A Car Battery Monitor

A close call on the road can really focus your mind on the importance of having a battery monitor in a car. I had been enjoying a pleasant week of travelling around the countryside at a leisurely pace and taking in the beautiful scenery each day. It wasnt until the final day, with the big rush to return home, that I had to drive at night.My home is deep in the country and on the road I was travelling the closest petrol station may be 80km away. I was travelling through an area that is full of open-cut coal mines and large heavily loaded semi-trailers constantly pound the roads, travelling at quite high speeds. It was around 8pm at night and everything was very dark no street lights or house lights anywhere.

Just as I was going up a hill, the lights began to dim and the engine coughed. A large semi-trailer loomed in the rear-vision mirror as I pushed the clutch in and tried to restart. My speed was falling rapidly and my lights were blacked out - I was like a sitting duck in the middle of the road, as the semi-trailer came rapidly bearing down on me. I just managed to pull the car off the road, as the semi-trailer came screaming past, missing me by inches! After calling for assistance from the NRMA, the problem was found to be a fault in the alternator, which was failing to charge the battery. The battery voltage had been falling under the heavy load of the lights and at the worst possible time, there was not sufficient power for the lights or the motor.

After the initial shock wore off, I put on my thinking cap to come up with a PIC-based solution to the problem. What was really needed was a display and a buzzer, to get my attention should the voltage fall outside a specified range. So my design criteria was set, a series of LEDs could indicate the voltage and a buzzer would also be used to warn of problems.

Main Features:

• Visual indication of battery voltage
• Audible warning when voltage becomes low
• Screw terminals for easy connection
• Simple and easy to build

Circuit details:

The circuit is based on PIC16F819 18-pin microcontroller which has an analog-to-digital (A/D) input to monitor the battery voltage and outputs capable of driving LEDs directly, to keep the component count down. There are seven LEDs in all, giving a good range of voltage indication. The topmost LED, LED1, comes on for voltages above 14V which will occur when the battery is fully charged. LED2 indicates for voltages between 13.5V and 14V while LED3 indicates between 13V and 13.5V. Normally, one of these LEDs will be on. LED4 covers 12.5V to 13V while LED5 covers 12V to 12.5V. LED6 covers from 11.5V to 12V while LED7 comes on for voltages below 11.5V. These two LEDs are backed up by the piezo chime which beeps for voltages between 11.5V and 12V and becomes more insistent for voltages below 11.5V.

That might seem fairly conservative. After all, most cars will start with no troubles, even though the battery voltage might be a touch below 12V, wont they? Well, no. Some modern cars will happily crank the motor at voltages below 11V but their engine management will not let the motor start unless the voltage is above 11V. So dont think that a modern car will always start reliably. This little battery monitor could easily prevent a very inconvenient failure to start! So lets describe the rest of the circuit. The incoming supply is connected via diode D1 which provides protection against reverse polarity while zener diode ZD1 provides protection from spike voltages.

A standard 7805 3-terminal regulator is then used to provide a stable 5V to the microcontroller. The battery voltage is sensed via a voltage divider using 33kΩ and 100kΩ resistors. This brings the voltage down to within the 0-5V range for the A/D input of the PIC16F819. Port B (RB0 to RB7) of the microcontroller is then used to drive the various LEDs, with current limiting provided via the 330Ω resistor network. RB7, pin 13, drives a switching transistor for the piezo buzzer.

Software:

For the software, the design follows the basic template for a PIC microcontroller. Port A and its ADC (analog-to-digital converter) function are set up while port B functions as the output for the LEDs and buzzer. Once the set-up is complete, a reading will be taken at port RA2, the input for the A/D convertor. This reading is then compared with a series of values to determine the range of the voltage. This is similar to a series of "if" statements in Basic language. If the voltage is found to be within a certain range, the relevant port B pin will be turned on. If the voltage is below 12V, the buzzer will be turned on for a brief period, to signal a low battery condition. As the voltage falls below 11.5V, the frequency of the beeps will increase, to signal increased urgency.

Building it:

All the parts are mounted on a small PC board measuring 46 x 46mm (available from Futurlec). The starting point should be the IC socket for the PIC16F819, as this is easiest to mount while the board is bare. The next item can be the PC terminal block. The resistors and capacitors can then follow. Make sure the electrolytics are inserted with correct polarity.

Make sure that you do not confuse the zener (ZD1) with the diode when you are installing them; the diode is the larger package of the two.

Even more important, dont get the 78L05 3-terminal regulator and the 2N3906 transistor mixed up; they come in identical packages. The 78L05 will be labelled as such while the 2N3906 will be labelled "3906". And make sure you insert them the correct way around. The buzzer must also be installed with the correct polarity. The 330Ω current limiting resistors are all in a 10-pin in-line package. There are four green LEDs, two yellow and one red. They need to be installed in line and with the correct orientation.

Testing:

Before you insert the PIC16F819 microcontroller, do a voltage check. Connect a 12V source and check for the presence of 5V between pins 14 & 5 OF IC1. If 5V is not present, check the polarity of regulator REG1 and the polarity of the diode D1. If these tests are OK, insert the IC and test the unit over a range of voltage between 9V and 15V. Make sure that all LEDs come on in sequence and the piezo buzzer beeps for voltages below 12V. 

Now it is matter of installing the unit in your car. It is preferable to install the unit in a visible position for the driver. However, it should not obscure any other instruments. The unit should be connected to the cars 12V supply after the ignition switch. This will turn the unit off with the other instruments and prevent battery drain while the motor is not running.