In this article we will discuss about:- 1. Methods of Charging Lead Acid Battery 2. Types of Charging Lead Acid Battery 3. Precautions during Charging 4. Charging and Discharging Curves 5. Charging Indications.
Methods of Charging Lead Acid Battery:
Direct current is essential, and this may be obtained in some cases direct from the supply mains. In case the available source of supply is ac then it is converted into dc by some means such as motor-generator set, rotary convertor set or rectifier. The circuit shown in Fig. 16.31 may be used for charging the secondary batteries from dc supply main.
The charging current is given by the expression:
Where V is supply voltage, Eb is battery voltage, R is current limiting resistance and r is the internal resistance of the battery.
If the source voltage is only slightly greater than the battery voltage, small source voltage or battery voltage variations will cause large variations in charging current. It is, therefore, better, whenever possible, to select a higher source voltage and a large resistance. This provides a more uniform charging current flow despite major changes in the battery voltage.
Charging of Batteries from AC Power Source:
The basic requirements of common ac source chargers are like those of the dc power source, namely, the source voltage must be significantly greater than the battery voltage and the impedance must limit the current. The impedance of an ac power source can be reactive, resistive or combination of these. Limiting the current by reactance has the advantage that the reactive impedance component does not contribute to rectification.
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Rectification is used to convert ac into dc. Most chargers make use of a transformer which permits optimized matching of the source voltage to the battery charging voltage. Another very important feature of the transformer is that it isolates the charging output from the ac supply mains and thus avoids the hazard of electric shock.
The rectifier used for this purpose may be mercury vapour rectifier (for medium and heavy charging loads only), copper oxide or selenium rectifiers or silicon or germanium rectifiers. The unit is usually provided with tapping points to give different low voltages e.g., 6, 12, 18, 24 V. This is most economical and simple method of charging batteries. The circuit diagram is shown in Fig. 16.32.
DC supply for battery charging from ac supply source can also be made available by employing a dc shunt wound generator coupled to an ac motor. In this method of charging batteries, the charging current can be controlled simply by regulating the generator excitation. In the event of failure of supply to the motor during charging, say because of blown fuses, the generator would continue to run as a shunt motor in the same direction of rotation, with no damage to battery.
Types of Charging Lead Acid Battery:
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There are mainly two types of charging namely constant voltage charging and constant current charging:
(a) Constant Voltage Charging:
In this method, the charging voltage is kept constant throughout the charging process. In this method the charging current is high in the beginning when a battery is in discharged condition, and it gradually drops off as the battery picks up charge resulting in increased back emf. Charging at constant voltage may be carried out only when the batteries have the same voltage, for example, 6 or 12 or 24 V. In this case source of current should have a voltage of 7.5, 15 or 30 V; these batteries are connected in parallel to the charging circuit.
The convenience of charging at constant voltage is that it allows cells with different capacities and at different degrees of discharge to be charged. The large charging current at the beginning of the charge is of relatively short duration and will not harm the cells. At the end of the charge the charging current drops to almost zero because the voltage of the battery becomes nearly equal to the voltage of the supply circuit. This method, is however, not very suitable for old, badly sulphated batteries which need prolonged charging at a slow rate.
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This method is the most common method of charging lead- acid batteries and has been used successfully for over 50 years for different types of lead-acid batteries. With this method of charging, the charging time is almost reduced to half, capacity is increased by approximately 20% but efficiency is reduced by approximately 10%.
(b) Constant Current Charging:
In this method of charging of batteries, the batteries are connected in series so as to form groups and each group is charged from the dc supply mains through loading rheostats. The number of batteries in each group depends on the charging circuit voltage which should not be less than 2.7 V per cell. The chaining current is kept constant throughout the charging period by reducing the resistance in the circuit as the battery voltage goes up. This method is usually employed for initial charging of lead-acid batteries and for charging portable batteries in general.
In order to avoid excessive gassing or overheating, the charging may also be carried out in two steps, an initial charging of comparatively higher current and a finishing rate of low current. In this method the charge current is kept one-eighth of its ampere-hour rating.
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The excess voltage of the supply circuit is absorbed in the series resistance. However, the groups of batteries to be charged should be so connected that the series resistance absorbs as little energy as possible. The series resistance used should be of current carrying capacity equal to or greater than the charging current required; otherwise the series resistance will overheat and burn out. The groups of batteries should be selected so that they all have the same capacity. If the batteries have the different capacities the charging current will have to be set according to the battery having the least capacity. This would both retard and complicate the charging.
Precautions during Charging Lead Acid Battery:
(i) During the charging period the temperature of the electrolyte should not exceed beyond 40 to 45 °C because of the danger of plate buckling. Therefore, the electrolyte temperature is carefully watched and if the temperature approaches the danger limit before the end of the charge, the charging current should be reduced or the charging discontinued until the batteries cool down.
(ii) While the charging is in process the fans in the battery room should be kept operating continuously. This is especially important during the last period of the charging in order to exhaust from the premises the dangerous oxygen and hydrogen gas evolved by the batteries during charging.
(iii) The gases, hydrogen and oxygen evolving from the batteries under charge can explode if spark or flame is brought too near. No metal tool should be used on batteries without switching off the battery charger.
(iv) Before the end of the charging it is necessary to examine all the batteries and make sure that gassing begins simultaneously in all the cells and with the same intensity. It is also necessary to measure the density of the electrolyte and the voltage of the battery cells.
Battery Charging Regulator:
The basic components of the circuits are shown in Fig. 16.33. Diodes D1 and D2 are to establish a full-wave rectified signal across SCR1 and the 12-V battery to be charged.
When the full-wave rectified input is large enough to give the required turn-on gate current (controlled by resistor R1), SCR1 will turn on and the charging of the battery will commence. At the commencement of charging of battery, voltage VR determined by the simple voltage-divider circuit R4 and R5 is too small to cause 11.0 V zener conduction. In the off state, zener diode is effectively an open circuit maintaining SCR2 in the off state because of zero gates current.
The capacitor C is included in the circuit to prevent any voltage transients in the circuit from accidentally turning on of the SCR2. As charging continues, the battery voltage increases to a point where VR is large enough to both turn on the 11.0 V zener diode and fire SCR2. Once SCR2 has fired, the short circuit representation for SCR2 will result in a voltage-divider circuit determined by R1 and R2 that will maintain V2 at a level too small to turn SCR1 on. When this occurs, the battery is fully charged and the open- circuit state of SCR1 will cut off the charging current. Thus the regulator recharges the battery whenever the voltage drops and prevents overcharging when fully charged.
Charging and Discharging Curves:
Typical charge and discharge curves (variations in terminal voltage) of a lead-acid accumulator are shown in Fig. 16.34. When the cell is charged, the voltage of the cell increases from 1.8 V to 2.2 V during first two hours, then increases very slowly, rather remains almost constant for sufficient time and finally rises to 2.5 to 2.7 V.
When a charged storage cell has just been disconnected from a charging source, its terminal voltage falls rapidly to 2.2 V. On discharge, the voltage of the cell drops to 2.0 V in the beginning; remains constant for sufficient time and falls to 1.8 V finally, as illustrated in Fig. 16.34.
Caution:
The cell should never be allowed to discharge beyond 1.75 V otherwise lead sulphate will be formed on the electrodes which is hard, insoluble and increases the internal resistance of the cell. The conversion of active material into lead sulphate is termed sulphatization.
Charging Indications for Lead Acid Battery:
Full charging of lead-acid accumulator (or cells) can be judged from the following indications:
1. Gassing:
When the cell is fully charged, the hydrogen and oxygen gases are liberated at the cathode and anode respectively, so liberation of gases (hydrogen and oxygen), known as gassing, on the electrodes indicates that the cells are fully charged.
2. Colour:
When the cell is fully charged, the lead sulphate anode gets converted into lead per oxide (PbO2) dark chocolate brown in colour and lead sulphate cathode gets converted into lead (Pb), grey in colour. It is considered one of the best tests for ascertaining the condition of a battery.
3. Voltage:
When the cell is fully charged its terminal potential will be approximately 2.6 volts.
4. Specific Gravity of the Electrolyte:
When the cell is fully charged, the specific gravity of the electrolyte will be approximately 1.21. When the cell is fully discharged its value falls to 1.17.
Cells are considered to be fully charged once three successive hourly readings of cell voltage and electrolyte gravity are found to be constant. However, the minimum total ampere- hours input, as mentioned by the manufacturers, must be provided to the cells even if the voltages and specific gravities are observed to be constant before that.