Rechargeable batteries are devices in which electrical energy is converted into chemical and vice versa, unlike standard batteries that can not be recharged after they have been used up. When the battery terminals are connected to a DC power source, it begins to charge. Chemical energy is accumulated. During the battery’s operation, the chemical energy is converted into electrical by means of power-generating processes and the battery discharges, i.e. works as a galvanic element. It may then be charged again. Each battery consists of positive and negative plates placed in a container, where there is electrolyte. The battery works as a chemical current source with a reversible multiple operation. The highest possible effective battery charge is called its charging capacity or just capacity. In the international system for indicating electrical quantities the battery capacity is measured in coulombs C but in everyday life more popular is the informal unit ampere-hour – Ah. The relation between the two units is 1 C = 1/3600 ampere-hours or 1Ah = 3600C. Although by definition the rechargeable battery can be repeatedly charged and discharged, its current and voltage fall over time, when the chemical energy generated by the battery’s cells is depleted, because we know that batteries are secondary sources of electricity.
At the end of its useful life the battery simply stops working. When charging the battery we need to take into account several factors. The most common rule is that the charging current must be one-tenth of the battery capacity in ampere-hours. Example: If the battery is 62Ah, and the current of the charger is 1A, the full charge time is 62 hours. The second factor is that the charging current must not be greater than 6.2A for the case of this example. The charging voltage must be several volts higher than the rated voltage of the battery at rest. If it is slightly higher, the charging will proceed more slowly, but the saturation of the cells will be more complete. Both overcharging and incomplete charging are not good for the battery. We will not discuss all the details because there are a lot of things written on the topic. Basic parameters of the batteries are EMF (electromotive force), capacity, efficiency, durability, time of self-discharge, internal resistance, voltage of the terminals, weight and sizes.
Types of rechargeable batteries
1. Lead-acid batteries
2. Alkaline batteries
– iron-nickel batteries;
– cadmium – nickel;
1. Based on lithium (except for the basic type):
- lithium- chlorine ;
- lithium- sulfur;
2 . Based on nickel (except for the basic type):
- nickel – zinc ;
- sodium – nickel – chloride;
- nickel- cadmium ;
- nickel-salt ;
- nickel -metal hydride ;
- nickel – hydrogen
3. Based on lead (except for the basic type):
- lead – hydrogen
4. Based on zinc:
- zinc – bromine;
- zinc – chlorine
5. Based on silver: silver- cadmium; silver- zinc, etc. Each type of battery has its advantages and disadvantages. We will discuss the most common types of rechargeable batteries: lead- acid and alkaline.
Lead-acid batteries consist of positive and negative lead plates containing antimony, placed in a suitable container, filled with electrolyte. The electrolyte contains sulfuric acid diluted with distilled water. In the plates there are cells filled with lead paste. It is lead oxide in the negative plates and minium in the positive plates. After the cells are filled, each plate undergoes further processing to convert it into a monolithic entity. You can distinguish the positive from the negative plate by their color: the positive plate is brown, the negative – gray. To reduce the size of the battery as well as its resistance, the plates are placed very close to each other, which may lead to short circuits. To avoid this problem we need to place acid and heat-resistant insulating separators, having a porous structure. The electrolyte is produced by chemically (not technically) pure concentrated sulfuric acid (92-94 %), which is slowly poured into distilled water (not vice versa) while constantly stirring the solution. The electrolyte must be poured on the plates and reach at least 10 mm above their highest point.
The casing of the rechargeable battery is made of rubellite , decillite, mipolam, etc. During the operation (discharging) of batteries the following processes occur: the electrolyte breaks down into ions – H2and SO4. H2 goes to the positive plate to form lead sulfate and SO4 goes to the negative plate and also forms lead sulfate, and the electrolyte is diluted and forms water. During charging, the following processes occur in the battery: SO4 goes to the positive plate, combines with the hydrogen from the water and forms sulfuric acid. The positive plate is of lead dioxide, too. Accordingly, H2 goes to the negative plate, combines with SO4 to form sulfuric acid, the active mass of the plate is again pure Pb. The density of the electrolyte increases.
Each battery, left without a load connected to its terminals, self-discharges over time for the following reasons: the emergence of internal parasitic currents between the plates coming from the difference in density of the electrolyte at the top and the bottom of the battery, derived from precipitation; the presence of metallic and other impurities in the electrolyte and plates; self-discharge increases at higher temperature. Another problem with lead batteries is the emergence of sulfatization. The process begins during the battery’s operation. Through a chemical reaction producing electricity, the active part of the plates is converted into lead sulfate in the form of small crystals. When recharging, this sulfate is easily broken down, but if no charging follows or charging is incomplete, the crystals of the sulfate become larger and adhere to the plates. Sulfated plates have higher resistance, they are more quickly discharged, their capacity is reduced and the voltage of their poles is reduced.
Alkaline batteries have certain advantages over lead batteries and they are: the electrolyte is chemically inert to the material of which the plates are made, therefore the self-discharge is considerably less; they are not sensitive to overloads, short circuits, higher operating temperature; the off-gases are considerably less harmful than in lead-acid batteries; they are produced of less scarce materials. Alkaline batteries have some disadvantages such as: less capacity; less internal resistance, etc. They are called alkaline because their electrolyte is alkaline. It is a 21% solution of potassium hydroxide in distilled water, plus about 20g/l LiOH. The active mass is placed in perforated iron packs, each of them insulated with nickel plates. When properly used, alkaline batteries have very long life (decades).
The latest use of accumulator batteries is in electric cars, where due to the growing competition in this field, the quality of the batteries is constantly improved, the main indicator being the mileage of the car per one charging, the number of charge cycles as well as the charging speed. Rechargeable batteries are also used to store the energy produced by solar panels, so that it can be used later. They are also used in central heating to ensure the safe operation of the system in case of a sudden power failure. The application of these batteries in the future will further increase with the growing use of renewable energy sources in everyday life and the scarcity of fossil energy sources.