Most car battery chargers are simple devices that continuously charge the battery with a few amperes for the duration it is ON. If the charger is not switched OFF in time, the battery will overcharge, its electrolyte lost due to evaporation, and its plate-element will likely be destroyed.
The circuit above will eliminate these problems by monitoring the battery's condition of charge through its retroactive control circuit by applying a high charge current until the battery is completely charged. When charging is complete, it turns on the red LED (LD2) and deactivates the charging circuit.
This circuit is drawn to charge 12V batteries ONLY. Certain emphasis should be taken when wiring up this circuit. They are the connections of the transformer to the circuit board, and those supplying current to the battery being charged. These connections should be made with cables having a large cross-sectional area to prevent voltage-drop and heat build-up when current flows through them.
After assembling of the circuit, adjust TR1 to null value, power-up and make the following adjustments :
- Without connecting the battery check that the 2 LEDs are turned on.
- Connect a car battery to the circuit and check that LD2 is OFF and a current (normally 2A to 4A) is flowing to the battery.
- Adjust TR1 until LD2 turns ON and the charge current is cut.
- Adjust TR1 to null value and charge the battery using the hydrometer technique (if you do not have or do not know how to use a hydrometer, then use a good condition battery and charge).
Carefully adjust TR1 so that LD2 begins to turn ON and the charge current falls to a few hundred milliamps (mA). If TR1 is set correctly then in the next round of charging you will noticed LD2 begin to flicker as the battery is being charged.
When battery is completely charged, LD2 turns ON completely.TR1 does not need further adjustment anymore. Q1 is connected in line with the battery and is fired by R3, R4 and LD2. The R2, C1, TR1 and D2 sense the voltage of the battery terminal and activate Q2 when the voltage of the battery terminal exceeds the value predetermined by TR1.
When an uncharged battery is connected, the terminal voltage is low. Under this circumstance, Q2 is turned OFF and Q1 is fired in each half cycle by R3, R4 and LD2.
The Q1 functions as a simple rectifier and charges the battery. If the battery terminal voltage is increased above the level that had been fixed by TR1, then Q2 shifts the control of Q1 gate. This deactivates Q1 and cuts off the current supply to the battery and turns LD2 ON indicating that the charge has been completed.
Q1 and bridge rectifier GR1 should be mounted on heatsinks to prevent overheating. M1 is a 5A DC ammeter to measure the charge current. Optionally a voltmeter can be connected in parallel with the battery, however it must have a high input resistance so as not to influence the measurement.
|R1= 1Kohms||D1= 1N4001||T1= 220V/17V 4A Transformer|
|R2= 1.2Kohms||D2= 6.8V 0.5W zener||LD1= Green LED|
|R3= 470 ohms||TR1= 4.7Kohms trimmer||LD2= Red LED|
|R4= 470 ohms||Q1= BTY79 or similar 6A SCR||M1= 0-5A DC Ampere meter|
|R5= 10Kohms||Q2= C106D SCR||S1= 10A D/P On-Off Switch|
|C1= 10uF 25V||GR1= 50V 6A Bridge Rectifier||F= 5A Fuse|
Kebanyakan alat pencharge aki merupakan rangkaian sederhana yang terus menerus mengisi aki selama rangkaian dihidupkan (ON). Jika aki sudah penuh dan rangkaian tidak dimatikan (OFF) maka akan terjadi overcharge, yang bisa menyebabkan penguapan cairan elektrolit dan merusak elemen-elemennya.
Rangkaian di atas dapat mengatasi hal seperti itu dengan memonitor kondisi pengisian aki melalui rangkaian kontrol retroaktif yang mengalirkan arus besar ke aki hingga aki terisi penuh. Ketika aki sudah terisi penuh, LED LD2 akan menyala dan mematikan rangkaian pengisi.
Hal-hal yang perlu diperhatikan:
- Rangkaian ini hanya untuk mencharge aki 12 V
- Semua kabel yang digunakan untuk menyambung komponen termasuk ke aki harus menggunakan kabel dengan kapasitas arus besar agar tidak ada tegangan jatuh atau rugi-rugi panas yang terjadi
Setelah rangkaian siap, trimpot TR1 dibuat nol kemudian rangkaian disetel:
- Sebelum menyambung ke aki, pastikan kedua LED menyala dengan baik.
- Pasang aki, pastikan LED LD2 mati dan arus yang mengalir sebesar 2-4 A.
- Atur trimpot TR1 hingga LED LD2 hidup dan arus berhenti mengalir.