Lead Acid Battery Charge Monitor

As per manufacturer’s data sheets, a 12V rechargeable lead-acid battery should be operated within 10.1V and 13.8V. When the battery charges higher than 13.8V it is said to be overcharged, and when it discharges below 10.1V it can be deeply discharged. A single event of overcharge or deep discharge can bring down the charge-holding capacity of a battery by 15 to 20 per cent.

It is therefore necessary for all concerned to monitor the charge level of their batteries continuously. But, in practice, many of the battery users are unable to do so because of non-availability of reasonably-priced monitoring equipment. The circuit idea presented here will fill this void by providing a circuit for monitoring the charge level of lead-acid batteries continuously. The circuit possesses two vital features:

• First, it reduces the requirement of human attention by about 85 per cent.
• Second, it is a highly accurate and sophisticated method.

Lead Acid Battery Charge Monitor Circuit Diagram:
 

The circuit is powered by the battery under test, via a voltage regulator IC. The circuit takes about 100 mA for its operation.

For calibrating the upper and lower reference levels, a digital multimeter and a variable regulated power supply source are required. For calibrating the lower reference voltage, follow the steps given below:

• Set the output of power supply source to 10.1V.
• Connect the power supply source in place of the battery.
• Now the display will show some reading. At this point vary preset VR2 until the reading on the display just changes from 1 to 0.

The higher reference voltage is calibrated similarly by setting the power supply to 13.8V and varying preset VR1 until reading on the display just changes from 8 to 9.
After the calibration is completed, the circuit may be housed in a suitable enclosure. The cost of all the components, including the enclosure, would be around Rs 200.