12V 7.2Ah SMF Battery Charger: Smart Approach for Smart devices by Smart People !!

Difficulty Rating: 5/10

Abstract:

This project gives an overview of the Sealed Maintenance-Free (SMF) Lead-Acid Rechargeable Battery charger using 3-pin  IC LM-317 as voltage regulator and opamp IC LM-324.

In case of a simple battery charger, when the battery is fully charged and is left plugged in for more than a day or two, the battery may get damaged. In order to protect the battery from this major risk, the proposed battery charger is designed in such a way that the charging current falls gradually as the charging progresses. So the battery can be left plugged into this charger indefinitely, and it won't bother it in the slightest.

Designing the Circuit: 

Click the image to view the enlarged version

The LM317 is an adjustable three-terminal positive-voltage regulator capable of supplying more than 1.5A over an output-voltage range of 1.25 V to 37 V. It is exceptionally easy to use and requires only two external resistors, R₂^’ and R₂”  (R₂= R₂^’+ R₂”) to set the output voltage. Furthermore, both line and load regulation is better than standard fixed regulators. In addition to having higher performance than fixed regulators, this device includes on-chip current limiting thermal overload protection, and safe-operating-area protection. All overload protection remains fully functional, even if the ADJUST terminal is disconnected. By connecting a fixed resistor, R₁  the ADJUST and OUTPUT terminals, the LM317 can function as a precision current regulator. An optional output capacitor can be added to improve transient response. 

Determining the values: 

Vo is calculated by following formula ,  Vo = Vref * ( 1 + R₂/R₁ )

Testing of the Circuit: 

The best way to check that the circuit is working is to monitor the current as it flows into your battery. Recall that all the current goes through resistor R3. So you can get a good estimate of the current going into your battery by measuring the voltage drop across the resistor. Use the formula I = V / R (where I = current in amps, V = voltage in volts, R = resistance in ohms) to get the current flowing through the resistor .

Having connected the multimeter probes to the OUTPUT,by adjusting the 500 Ώ Trimpot, output voltage can be varied and optained as wanted( between 12v and 15v ,minimum and maximum charging voltages).To check the regulation of voltage regulator IC LM317,connect the negative probe of the multimeter to the ground ,and observe the voltage at INPUT , OUTPUT and ADJUST pins.While doing so take enough precaution as not to short OUTPUT and ADJUST pins as it may damage the transistor BC 547,whose collector is connected to adjustment pin.

ADJUST	OUTPUT	INPUT	Vref
13.7 V	12.47 V	15.01 V	1.23

The difference between voltage at output pin and adjustment pin is 1.23 (~1.25) which is the reference voltage Vref.

Current rating of battery to be charged  7.2 Ah 12v, short circuit curent Isc = 720mA .Using multimeter check the short circuit current.If the Isc shows a different value than expected,it can be changed by increasing or decreasing the load connected between the emitter of the transistor T1 and ground.

Circuit Working:

 The circuit uses two LEDs as indicators ; one for signalling charging ON condition ,and the other as an indicator , when charging voltage falls below  its terminal voltage (~12 volts). Terminal voltage can be adjusted by adjusting the 1k Trimpot. The output voltage range can be adjusted by 500 ohm Trimpot.The charging voltage is adjusted such that each cell gets charged at 2.25 – 2.3 v/cell by limiting the charging voltage between 12v and 15v,initial current being less than 0.3CA.

LM324 is  used in comparator circuit ,after the rectifier circuit.Comparator will compare the voltage levels,and if the output voltage is less than the charging voltage ,the voltage across the red LED will go high thus indicating drop in charging voltage.

In the proposed battery charger circuit, minimum voltage is set at ~12v.But if we want to set a different voltage as terminal voltage that can be done by adjusting the 1k Trimpot.

All the details of the project including circuit designing, circuit operation, circuit testing & debugging has been mentioned in detail at this link. Click here