Battery Charger Circuit using SCR and LM311

A battery is a device which can store energy in chemical form and convert that energy into electrical energy when needed. It can be secondary (rechargeable) or primary (Non rechargeable) Rechargeable batteries have an advantage over primary batteries with the fact that they can be reused. This is possible by charging a battery, i.e. feeding energy to the secondary cell by forcing electric current through it.

A battery basically consists of number of cells, with each cell consisting of two electrodes immersed in an electrolyte solution consisting of an acid and distilled water. In charged state, one of the electrodes consists of a metal whereas the other consists of metal oxide. The acid enables flow of ions between the two electrodes.

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When the battery starts discharging, electrons flow from one electrode (anode) and get accumulated at another electrode (cathode). This happens by the process of oxidation and reduction and continues till both the electrodes are devoid of reagents and the battery is completely discharged. The charging is then done externally by providing external electric current so that the anode receives electrons and the excess electrons are removed from the cathode. However a problem arises when the battery gets overcharged, this leads to more gassing effect. Even excessive discharging of a battery reduces its age.

A solution to this problem is to ensure a controlled charging method such that the battery charging and discharging is done in a controlled manner. This article describes a simple controlled battery charger using an SCR and LM311. The AC signal is rectified using a SCR and a comparator is used to detect the battery charge voltage with respect to a reference voltage so as to control the switching of the SCR.

Principle Behind this Circuit:

The principle behind the circuit lies in controlling the switching of an SCR based on the charging and discharging of the battery. Here the SCR acts as a rectifier as well as a switch to allow the rectified DC voltage to be fed to charge the battery. In case the battery gets fully charged, this situation is detected using a comparator circuit and SCR is turned off.

When the battery charge drops below a threshold level, the comparator output is so as to turn the SCR on and the battery gets charging again. Here the comparator compares the voltage across the battery with a reference voltage.

Circuit Diagram of Battery Charger Circuit using SCR and LM311:

Circuit Diagram of Battery voltage charger using LM311 and SCR – ElectronicsHub.Org

Circuit Design of Battery Charger using SCR and LM311:

Designing the whole circuit depends on the kind of battery used to be recharged.  Suppose we are using a 6 cell, 9V Ni-Cd battery with an ampere hour rating of 20Ah and a single cell voltage of 1.5V. This would set the required optimum battery voltage to be around 9V. For a voltage of 9V across the potential divider, the voltage across the pot and resistor should be above 5.2V (reference voltage level). For this purpose we select a potential divider arrangement consisting of 22K resistor, 40K resistor and a 20k pot. Output current from LM311 is about 50mA and since here we are using transistor BC547 with a low base current, we require a resistor of about 150 ohms. The transformer used is a 230/12V transformer. Primary of the transformer is connected to 230V AC supply whereas the secondary is connected to the rectifier.

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How to Operate Battery Charger Circuit?

Initially when the circuit is powered and the battery level is below the threshold voltage, the circuit performs the task of charging the battery. The SCR gets triggered with a voltage at its Gate terminal through the resistor R1 and diode D1. It then starts rectifying the AC voltage, though only for the half cycle. As the DC current starts flowing to the battery through the resistor R2, the battery gets charged. The voltage across the potential divider consisting of the pot RV1 and resistor R4 depends upon the voltage across the battery. This voltage is applied to the inverting terminal of the OPAMP LM311.

The non inverting terminal is given a reference voltage of 5.2V using a Zener diode.  For normal charging operation, this reference voltage is more than the voltage across the potential divider and the output of the comparator is less than the threshold voltage required to trigger the NPN transistor into conduction. The transistor and the diode D3 thus remains off and the SCR gate gets triggering voltage through R1 and D1.

Now when the battery starts charging and at a certain point when it is fully charged, the voltage across the potential divider reaches a value above the reference voltage. This implies the voltage at the inverting terminal is less than the voltage at non inverting terminal and the output of the comparator is more than the threshold base emitter voltage for the transistor.

This causes the transistor to conduct and it is switched on. At the same time as the diode D3 is forward biased, it starts conducting and this blocks the triggering the SCR gate voltage as it is now connected to low potential or ground. The SCR thus gets switched off and the charging operation is stopped or paused. Again when the battery charge drops below the threshold level, the charging operation resumes in the manner described above. The resistor R7 and diode D4 are to ensure a small amount of trickle charging takes place in case of the SCR being in off condition.

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Applications of Battery Charger Circuit using SCR and LM311:

1. It can be used to charge batteries used for toys.
2. It is a portable circuit and can be carried anywhere.
3. It can be used as an automatic battery charger, used specially during driving.

Limitations of Battery Charger Circuit:

1. The AC to DC conversion here uses only the rectifier and may contain AC ripples as there is no filter.
2. The half wave rectifier makes the charging and discharging quite slow.
3. This circuit cannot be used for batteries with higher Ampere-hour rating.
4. The battery charging may take longer time.