Car Battery Voltage Monitoring in a Relay Form Factor

You probably know that your car has a big battery under the hood, and that you can charge your phone through the cigarette lighter plugs. But, do you know how the system actually works? The 12V car battery is used to start the engine, which requires a lot of current. That power is replaced by your car’s alternator, which is turned by the engine and recharges the battery as it’s running. But, a bad alternator, battery, or unexpected current draw can cause that system to fail. The easiest way to prevent that is to monitor the battery voltage, and this DIY monitor fits into a standard automotive relay package.

Your car likely already has a voltmeter in the dashboard, and you may have noticed that it hovers around 15V when the engine is running, but then dips down if the engine is off but the lights or stereo are on. If it dips down too low, your battery will go flat and you won’t be able to start your car. This battery monitor, created by Jesus Echavarria for a client, activates an alarm when that happens. With this setup, the client will know if their battery voltage has dropped below a safe level without having to actually stare at the voltmeter in the car.

Inside that relay package is an MC78M05CDTRKG voltage regular that powers a Microchip PIC16F1824 8-bit microcontroller. A voltage divider is used to bring the car’s battery voltage down to a safe level for the microcontroller. When it detects that the voltage is too high or too low, it toggles a small solid state relay. That, in turn, can be used to activate any kind of alarm the client chooses. The whole thing is encased in a 3D-printed enclosure, so it can be conveniently placed in a spare slot of the car’s relay box.