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This Lead Acid Battery Charger is suitable for all types of lead acid batteries. LM3914 and NE555 are used together to perform task that make it independent of human attention. While LM3914 is used to monitor status of the battery during the charging process, NE555 receives reports (signals) from LM3914 and act accordingly.
The operation description is simple once you have the understanding of the connection of main components of NE555 timer. The connection is shown bellow.
555 internal connection
Setting of LM3914 circuit for battery level calibration is explained in my Battery Level Monitor Circuit. When the battery is fully charged, pin10 is low and '0' is sent to pin2 of 555. Look at the circuit above, with '0' sent to pin2 which is connected to inverting input of comparator 2, output of the comparator is high, that is, '1'. This set the flip-flop (F/F) and the output of F/F (i.e pin3 of 555) is '1'. '1' is sent to the base of T1 (PNP) and it blocks hence cut off the relay which breaks charging of the battery.
If a fully charged battery is left for some time, it will drain down even if not used. This circuit have provision for that as it charges up the battery whenever it discharges to a preset voltage level. This is how it works, pin13 of LM3914 is high whenever battery voltage drops below that preset value, therefore '1' is sent to pin6 (non-inverting input of comparator 1) of 555. With non-inverting input of higher potential, comparator output is high, '1', this reset the F/F to zero so pin3 of 555 is low, '0'. This turns on T1 and the relay is activated again. At this time, LED 5 lights up indicating that the battery is charging.
LED 1, LED 2, LED 3 and LED 4 are connected for visual display of of the battery status. When no LED is lit, the battery is considered empty. When only LED1 is lit the battery is 10% charged; LED1 & LED2, 30%; LED1 LED2 & LED3, 60% and finally when all LEDs are lit, it is 90% charged.
All the components are sized to deliver current of 20Amp. Nevertheless it should not be loaded with battery that draw more than 15Amp.
Bridge rectifier GBPC25-1
Step down transformer 15V secondary, 450VA
Transistor:T1= MPS2907A, T2=BC337
Resistors: R1= 1K2
R3, R6, R7= 10K
R11, R12, R13, R14, R15, R16= 100K
Variable resistors: VR1= 20K
Diode: 2 1N4001
Capacitors: C1= 10µF, C2= 100 µF
Relay 12V DC, 20A contact current