Most battery chargers stop charging the battery when it attains its maximum charging voltage set by the circuit. This circuit charges the battery at a particular voltage, that is, absorption voltage, and once the maximum charging voltage is attained, the charger changes the output voltage to float voltage for maintaining the battery at that voltage. Absorption and floating voltages are dependent on the type of battery.
For this charger, voltages are set for a sealed lead-acid (SLA) 12V, 7Ah battery, for which absorption voltage is 14.1V to 14.3V and floating voltage is 13.6V to 13.8V. For safe working and to avoid overcharging of battery, absorption voltage is selected as 14.1V and floating voltage is selected as 13.6V. These values are to be set as specified by the battery manufacturer.
Circuit and working
Circuit diagram of the 12V battery absorb and float charger is shown in Fig. 1. It is built around step-down transformer X1, adjustable voltage regulator LM317 (IC1), op-amp comparator LM358 (IC2) and a few other components.
The 230V AC primary to 15V-0-15V, 1A secondary transformer used in this circuit steps down mains voltage, which is rectified by diodes D1 and D2 and smoothened by capacitor C1.This voltage is given to the input of LM317 for regulation.
The basic circuit is a regulated power supply using LM317, with a control on output by changing resistance at adjust pin 1. A good heat-sink is required for LM317. LM358 is a dual-operation amplifier that is used here to control overcharging of the battery. Capacitor C4 should be as near as possible to pin 1 of IC2.
Jumper J1 is used for calibration (set-up). While setting the charging voltage, remove the jumper and connect it back after calibration.
For initial setup, remove jumper J1, switch off S2, switch on S1 and adjust potmeter VR2 to get 13.6V at test point TP2. Adjust potmeter VR3, so that LED2 begins to glow. Adjust potmeter VR1 to read 0.5V (difference of 14.1V and 13.6V) at test point TP1. Adjust VR2 to read 14.1V at test point TP2.
With these settings, TP2 should read 14.1V when there is low voltage at test point TP3, and 13.6V when there is high voltage at test point TP3. Connect jumper J1. The charger is now ready for use.
Connect the 12V battery under charging (BUC), with correct polarity, at CON2. Switch on S2; one of the LEDs out of LED2 and LED3 will light up (most likely it would be LED2). If neither of these light up, check the connections; battery could be dead. Switch on S1 for charging. Fully charged status of the battery will be indicated by glowing of LED3.
Do not worry if you forget to switch off the charger. The charger is on floating voltage (13.6V) now and it can be kept in this charging mode forever.
Construction and testing
An actual-size, single-side PCB for the 12V battery absorb and float charger circuit is shown in Fig. 2 and its component layout in Fig. 3. Assemble the circuit on the PCB, except transformer X1 and the battery under charge (BUC).
Download the PCB and component layout PDFs: click here
Enclose the PCB in a small box. Fix the battery terminal on the front of the box for connecting the BUC. Connect switches S1 and S2, potmeters VR1 through VR3, etc on the body of the box.
EFY notes. 1. Switch off S2 or disconnect battery terminals to avoid unnecessary discharge of battery when not charging, that is, when S1 is switched off.
2. Connect the battery with correct polarity.
3. Casing of IC1 should not be connected to ground, so use insulation.
Fayaz Hassan is manager at Visakhapatnam Steel Plant, Visakhapatnam, and is interested in microcontoller projects, mechatronics and robotics