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Consider the impact of the discharge current, temperature on the capacitance and discharge voltage of the lead-acid batteries.




The active agents of the accumulator are concentrated in an electrolyte and the positive and negative electrodes, and set of these substances is called electrochemical system. In lead-acid rechargeable batteries an electrolyte is solution of sulfuric acid (H2SO4), the active agent of the positive plates - dioxide of lead (PbO2), the negative plates - lead (Pb).

The main processes which are taking place on electrodes describe reactions: On a negative electrode: Pb + HSO4-→ PbSO4 + H + + 2e-(charge); PbSO4 + H + + 2e-→ Pb + HSO4-(discharge). On a positive electrode: PbO2 + HSO4-+ 3H + + 2e-→ PbSO4 + 2H2O (charge); PbSO4 + 2H2O → PbO2 + HSO4-+ 3H + + 2e-(discharge). Total reaction in the lead accumulator has an appearance: PbO2 + Pb + 2H2SO4 → 2PbSO4 + 2H2O (charge); 2PbSO4 + 2H2O → PbO2 + Pb + 2H2SO4 (discharge). Nominal capacity of the lead-acid accumulator the capacity received at the category during 20 h i.e. current 0,05C is considered. The capacity given by the accumulator considerably depends on current of the category which can reach several C. The greatest influence on service life of the pressurized lead-acid accumulator is exerted: working temperature, depth of the category and size of a recharge, and also frequency of operation of the valve for gas dumping. The pressurized lead-acid accumulators are very sensitive to a recharge. At a long recharge (current of 0,25 Sn) as fresh accumulators, and after one and a half years of operation in the mode of a constant subcharge, and also at a charge of accumulators at the overestimated tension (2,6B), the extraordinary warming up of accumulators didn't occur. Temperature is stabilized later 4-6 h at the level of 50-70 °C or then slowly goes down. But because of emission of gases via the emergency valve there is a drainage of accumulators and their fast degradation.

The sealed lead rechargeable batteries are efficient in the range of temperatures from-30 to +50 °C, working capacity is guaranteed to a thicket at a temperature not below -15 °C. At lower temperatures of a possibility of discharge freezing of an electrolyte hinders. Operability of accumulators at low temperatures can be provided with increase in concentration of an electrolyte as it and becomes in special accumulators.

 

 

29. Describe the main characteristics of the battery: voltage, capacity, specific energy.

The amount of energy that can be stored in the battery, is called its capacity. It is measured in ampere-hours. A AB 100 Ah capacity can supply the load current of 1 A for 100 hours, or 4 amps for 25 hours, and the like, although the capacity of the battery decreases with increasing discharge current. In the market sold the batteries with capacities from 1 to 2000 Ah.

To extend the life of a lead acid AB desirable to use only a small fraction of its capacity before recharging. Each process is called discharge-charge cycle battery, but not necessarily completely discharge the battery. For example, if you discharge the battery for 5 or 10%, and then re-charge it - is also counted as 1 cycle. Of course, the number of possible cycles will be very different at different depths of discharge. If it is possible to use more than 50% of the energy stored in the AB before its charge, without noticeable degradation of its parameters, such battery called battery "deep discharge".

The voltage on the battery is often the main parameter by which to judge the state and the battery's charge. This applies particularly to sealed batteries that have not possible to measure the density of the electrolyte.

The voltage at a charge, discharge and lack of power are very different. To determine the degree of charge of the battery voltage is measured at its terminals in the absence of a charging and discharge currents for at least 3-4 hours. During this time, the voltage usually has time to stabilize. The value of the voltage when charging or discharging will not say anything against the state or the state of charge of the battery. Approximate dependence of the degree of charge of the battery voltage at its terminals in idle mode is shown in the table below. These are typical values ​​for starter batteries with liquid electrolyte. For sealed batteries (AGM and gel) is usually the voltage is slightly higher (you need to ask the manufacturer) - for example, AGM batteries are fully charged when the voltage is 13-13,2V (compare with the voltage of starter batteries with liquid electrolyte 12,5-12,7V).

Specific energy, or gravimetric energy density, defines battery capacity in weight (Wh/kg); energy density, or volumetric energy density, reflects volume in liters (Wh/l). Products requiring long runtimes at moderate load are optimized for high specific energy; the ability to deliver high current loads can be ignored.

The C-rate specifies the speed a battery is charged or discharged. At 1C, the battery charges and discharges at a current that is on par with the marked Ah rating. At 0.5C, the current is half and the time is doubled, and at 0.1C the current is one-tenth and the time is 10-fold.

A load defines the current that is drawn from the battery. Internal battery resistance and depleting state-of-charge (SoC) cause the voltage to drop under load, triggering end of discharge. Power relates to current delivery measured in watts (W); energy is the physical work over time measured in watt-hours (Wh).

Watts and Volt-amps (VA). Watt is real power that is being metered; VA is the apparent power that is affected by a reactive load. On a purely resistive load, watt and VA readings are alike; a reactive load such as an inductive motor or fluorescent light causes a phase shift between voltage and current that lowers the power factor (pf) from the ideal one (1) to 0.7 or lower. The sizing of electrical wiring and the circuit breakers must be based on VA power.


 

30. The internal resistance of chemical power sources.

A practical electrical power source, which is a linear electric circuit, may, according to Thévenin's theorem, be represented as an ideal voltage source in series with an impedance. This impedance is termed the internal resistance of the source. When the power source delivers current, the measured voltage output is lower than the no-load voltage; the difference is the voltage drop (the product of current and resistance) caused by the internal resistance. The concept of internal resistance applies to all kinds of electrical sources and is useful for analyzing many types of electrical circuits.

A battery may be modeled as a voltage source in series with a resistance. In practice, the internal resistance of a battery is dependent on its size, chemical properties, age, temperature, and the discharge current. It has an electronic component due to the resistivity of the component materials and an ionic component due to electrochemical factors such as electrolyte conductivity, ion mobility, and electrode surface area. Measurement of the internal resistance of a battery is a guide to its condition, but may not apply at other than the test conditions. Measurement with an alternating current, typically at a frequency of 1 kHz, may underestimate the resistance, as the frequency may be too high to take into account slower electrochemical processes. Internal resistance depends on temperature; for example, a fresh Energizer E91 AA alkaline primary battery drops from about 0.9 Ω at -40 °C, when the low temperature reduces ion mobility, to about 0.15 Ω at room temperature and about 0.1 Ω at 40 °C The internal resistance of a battery may be calculated from its open circuit voltage V NL, load voltage V FL, and the load resistance R L: . {\displaystyle R_{\text{int}}=\left({{\frac {V_{\text{NL}}}{V_{\text{FL}}}}-1}\right){R_{\text{L}}}}

Many equivalent series resistance (ESR) meters, essentially AC milliohm-meters normally used to measure the ESR of capacitors, can be used to estimate battery internal resistance, particularly to check the state of discharge of a battery rather than obtain an accurate DC value. Some chargers for rechargeable batteries indicate the ESR. In use, the voltage across the terminals of a disposable battery driving a load decreases until it drops too low to be useful; this is largely due to an increase in internal resistance rather than a drop in the voltage of the equivalent source. In rechargeable lithium polymer batteries, the internal resistance is largely independent of the state of charge but increases as the battery ages; thus, it is a good indicator of expected life.

 

 





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