Automotive battery voltage monitor


[Rajendra’s] car had just about all the bells, whistles, and gauges he could dream of, but he thought it was missing one important item. In an age where cars are heavily reliant on intricate electrical systems, he felt that he should have some way of monitoring the car’s battery and charging system.

To keep tabs on his car’s electrical system, he built a simple device that allows him to monitor the battery’s instantaneous voltage when the car is powered off, as well as the charging voltage across the battery when the car is running. A PIC16F1827 runs the show, using a simple voltage divider network to step the input voltage down to an acceptable level for use with the PIC’s A/D conversion channel. The resultant measurements are output to a four digit 7 segment display, mounted on the front of the device.

He says that the voltage monitor works quite well, and we’re sure he feels a lot better about the health of his car’s charging system. For anyone interested in keeping closer tabs on their car, he has a circuit diagram as well as code available on his site.


  1. Bobby J says:

    Or you could buy a ScanGuage and read it directly from the ODB port as well as a bunch of other information…

  2. Jobby B says:

    So wait.

    I can buy a $100+ device that does more than I need, or I can scrounge parts/pay about $20 to build this?

    Riight. Let me get right on that scangauge.

    Money must grow on trees in your neck of the woods, partner.

  3. Nick says:

    Or he can have the pleasure of doing it himself, and it’s nice to have a build like this, i don’t know…maybe for people who’s car does not have an ODB port like mine. Not to mention, im sure his is a helluva lot cheaper then a scanguage

  4. Tom says:

    but that doesn’t take any brains…

  5. Tom says:

    my comment was directed @ Bobby J, oops!

  6. fartface says:

    Battery Voltage Monitor Circuit

    Better easier and looks far less nerdy in the car. mount one LED right in the instrument cluster for a “OEM” look. plus you can modify the color changes for specific voltage readings.

  7. damox says:

    I hope nobody tries to use the modified USB adapter he uses to charge their phone!

  8. Ivan says:

    Wouldn’t be great to monitor current instead of voltage? :)

  9. Nick says:

    That would be the icing on the cake if it could even be added to this ;)

  10. Chad B says:

    Now he just needs it to spit out logs for when its time to get that battery replaced under warranty :-P

  11. Chris says:

    I’m fairly certain the BMW pictured in this article is already monitoring the battery voltage 5 times over and would happily report this to you if it was out of spec.

  12. Sebastian Heyn says:

    I doubt that its automotive. I bet it would fail radiation tests!

  13. Ivan says:

    @Nick, yes. But not easy from the DC outlet inside the car. That probably would require direct access to the battery and may be messy. But both voltage/current (power) would be great to monitor actual load of the electrical system of the car, and of course it would help to early detect failures in the alternator and perhaps the battery itself.

  14. Nick says:

    Eactly, I wouldn’t mind having for it to go to the battery, heck, my car still has an ammeter, however no longer functional, I think somewhere in the harness is screwed up :/

  15. Scott says:

    The big application for this is battery voltage in other vehicles and installations. Sailboat owners obsess about their storage batteries since charging can be troublesome (running the engine or using solar cells/wind generator) and off-the-grid sorts use these all the time.

  16. Ivan says:

    Why not just a panel meter and a passive low pass filter for surges? It even matches the rest of the gauges *FACEPALM*

  17. Ivan says:

    There are 1999 count voltmeters with LED display interface out there.

  18. Ivan says:

    “Wouldn’t be great to monitor current instead of voltage? :)”

    That wasn’t me, by the way.
    Measuring current is a lot harder, you have to break the circuit and there are huge spikes. Measuring DC current is a hassle. You can’t fit a reasonably accurate current shunt and 99% of the time you are not using 60 amps like the range on your display.

  19. Anonymous says:

    Huh, I’d like something that logs the battery level to an SD card or something, even when off though… I might have to get on that.

  20. Bobby J says:

    Fine, don’t want a $100 scanguage, here’s a $20 voltage meter:

  21. Buddy says:

    @Ivan – There are non-intrusive current detectors that wrap around the cables. Apparently, they’re reasonably accurate.

  22. password says:

    @buddy do they work for dc current?

  23. George says:

    This little diddty will get you started on your Hi current gauge.

  24. eyesaid says:

    I totally understand the satisfaction of the build but sometimes you need to keep thing in perspective.
    This is one I purchased on Ebay and it works great.

  25. Soundwavehi says:

    It would be better to know the amperage of the battery… I’ve started cars with 10v but the amperage is what cranks the starter. That’s why you have to pay attention to the cold cranking amperage when buying a battery, like for larger trucks, and cars that have superchargers/turbos.

  26. Soundwavehi says:

    Props on the build though…

  27. wernicke says:

    BMW makes the Equinox now?

    I’ve got a voltmeter on my GMC Acadia (nearly the same gearshift handle as in the picture, btw), but I suspect it isn’t much more than a 3 position Hi/OK/Lo indicator. Many current analog gauges don’t read linearly, but are setup to give easy-to-read feedback at the expense of accuracy.

  28. john says:

    good stuff

    i wonder how it would need to be changed to work in my mgb, which is positive earth and has two 6 volt batteries

  29. Alexander says:

    Wouldnt a simple multimeter do exactly what he’s doing?

  30. Buddy says:

    @password – yeppers, but expensive

    e.g. http

    Personally, I would buy a set of generic battery cables (keep the originals intact!), and chop up those.

  31. cutandpaste says:

    My 1995 E36 BMW does battery monitoring right on the dashboard: Just press 1 and 1000 at the same time, dial in “9,” and press Set.

    Bingo! A digital voltmeter on the dash.

    (There’s a lot of other “hidden” features on the on-board computer, too, but that’s one of the most useful ones for me.)

    I haven’t looked, but I would be genuinely alarmed if newer BMWs weren’t similarly-equipped. They started doing this at least 20 years ago. :)

  32. fazli says:

    please give the automotive voltage monitor ic s micro c programme …

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