06-05-2011, 12:27 PM
Nowadays maintenance-free lead-acidbatteries are common in vehicles,inverters, and UPS systems. If thebattery is left in a poor state of charge, itsuseful life is shortened. It also reduces thecapacity and rechargeability of the battery.For older types of batteries, a hygrometercan be used to check the specific gravityof the acid, which, in turn, indicates thecharge condition of the battery. However,you cannot use a hygrometer for sealedtypemaintenance-free batteries. The onlyway to know their charge level is by checkingtheir terminal voltage.The circuit presented here can replenishthe charge in a battery within6-8 hours. It also has a voltage analysingcircuit for quick checking of voltage beforestart of charging, since overcharging maydamage the battery. The voltage analysergives an audio-visual indication of the batteryvoltage level and also warns about thecritical voltage level at which the batteryrequires immediate charging.The charger circuit consists of a standardstep-down 12V AC (2-amp) transformerand a bridge rectifier comprising diodesD1 through D4. Capacitor C1 smoothes theAC ripples to provide a clean DC for chargingthe battery.The battery voltage analyser circuitis built around the popular quad op-ampLM324 that has four separate op-amps(A through D) with differential inputs.Op-amps have been used here as comparators.Switch S2 is a pushswitch, which ispressed momentarily to check the batteryvoltage level before charging the battery.The non-inverting terminals of op-ampsA through D are connected to the positivesupply rail via a potential divider chaincomprising resistors R1 through R5. Thusthe voltage applied to any non-invertinginput is the ratio of the resistance betweenthat non-inverting terminal and ground tothe total resistance (R1+R2+R3+R4+R5).The resistor chain provides a positive voltageof above 5V to the non-inverting inputsof all op-amps when battery voltage is 12.5Vor more. A reference voltage of 5V is appliedto the inverting inputs of op-amps via 5Vzener diode ZD1.When the circuit is connected to thebattery and pushswitch S2 is pressed (withS1 open), the battery voltage is sampled bythe analyser circuit. If the supply voltagesample applied to the non-inverting inputof an op-amp exceeds the reference voltageapplied to the inverting inputs, the outputof the op-amp goes high and the LED connectedat its output lights up.The different levels of battery voltagesare indicated by LED1 through LED4. Allthe LEDs remain lit when the battery isfully charged (above 12.5V). The buzzerconnected to the output of IC1 also sounds(when S2 is pressed with S1 kept open) aslong as the voltage of battery is above 9.8V.If the voltage level goes below 9.8V, thebuzzer goes off, which indicates that it’stime to replacethe battery.The status ofLEDs for differentbatteryvol t a g e s i sshown in thetable.
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