I had a 6V 4.5Ah Sealed Lead Acid rechargeable battery lying around for sometime now.I don't remember from where I got this but ever since I saw it I wanted to build a charger for it. I was working on learning power supply circuits such as series voltage regulator, shunt voltage regulators, integrated circuit based voltage regulator and the switch mode power supply etc. I came the series voltage regulator designed with op-amp again. The circuit also has added current limiting circuitry which is crucial for effective and reliable rechargeable battery charging circuit. So I build and tested the battery charging circuit which is shown below.
The rechargeable battery charging circuit was build on a breadboard as can be seen in the above image.
The circuit diagram of the rechargeable battery charging circuit is shown below.
Here, the input voltage is +9V and the output is around 6.4V which is required by the Lead Acid battery charger. The input voltage is from either regulated or unregulated +9V since this circuit itself is a voltage regulators. The zener diode D1 with zener breakdown voltage of 7.5V is used to set the reference voltage from which together the voltage divider resistors will set the output voltage given by the following equation.
\[ V_o = \frac{R_1 + R_2 }{R_1} V_z \]
since Vz = 7.5V, we can see we can set the output voltage Vo by changing the R1 and R2 values.
In the above circuit implementation I have used a 10KOhm POT instead of using R2 and R3. The standard form of the series voltage regulator with op-amp is shown below with R2 and R3 as the feedback signal voltage divider circuit,.
By changing the POT we can set the desired output voltage which ranges from around 0.6V to around 6.8V. As per the sealed lead acid rechargeable battery specification I needed 6.4V to charge the battery. This value is achieved by using the POT.
The resistor R6 is the current sense resistor which controls the base-emitter voltage of transistor Q2.
The short circuited load current is given by,
\[ I_{SL} = \frac{V_{BE}}{R_6} \]
Here \(R_6\) is 1Ohm and \( V_{BE} \) is 0.65V(say or measure) then the short circuit load current \(I_{SL}\) is 650mA. So as long as the load current is below 650mA the transistor is cutoff(OFF) and the output voltage remains the desired voltage set. But as soon as the load current exceeds 650mA the base-emitter voltage of Q2 exceeds the 0.65V and this turns ON the transistor Q2. Since it is turned on the current flows from emitter of Q1 into the base of Q2 then into collector of Q2. So the base voltage of Q1 increases which reduces the load current and load voltage. In this way, voltage regulation with current limiting is achieved by this series op-amp based voltage regulator.
Watch the video below which demonstrates how the diy sealed lead acid rechargeable battery voltage regulator works.
This simple and reliable voltage regulator is not only useful for making charging circuit but it can also be used to power electronics circuits, portable electronics circuit driven by battery, to power micrcontrollers or Arduino as was demonstrated in Simple Voltage Regulator (Transistor + Diode) for Powering Arduino.
