Mostly in broad projects, we have seen lead-acid storage cells are often used as back-up source of electrical energy during the periods of supply line failure so that the system operates efficiently. In case of mains supply failure, the energy (i.e. electrical) stored in the cell is used and later when the mains supply is available, the energy in the cell is restored from the mains through a device named as charger. The cell undergoes chemical processes during charging as well as discharging process, one in the forward direction and the other in the reverse direction. As the charger circuit continues to ‘on’ state even after the cell restores complete energy, it might create some serious issues, didn’t think about this case? Don’t worry, the circuit is designed carefully and it also includes solutions to effects of battery overcharging.
Observing the ampere-hour capacity of the battery specified by the manufacturer, we can approximate the duration a battery can be used after charging it completely. If the battery reads 90 ampere-hour, it can deliver current at the rate of 9 amperes for about 10 hours. If we use the battery beyond its capacity, then the battery will get damaged completely; the plates will be coated with insoluble molecules of lead sulphate which cannot be removed. This state is known as deep-discharge of the battery after which the battery is of no use. This state can be clearly predicted from the battery voltage. For the most common 12-volt batteries, this level is 10.8 volts. So the battery voltage should not be allowed to fall below 11 volts to avoid possible damage.
Here we have presented a reliable and efficient method to ensure proper utilization of the battery capacity and at the same time protect the battery. The circuit “12 V Battery Charger with overcharge and deep-discharge protecting” is so designed that it cuts-off the charging of a 12-volt battery at full charge condition and also the discharges battery when it reaches a safe limit.
A transformer in a metallic box is what forms the charging section. The primary of the transformer is connected to the AC mains (Fig. 1). The secondary of the transformer usually has a centre terminal and the two terminals at its sides are 12 volts AC with respect to it.
The centre-tapped terminal is directly connected to the negative terminal of the battery. The other two terminals are connected to the anode ends of two diodes. The other ends, i.e. the cathode ends of the diodes, are joined together and are led to the positive terminal of the battery through an ammeter and a safety fuse.
To cut off the charging, as the battery reaches full charge, i.e. 13.2 volts, a circuit board with a relay can be used. The circuit, board has a zener diode which gives a reference voltage of 5.1 volts (Fig.2). The current through the zener is maintained at about 5mA through a resistance of 1.8k from the supply voltage which is around 12-15 volts.
This reference voltage is put at the emitter of transistor T1 whose base senses the battery voltage through a potential divider formed by 4.7k and 3.3k resistors, and a preset of 2.2k. If the base voltage doesn’t exceed 0.6 volt than the emitter voltage, transistor T1 does not conduct; the collector voltage of T1 remains at Vs. Transistor T2 also does not conduct since the voltages at its emitter and base are the same and equal to Vs. As the base of T1 rises above 5.7 volts, T1 starts conducting and the collector voltage falls below Vs. The base voltage of T2, a pnp transistor, becomes lower than that of its emitter–eventually T2 also conducts and the relay is energised.
There is another potential divider formed by two 4.7k resistors and the 2.2k preset. Transistor T3 senses the same zener voltage of 5.1 volts at its emitter and the battery voltage at the base. If the battery voltage is about 11 volts, the transistor conducts fully, the collector voltage remains around 5.2 volts. The collector of T3 is linked to the base of T1 through diode D3. If the battery voltage falls below 11 volts, conduction of T3 decreases and the collector voltage rises. D3 then conducts to raise the base voltage of T1. Transistors T1 and T2 conduct sequentially and the relay is energised.
To avoid flickering in the relay contacts during overcharge or deep-discharge cut-off, a feedback resistance of 47k is placed between the collector of T2 and the base of T1. A condenser of 100uF across the relay also reduces the flickering. If further damping is needed, a condenser should be set at the base of T1.
Both the overcharge and deep-discharge cut-offs are provided by a single relay. Hence, it has to be a double contact relay. Normally, the DC output of the charger at point A charges the battery through one normal contact (Fig.3). The battery is also connected to the pole of the other contact and the battery output is taken from its normal contact. A double LED serves as both t he overcharge and deep-discharge cutoff indicator. When the relay is energised due to overcharge, the voltage at A lights up green part of the LED, at the same time reverse-biasing the red part. During discharge, if the relay is activated, the red part will glow since the voltage at A is absent.
For setting, first set the preset VR2 at the highest towards supply. Then let the battery voltage rise to 13.2 volts by charging. Set the preset VR1 to cut the charging at a voltage slightly above 13.2 volts.
A satisfactory check mechanism for the setting will be as follows. First discharge the battery through an ammeter for a certain time. For example, let the current be 5 amperes and time 8 minutes. Then let the battery be charged. If the charging current is 4 amperes, then the minimum charging time required is given by 4t = 5×8.
Here is a problem. The charging current is not a steady one. So you have to find the average current. By repeating the above procedure several times you have to fix the preset, keeping in mind that the power supplied during charging should be somewhat greater than the power delivered by the battery during discharge.
While the battery voltage is at 13.2 volts, shift the battery clip from the positive terminal to the positive of the preceding unit (6 units in 12-volt batteries). Then the voltage reaching the circuit board is 13.2×5/6= 11 volts. Use this voltage to set preset VR2 for discharge cut-off.
|Resistor (all ¼-watt, ± 5% Carbon)|
|R1, R7, R8 = 4.7KΩ
R2, R4 = 3.3 KΩ
R3, R9 = 1.5 KΩ
R5 = 47Ω
R6 = 47 KΩ
R10 = 1 KΩ
VR1, VR2 = 2.2 KΩ
|C1 = 1000 µF/25V
C2 = 200 µF/16V
C3 = 100 µF/16V
C4 = 0.01 µF TO 100 µF/16V
|T1, T3 = BC147
T2 = SK 100
D1, D2 = 1N5401
D3 = 1N4001
D4 = 1N4148
ZD1 = 5.1 V/250mW
LED1 = Dual color LED
|X1 = 230V AC Primary to 12V-12V, 3 AMP Secondary Transformer
RL1 = 12V, 100Ω relay