Saturday, April 6, 2013

Build an Automatic 6 volt 12 volt 24 volt Lead Acid Battery Charger Circuit

A simple yet accurate automatic, regulated 6/12/24 volt lead acid battery charger circuit is explained in this article. The circuit switches off the current to the battery as soon as the battery reaches full charge. An illuminated LED at the output indiacates the fully charged condition of the battery.

The circuit diagram may be understood from the following points:

Fundamentally the voltage control  and regulation is done by the versatile, work horse IC LM 338. An input DC supply volt in the range of 30 is applied to the input of the IC. The voltage may be derived from a transformer, bridge and capacitor network.

The value of R2 is set to get the required output voltage, depeNding upon the battery voltage to be charged.
If a 6 volt battery needs to charged, R2 is selected to produce a voltage of around 7 volts at the output, for a 12 volt battery it becomes 14 volts and for a 24 volt battery, the setting is done at around 28 volts.

The above settings take care of the voltage that needs to be applied to the battery under charge, however the tripping voltage or the voltage at which the circuit should cut off is set by adjusting the 10 K pot or preset.
The 10K preset is associated with the circuit involving the IC 741 which is basically configured as a comparator.

The inverting input of the IC 741 is clamped at a fixed reference voltage of 6 via a 10K resistor.
With reference to this voltage the tripping point is set via the 10 K preset connected across the non inverting input of the IC.

The output supply from the IC LM 338 goes to the battery positive for charging it. This voltage also acts as the sensing as well as the operating voltage for the IC 741.
As per the setting of the 10 K preset when the battery voltage during the charging process reaches or crosses the threshold, the output of the IC 741 goes high.

The voltage passes through the LED and reaches the base of the transistor which in turn conducts and switches off the IC LM 338.

The supply to the battery is immediately cut off.


The illuminated LED indicates the charged condition of the connected battery. 

This automatic battery charger circuit can be used for charging all lead acid or SMF batteries having voltages in between 3 and 24 volts. Link