Define the characteristics, which allow to compare electrochemical cells with each other.

At the heart of any power source, and battery is not that other, as the source current on a simple circuit: anode and cathode and between the electrolyte. Due to the different nature of the material of the anode and cathode when immersed in electrolyte a potential difference – the voltage which an electric current. Chemical current sources are its name because of the nature of power: the chemical energy of active substances is converted directly into electrical energy. They are divided into two groups – primary and secondary. In the primary power sources (batteries), the process proceeds irreversibly. Secondary power sources include batteries that can be recharged after they are exhausted itself. Different devices work with different voltage, so the battery it should be different. In addition, the voltage different types of batteries depends on the used electrolyte. For example, lithium batteries have a nominal voltage of 3 V, alkaline is 1.5 V. the capacity of the battery is calculated from the volume of active elements placed in the housing of the battery. However, calculated in this way capacity can not be used to determine the health of batteries and has a title of "design capacity". Actual capacity depends on many factors: level of charging; mode of use; ambient temperature; cut-off current (Voltage at which the device does not work even though saved battery life. For example, a battery which no longer works in the camera often continues working in hours or control panels).

Each cell batteries produces 1.5 volts TOKK that a little compared to the 220-volt household voltage' the mains. So the batteries are not dangerous for the consumer. Any battery, the voltage which is higher than 1.5 volts (e.g., 6 volts) is, in essence, a set of batteries connected in series at 1.5 volts. Exceptions are the rechargeable Nickel-cadmium batteries, the voltage which is charged only 1.2 volts. Electric battery power. The quantity of electricity in batteries is measured in ampere or milliampere-hours. If for example, the battery would be 1.0 ampere-hour and an electrical device in which it operates, requires a current of 200 milliamps (i.e., 0.2 ampere), the validity of the battery is calculated according to the following formula: = validity period (in hours)

In the example, this period will be five hours (1,0 : 0,2 = 5). Self-discharge is a consequence of the non-working state of the battery, which leads to capacity loss. The storage mode can occur for two reasons. First, it concerns a new product since the release of before using. Second, if you use the resource of batteries with sufficiently long intervals-breaks. The reason lies in the self-discharge of the battery of the instability of the electrode, contamination of the electrolyte. Usually, the normal shelf life of the battery loses about 30% of its initial capacity. The most severely discharged battery at the beginning of storage. Also the self discharge rate increases with increasing temperature.

 

 

15. Define the charge\discharge curves of the batteries. Give examples.

The Charge/Discharge Curve. The measured terminal voltage of any battery will vary as it is charged and discharged. The MPV (mid-point voltage) is the nominal voltage of the cell during charge or discharge. The maximum and minimum voltage excursion from the nominal value is an important design consideration: a "flatter" discharge curve means less voltage variation that the design must tolerate. When peak charged, the actual cell voltage will be higher than the MPV. When nearing the EODV (end of discharge voltage) point, the cell voltage will be less than the MPV. The EODV is sometimes referred to as the EOL (end of life) voltage by manufacturers.

 

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Lead-Acid Batteries: Discharging. Connection of an electrical load allows electrons to flow from negative to positive terminals. This reduces the charge and the voltages at the electrodes. The chemical reactions are able to proceed, generating new electrons and generating the power that is converted to electrical form to drive the external electrical load. As the battery is discharged, the electrodes become coated with lead sulfate and the acid electrolyte becomes weaker. Charging. Connection of an electrical power source forces electrons to flow from positive to negative terminals. This increases the charge and the voltages at the electrodes The chemical reactions are driven in the reverse direction, converting electrical energy into stored chemical energy. As the battery is charged, the lead sulfate coating on the electrodes is removed, and the acid electrolyte becomes stronger.

 

 

 

 

16. Consider processes what is occur on the negative electrode during discharge and charge lead battery?

Discharge. In the discharged state both the positive and negative plates become lead (II) sulfate (PbSO₄), and the electrolyte loses much of its dissolved sulfuric acid and becomes primarily water. The discharge process is driven by the conduction of electrons from the negative plate back into the cell at the positive plate in the external circuit. Negative plate reaction Pb(s) + (aq) PbSO₄ (s) + H⁺ (aq) + 2e. Release of two conducting electrons gives lead electrode a net negative charge. As electrons accumulate they create an electric field which attracts hydrogen ions and repels sulfate ions, leading to a double-layer near the surface. The hydrogen ions screen the charged electrode from the solution which limits further reactions unless charge is allowed to flow out of electrode. Positive plate reaction: PbO₂ (s) + (aq) + 3 H⁺ (aq) + 2e PbSO₄ (s) + 2H₂O. The total reaction can be written as Pb(s) + PbO₂ (s) + 2H₂SO₄ (aq) 2PbSO₄ (s) + 2H₂O. The sum of the molecular masses of the reactants is 642.6 g/mol, so theoretically a cell can produce two faradays of charge (192,971 coulombs) from 642.6 g of reactants, or 83.4 ampere-hours per kilo-gram (or 13.9 ampere-hours per kilogram for a 12-volt battery). For a 2 volts cell, this comes to 167 watt-hours per kilogram of reac-tants, but a lea-acid cell in practice gives only 30–40 watt-hours per kilogram of battery, due to the mass of the water and other consti-tuent parts. Charging. Fully recharged: Lead anode, Lead oxide cat-hode and sulfuric acid electrolyte. In the fully charged state, the negative plate consists of lead, and the positive plate lead dioxide, with the electrolyte of concentrated sulfuric acid. Overcharging with high charging voltages generates oxygen and hydrogen gas by elec-trolysis of water, which is lost to the cell. The design of some types of lead-acid battery allow the electrolyte level to be inspected and topped up with any water that has been lost.

17. What are the curves shown in this figure? Compare them and make assumtions about their nature?

This picture is shown the curves of discharging of batteries. This red curve of lithium battery as lithium battery’s voltage is at 3.4 V. Second curve is may be lead as it’s voltage is 2 V. Lithium ion cells is more stable then lead cell. The advantages of Li-ion batteries. High voltage in the range of 2.5-4.2; 500-1000 life cycles and more; high specific energy and power; low self-discharge; no memory effect ; possibility of operating in a wide temperature rang at: a charge of 20 to 60 ° C; discharge at t -40 to +65 ° C. The advantages of lithium anode: Lithium has the most negative electrode potential of all metals: The water -3,055; The propylene carbonate -2,887; Lithium has a high energy density:11760 W · h / kg; lithium is very active metal; Thermodynamic calculations show the fundamental possibility of lithium recovery of all substances that could be used as an electrolyte solvent.

 

 

18. Which batteries provide high discharge current, nickel cadmium or lithium? Give examples.

Discharge characteristics of accumulators in case of different current densities are defined by the features of accumulators influencing value of their internal resistance. To such features first of all thickness of electrodes and their structural characteristics, density of assembly of a packet of electrodes, thickness and structure of a separator, quantity of an electrolyte and separate parameters of construction of the accumulator belong. The modern cylindrical Ni-cds accumulators with rolled electrodes allow higher discharge currents: for some types of accumulators the maximum long-time current makes 7-10C.

Fig 1. The discharge characteristic of the nickel - cadmium accumulator. From a figure it is visible how essential factor of external influence on electrical characteristics of accumulators is ambient temperature. Capacity, which can be received from the accumulator in case of 20 °C, the greatest. It almost doesn't decrease also in case of discharge in case of more high temperature. But at a temperature below 0 °C bit capacity decreases, and the it is more, than more discharge current. All lithium accumulators are characterized by rather good safety. Loss of capacity at the expense of a self-discharge of 5-10% a year. The self-discharge of Li-ion of accumulators constitutes 4-6% for the first month, then - it is significantly less: in 12 months accumulators lose 10-20% of the stocked reservoir. Li-ion of accumulators several times has less loss of reservoir, than at nickel - cadmium accumulators, both in case of 20 °C, and in case of 40 °C. Resource-500-1000 of cycles.