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.

32 thoughts on “Automotive Battery Voltage Monitor

  1. 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.

  2. 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

  3. 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.

  4. @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.

  5. @Ivan
    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 :/

  6. 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.

  7. “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.

  8. 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.

  9. @Chris
    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.

  10. @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.

  11. 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. :)

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