The circuit presented here can charge a 12V lead-acid battery of 50Ah to 80Ah (even up to 100Ah) capacity and can even be used as up to 18V DC variable power supply of maximum 5A capacity, which is useful for a test bench. The circuit can automatically detect the presence of a battery connection and start charging. At that time, it disconnects the output provided as variable power supply. It also detects the wrong/reverse polarity connection of the battery terminals and raises an alarm. The charger initially charges the battery at a higher voltage (about 14.2V), and once it is fully charged, it maintains the battery charge at a constant voltage (about 13.4V).
Circuit and working
Circuit of the 12V battery charger-cum-variable power supply is shown in Fig. 1. It is built around two LM138 variable voltage regulators (IC1 and IC2), dual op-amp LM358 (IC3), 12V voltage regulator 7812 (IC4), two relays (with normal 12V, 1C/O PCB-mounted and 12V, 1C/O, 10A contact-current rating) and a couple of transistors.
LM138 is a 3-pin, 5A positive-voltage regulator available in TO-220 or TO-3 package. But continuous current flow of 5A generates a high temperature, which shuts down output of LM138 automatically due to its internal thermal protection.
This circuit simplifies higher current handling of LM138 by using IC1 and IC2 in parallel, but still output voltage can be regulated by a single variable resistance. Care should be taken for handling the lines carrying 5A current. Two separate relays (RL1 and RL2) are used to reduce the cost of the project.
Transformer X1 steps down 230V AC to 15V-0-15V AC that is then rectified by diodes D1 and D2 and smoothened by capacitor C1. This voltage, which is around 20V DC, is fed to IC1 and IC2 that are wired in parallel. Their output voltage is regulated either by VR1 (in case of variable power supply) or VR2 (in case of battery charger), which is selected by relay RL2. Output so obtained is available for the variable power supply or battery charger through RL1.
Dual op-amp LM358 (IC3) is used to control the relays and select the type of output, that is, whether for battery charger or variable power supply. When no battery is connected for charging, no power is given to IC3. RL1 and RL2 are in non-energised state, and potmeter VR1 can be used to get variable voltage output across CON3 fitted on the cabinet.
When a 12V battery under charging (BUC#) is connected properly to terminal CON4 for charging, IC3 gets power supply from the battery through diode D10. If battery voltage is below ‘dead’ voltage (say, 6-9V and at least above 6V), pin 7 of IC3 pulls low and LED6 glows. If the battery is healthy (say, more than 9V), pin 7 of IC3 goes high and switches on LED2 and conducts transistor T2, and RL1 and RL2 energise.
Output voltages of IC1 and IC2 are regulated by VR2 and are available for charging at CON4. Once the battery under charging attains its full charging voltage, pin 1 of IC3 goes high, as indicated by the glowing of LED4, and T1 conducts, which reduces the voltage at pin 1 of IC1 and IC2.
When a 12V battery is connected in reverse polarity, D11 conducts, which, in turn, switches on the piezo buzzer and LED5 glows. This protection is important while charging batteries externally.