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Battery monitor

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  • Battery monitor

    I want to use a battery monitor to monitor how much juice is left in my battery. Something like what you get on a cell phone display, i.e. the more time left in the battery the more bars are displayed. Even better would be a display like the one for my laptop, it tells me how many mins I have left to power it.

    I will be using it on a second battety used purely for the carpc, display, and anything else to do with the car pc.

    Does anybody know where to buy one or how to make one

  • #2
    i have an escort 8500 radar detector and it has a digital voltage display option and will also beep when the car is getting to low of a voltage. costs about $300 to buy one.


    • #3
      I think your only real choice would be a voltmeter. You can get analog ones from any gauge manufacturer. Autometer ( is starting to make audio gauges (voltmeters , amp ammeter and amp temp) in digital readout.
      Cobalt gauges
      Dakota Digital makes them too.
      Otherwise, you can use a digital multimeter set for just volts, and permanently mount it in the car. It'll cost you about $10 for a cheap one.

      If you're looking for a time remaining readout, I don't think there's really such a thing for automotive situations. You'd need to program a micro or something to read the voltage, determine how much power is currently being drawn, and have it calculate an estimated time remaining. It's much easier for laptops and small devices, as the current draws and battery size are much more constant.

      There are too many variables in the carPC environment. Battery size and reserve capacity, quality of interconnects, amperage and quantity of devices connect to the battery, etc. They best measure of remaining battery life is a voltmeter for this case. A battery at full charge will have 12.6 volts (six plates at 2.1 volts per plate), and while being charged, should be around 14.4 volts.

      That aside, it's more than possible someone has made a simple plug-in solution that does just what you're asking for. Try google or some electronics or car-audio sites.

      Good luck,

      How Much Horsepower Can I have And Still Go To Heaven?


      • #4
        Analog Voltmeters are ~$15 @ any car store.
        2007 Honda Fit Sport 1.5L SOHC-VTEC


        • #5
          check (Australia) webssite for one to DIY, cost about $10-$15 US ship to your door. This same device can be use for air/fuel mixture monitor in the car by replacing couple resistor. You can configure it to have just one light on or the whole bar (10 leds).
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          • #6
            Thanks for the posts guys. I already Have a analogue meter and an amp meter for it but find they fluctuate a lot and canít tell till the last few minuets when the battery goes dead.


            • #7
              Well, what about calculating it scientifically?

              Do the math.

              Determine Reserve Capacity (in minutes). It should say it on the battery.

              Reserve Capacity is the amount of time the battery will output a 25 amp load before it drops below 10.5 volts (fully discharged).

              Determine amps being drawn by your system. You can use an ammeter or high-current multimeter for this.
              Find out how long your system will run before the battery dies.
              Make a timer that starts when car is off and system is on.
              That'll give you a fairly rough idea how long you've got left.

              By the way, when looking for a secondary battery for a CarPC, note that the higher the cranking amps of a battery, the lower the reserve capacity. Aim for deep-cycle if you're good at completely killing the battery on a regular basis.


              How Much Horsepower Can I have And Still Go To Heaven?


              • #8
                Some newer vehicles have a battery monitor circuit that shuts down non critical to operation circuits to preserve the ability to start the vehicle in event of a low battery condition. Things like navigation, radio, clock, ECUs that require voltage for memory. Because of all the ECUs and devices now in vehicles, and the desire for lightweight vehicles (smaller batteries and charging systems) is why some vehicles have incorporated this technology.

                BTW, this shouldn't be involved at all if you wire direct to the battery Positive. Then again, you may be able to take advantage of this vehicle feature by putting a relay to control your voltage in the "power down" circuit to isolate your CarPC, if your vehicle is equipped with this.

                Best way to determine if yours has it, is to consult the service manual which should have this documented for your service technician.


                • #9
                  I got one of these really cool DVM's from Datel, about $30. It monitors the voltage on the second battery I installed. Part Number DMS-20LCD-0-DCM
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                  • #10
                    take a look at
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                    • #11
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                      • #12
                        Does anyone know of a way getting the battery information to the pc. Eg battery voltage and the amps used by the carputer?


                        • #13
                          Originally posted by hapination
                          Does anyone know of a way getting the battery information to the pc. Eg battery voltage and the amps used by the carputer?
                          You could get the voltage of the car battery to the computer via the serial port, and use a standard UPS monitoring program to watch it, but you will have to find out what pins it watches and uses for each function.

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