Alkaline battery
The alkali manganese cell, colloquially alkaline manganese battery or Alkaline (sometimes more accurately referred to as zinc - manganese oxide cell) is a galvanic cell and is among the most important electrochemical energy storage devices. It counts as the zinc-carbon cell to the family of zinc - manganese dioxide cells, but the zinc-carbon cell displaced from most applications due to higher capacity, better durability and longer shelf life.
The alkaline-manganese cell is to the primary elements, i.e. counting the non- rechargeable batteries, although it is in principle limited rechargeable. There are provided for recharging versions, the RAM cells ( Rechargeable Alkaline Manganese ), which are counted among the secondary elements (accumulators). However, these have not been widely distributed.
- 4.1 Power
- 4.2 discharge
- 4.3 leakage
- 4.4 Comparison alkaline manganese and zinc carbon cell (type: AA )
- 5.1 Normal alkaline batteries
- 5.2 RAM cells
General
The alkaline manganese cell was described aptly as a zinc - manganese dioxide cell with alkaline electrolyte, as the electrochemically active material in the negative electrode zinc, and manganese dioxide in the positive electrode ( terminal member of the manganese dioxide minerals group) is. As an electrolyte, an alkaline solution, and in concentrated caustic potash (potassium hydroxide solution) is used.
The main types are cylindrical round cells ( for example, = LR6 alkaline AA or AA ) and button cells (eg LR44). Several individual cells can also be combined to batteries (such as alkaline 6LR61 = 9 volt block of six cells). In Germany 2004, about 800 million alkaline round cells and some 400 million alkaline manganese button cells were placed on the market.
Electrochemistry
As with the zinc -carbon battery, the oxidation of zinc and the reduction of manganese dioxide ( pyrolusite ) supplies the electrical energy. The electrons released during the oxidation of the cell migrate from the anode, which is the negative electrode in this case a battery, with power output by the external circuit to the cathode, which is the positive electrode in this case. To balance the charge migrate into the cell through the electrolyte OH - ions from the cathode to the anode. The process taking place in the alkaline manganese cell reactions are complex and are shown in simplified form below.
Anode reaction
Upon discharge, metallic zinc is (Zn ) is oxidized at the anode. Two electrons are released, the oxidation number of zinc is increased from ± 0 to II. The reaction product depends on the conditions under which the oxidation is carried out. At the beginning of the discharge, that is, at high OH - concentration is readily soluble in the alkaline electrolyte Tetrahydroxozinkat ion ( Zn (OH) 42 - ), via various intermediate steps just zincate formed.
If the electrolyte is supersaturated with zincate, begins zinc oxide ( ZnO) to fail.
With progressive discharge, that is at a lower OH - concentration, then zinc hydroxide (Zn (OH ) 2) is formed. From this arises under release of water slowly turn zinc oxide ( ZnO).
Cathode reaction
The manganese dioxide used as a cathode material is typically an electrolytic manganese dioxide ( γ - MnO2 ) having a high electrochemical activity. In the discharge is reduced in the cathode of manganese dioxide ( MnO2) initially Manganoxidhydroxid ( MnOOH ). This homogeneous solid phase reaction is referred to as the first discharge stage.
In the reaction, an electron is added, the oxidation state of the manganese is lowered from IV III and a proton (H ) is incorporated in the crystal lattice of the manganese dioxide. This reaction is unusual in that the product α - MnOOH ( Groutit ) has the same crystal structure as the starting material γ - MnO2.
Under certain conditions, can be further reduced at mild discharges in a slow reaction Manganoxidhydroxid ( MnOOH ). This reaction is referred to as the second discharge stage.
This reaction is a heterogeneous reaction, the actual reduction takes place in solution. The Mn3 ions are present as a complex [Mn (OH ) 4] - in solution and to [ Mn ( OH) 4] 2 - is reduced. The actual solid product Mn ( OH) 2 is then precipitated from the saturated [ Mn ( OH) 4] 2 - from solution.
Redox reaction
If only the first discharge stage taken into account, the result for the overall reaction in the alkaline manganese cell:
As can be seen from the overall reaction equation is consumed in the discharge water, a spent alkaline cell is thus "dry".
Side reactions
Zinc is thermodynamically unstable in strongly alkaline solution. As shown in the electrochemical series is therefore oxidized as a side reaction in the anode zinc (Zn ) and water ( H2O) to hydrogen gas (H2 ) is reduced.
Such as " gassing " described reaction proceeds during the storage of non-discharged and partially discharged cells. The reaction rate is relatively low for high purity zinc. However, even small amounts of impurities ( for example, heavy metals such as iron, copper, molybdenum, and nickel) can dramatically increase the gassing.
Construction
The picture shows the basic structure of a commercially available alkaline round cell. Evident is a constructive precaution against over-pressure cells, for example, by short circuit or overheating.
The picture on the right shows two cutaway alkaline manganese cells and the separated top. The positive pole is, unlike a zinc -carbon battery, part of the outer casing and electrically connected to it. Inside squeezed Braunstein is the cathode to recognize as a black ring inside the jacket in the picture. The anode comprises a paste of zinc and potassium hydroxide, wrapped in ion- permeable fiber paper. In the middle view, it extends beyond the cut cell. Left you can see the metal pin, which establishes the electrical contact with the bottom plate and forms the negative pole.
A 9-volt alkaline battery block of 6 individual internal cells may be constructed differently. Either it contains 6 round cells of size Piccolo AAAA LR61 (left image). These batteries carry the size designation 6LR61.
The second design uses 6 rectangular button cells in a stack, these types are designated with 6LF22 (right image).
The encapsulation of single cells in the 9V pack increases the leakage resistance, but reduces the packing density and thus the capacity.
Properties
Voltage
The rated voltage of the alkaline-manganese cell is 1.5 V. By connecting a plurality of cells, higher voltages are achieved. Thus, in the normal battery 3LR12 with three 4.5 V cells, the Flat Pack 4LR61 6V with four cells and the E- block 9V 6LR61 with six cells can be achieved.
The actual open-circuit voltage of a fresh alkaline cell at 20 ° C in the range of 1.57 V to 1.63 V. The mainly depends on the activity of the Mangandioxides used and the zinc oxide in the electrolyte solution. The (average ) load voltage depends on the load, it is the case of discharge with 0.4C typically 1.15 V to 1.18 V ( NiMH: 1.22 V to 1.25 V). As a final discharge usually 1.0 V is used.
Discharge
The chart compares the load curves at constant current of a zinc-carbon cell ( Zn -Mn (C )) and a NiMH rechargeable batteries with an alkaline cell ( Zn -Mn (KOH ) ). The voltage of the zinc-carbon cell drops after a short time under 0.8V. A battery keeps the voltage of 1.2 V for a long time. The time behavior of an alkaline - manganese cell is between the two curves, the voltage slowly decreases over time.
Battery Level Indicators in devices measure the change in voltage with time. While they work fairly reliably in primary cells, they fail to NiMH cells. Here, the voltage over a long period is almost constant to decay quickly deplete the battery.
Voltage - capacity curve at a load of about 300 mW (about 0.1 C)
The capacity of an alkaline-manganese cell is dependent on the load. At low load << 0.1 C batteries typically reach values of 3000 mAh, see left chart. The second graph shows the variation of capacity with a load of about 0.1 C. The battery is exhausted already at 2/ 3 of the rated capacity. In contrast to the behavior at low load, it regenerates itself after a short time. She is again in a position to provide a capacity of about 10% available, see bottom curve in the second graph.
Practice: alkaline batteries in high power consumers seem to be empty after a short time. After a break of several hours they work for a short time again. Slightly elevated temperature accelerates this regeneration. Therefore, the effect of the " charging" of batteries on the stove or in the sun touched. Then, however, these cells are no longer able to meet the high power requirements. From the perspective of high-performance consumer batteries are exhausted, although they can still have a residual capacity of 30%. Instead of disposing the cells, they should be used for consumers with lower power requirements, such as clocks or remote controls.
Leakage
Alkaline batteries are protected by a metal casing before departure. With the old zinc-carbon cells, the zinc cathode forms at the same time, the battery case. When unloading decomposes the zinc and often result in the leakage of the cell.
However, alkaline batteries may leak and leaking potassium hydroxide may corrode metallic components such as contacts and interconnects. The potassium hydroxide reacts with the carbon dioxide ( CO2) in air to potash ( K2CO3) and forms white crystalline hygroscopic deposits.
Comparative alkaline manganese and zinc carbon cell (type: AA )
Carbon-zinc cells have a much worse situation than voltage alkaline manganese cells, that is, the voltage falls partially discharged cells at an early stage (see discharge). The same applies to the high capacity makes them. Many modern devices that briefly high currents found (digital cameras, flash units, alarm systems ) are no longer usable High current drain can be even with fresh zinc - carbon cells to fall too low, the voltage in a few seconds to ensure safe operation of the equipment. The self-discharge of carbon-zinc battery cells is also significantly higher than that of alkaline manganese cells, which limits the shelf life.
Also in the economy and life cycle assessment, the zinc -carbon batteries are at a disadvantage, especially by the poorer rated capacity for the same size in addition lower usable capacity by the poorer voltage level. This significantly more cells are required for the same energy expenditure ( operating time of a device).
In addition, the leakage resistance is often worse. Expiring ammonium chloride solution of spent cells destroys the metal contacts and conductors of an electronic device much faster than the alkaline electrolyte of alkaline cells.
These significant differences have led to alkaline manganese cells now dominate the market and have replaced the zinc-carbon cells.
Recharging
Normal alkaline batteries
Most alkaline manganese batteries can be ( for example those for RAM cells, turning the next section) chargers suitable brush three to ten times. The prerequisite is that the battery is not discharged too deeply. Not suitable for charging battery chargers for standard NiMH batteries, as the threshold value for the charging voltage and the charging process will not fit.
RAM cells
RAM cells ( rechargeable alkaline manganese English ) are special, according to the supplier about 50 to 500 times (some even more ) rechargeable alkaline manganese cells. Commercial Chargers for RAM cells operate with a constant charging current is interrupted every second for a few milliseconds to normally measure the cell voltage. If it exceeds 1.73 V, the charging current is switched off until the cell voltage is 1.69 V below again. The constant voltage method is also safe and suitable, but slower. RAM cells are only suitable for low-power applications, such as clocks or remote controls. For high-current applications such as digital cameras, cordless tools or as traction batteries in model vehicles, they are not suitable and can be damaged.
RAM cells are allowed in order not to lose their ability to recharge, can not be discharged too deeply. Will RAM cells discharged to a discharge voltage per cell of 1.42 V, the achievable number of cycles is some 100 When discharged to 1.32 V reduces the number of cycles to some 10 In another discharge RAM cells can not more or loaded only with a significantly reduced capacity.
Price and performance
Commercial alkaline manganese batteries are offered with large price differences. In a consumer test was found for a brand battery that has been offered for six times the price of a cheap battery, a 25 % longer service life compared to this.
Disposal
Alkaline manganese batteries must be disposed of in Germany to the battery law. To this end, the relevant trade collecting the Foundation Common battery return system.
Batteries do not belong in other waste collection or in the environment because they contain environmentally harmful substances and retrievable.