 600 A). When buying a used battery, you load a heating resistor (low resistance) or operate the starter and measure the voltage. This does not drop so much and not to zero with a new battery, but with an old one. There are maintenance-free batteries that cannot be refilled with distilled water. The cells are filled with a mixture of sulfuric acid (about 37%) and (distilled) water. The water in the form of its constituents hydrogen and oxygen can escape as a gas through electrolytic decomposition. H. closed not. In the case of open batteries, the loss must be compensated for by topping up with distilled water. Impure water, including tap and mineral water in this case, would render the battery unusable within a short period of time (short circuit in the electrolyte). Poorly working voltage regulators of the alternator favor the decomposition of the water and require more maintenance of the battery.

The voltage of the voltage regulator should ideally be between 13.8 and 14.4 volts. If it is higher, maintenance-free batteries quickly lose too much water, which has a negative effect on their service life.If the voltage is lower, the battery may not be fully charged, which makes subsequent starting processes more difficult and shortens its service life. If the battery then has to use too much of its capacity, there is a risk of deep discharge and sulphation, which is harmful to the battery.

Starter batteries should also not be left uncharged for long periods of time (several months). If a battery has to stand still for a long time, it should be fully charged beforehand. Older starter batteries have an increased self-discharge, and if the battery is left standing without recharging there is an increased risk of harmful sulfation. Leaving it standing for too long will therefore damage the battery. The voltage of a 12 V lead-acid battery should not drop below 11.8 volts.

If the device is not used for a long time, so-called trickle charging with a low current that only compensates for self-discharge is also helpful.

The Charging voltage should be in the range of 14.2 to 14.4 volts at about 15 to 25 ° C. The Charging current Should be a tenth to a maximum of a fifth of the value of the battery capacity with unregulated chargers and not exceed a third of the value of the capacity even with fast charging. In the case of voltage-regulated chargers, it is not necessary to limit the charging current.

The Gassing voltage is around 14.4 volts and should not be exceeded, especially when charging maintenance-free starter batteries.

The terminal voltage shortly after the end of the charging of a starter battery that has just been fully charged will first drop quickly from the charging voltage to around 13.2 volts and from there on more slowly to around 12.7 volts.

The approximate capacity of a starter battery can also be estimated from the terminal voltage. The voltage is measured directly on the battery that has been unloaded and uncharged for several hours:

Clamping voltage Approximate capacity
12.65V 100 %
12.45V 75 %
12.24V 50 %
12.06V 25 %
11.89 V 0 %

The procedure only gives a reasonably usable capacity information if the battery has not become highly resistive. Or the open-circuit voltage is generally below the nominal voltage due to a cell short-circuit. A high-impedance battery can be recognized by the fact that it is "full" very quickly when charging (i.e. it no longer accepts any current), but the voltage immediately collapses again - even when small currents are drawn. If, on the other hand, the starter battery is still OK, it should be able to deliver around three times its nominal capacity for a few seconds without any problems and without the voltage dropping too much.

In the case of non-maintenance-free versions (those with screw plugs), an acid siphon with hydrometer can also be used to check the capacity.

Acid density Approximate capacity
1.28 kg / dm³ 100 %
1.24 kg / dm³ 50 %
1.10 kg / dm³ 0 %

Another problem that can lead to the discharge of the starter battery is due to leakage currents and corroded pole caps or connection terminals. Leakage currents can occur if the surface of the battery or the poles are dirty (for example due to environmental influences such as dirt and moisture). Corroded connections lead to increased contact resistance and have a negative effect on starting behavior. They also prevent the alternator from charging the battery evenly. Care should be taken to ensure that the connections are clean and that the contact surfaces are firmly connected to the poles of the battery. The use of pole grease also provides protection against corrosion.

Safety instructions:

  • When charging with the battery charger, remove the plug, avoid sparking (do not smoke). Risk of explosion as oxyhydrogen gas is produced.
  • Keep children away from batteries.
  • Battery acid is very corrosive, therefore wear protective goggles and protective gloves. Do not tilt, as acid can escape through the vent openings.
  • Immediately rinse out acid splashes in the eye with cold water, then consult an ophthalmologist immediately.

Take-back regulation for starter batteries

In the Ordinance on the return and disposal of used batteries and accumulators, Battery Ordinance for short (BattV), paragraph 6 stipulates that distributors of starter batteries who hand them over to end consumers are obliged to charge a deposit of 7.50 euros including sales tax, provided that no old battery is returned when purchased (status : January 1, 2002).

Batteries that have already been installed, for example in new cars, are not subject to this deposit ordinance.

See also

  • Jump start
  • Vehicle technology list of topics

Category: Battery