Download 12v battery charger

12V Battery Charger-Cum-Variable Power Supply
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.