Car Battery Charger

The objective here is to build a simple car battery charger , the one that can be used to charge 12 volt batteries of usual automobile battery and lead-acid type one.

The gadget provides you with metering of both voltage across the battery being charged as well as the charging current being drawn during the charging process, this lets you know the status of the battery.

Car Battery charger


R1 2.2 K , 1/4 W
C1 1000uf ,50V Electrolytic
C2,C3 0.1 uF (ceramic disc)
D1,D2,D3 BY 127
D4,D5 1N4001 or Equivalent
LED Any Colour
IC-1 78T12 (it is 7812 in TO-3 Package)
Transformer T1 15-0-15 , 2A
Ammeter 0-3A


The circuit is nothing but a AC/DC power supply that generates a regulated DC voltage of 13.2 volts from AC main, Transformer T-1, D1,D2 and capacitor C1 constitute the unregulated power supply portion of the power supply with diode D1 and D2 along with the transformer providing voltage transformation and rectification and the C1 providing the filtering action.

IC-1 is a three terminal voltage regulator of type 7812. The common terminal of tis regulator is lifted to a potential of about 1.2 volts by forward biased diodes D3 and D4 so as to give a regulated output voltage of 13.2 volts instead of 12V which would be the case if the common terminal was grounded. C2, C3 are decoupling capacitors. Diode D5 provides the charging path for the battery. The diode D5 also prevents the current to flow in the opposite direction i.e from battery towards the power supply.

Meter M-1 , which is a current meter , gives continuous reading of the charging current . The current reading becomes zero when the battery is fully charged. Meter M-2 gives the reading of the voltage across the battery as it is being charged. A fully charged battery is typically has an open circuit voltage of 12.5 volts or so.