Section : Dashboard and Instruments

Volt Meter or Ampmeter

Posted By : Rich Nurse, 08-Aug-2006, 12:45pm

• Volt Meter or Ampmeter

  Posted By : Chris Povey, 08-Aug-2006, 01:01pm

  I always prefer an ammeter. This tells you what is happening now - ie is current flowing to the battery or from the battery (battery charging or discharging) If the ammeter constantly reads very positive you will cook your battery and if negative you will have a flat battery.
  
  The volt meter tells you the charge state of the battery (Somewhere between 0 and 13.8 ish volts)
  
  So in my mind the voltmeter tells you the state of the battery and the ammeter tells you why it is in that state.
  
  In my opinion the info provided by an ammeter is a more useful than the voltmeter although you could argue the case for either instrument.
  
  The ammeter has to be fitted in the main feed (large brown ? cable connected to the starter solenoid).Some fairly hefty cable (50amp) needs to be run from here to the ammeter as it carries all the current for the vehicle except the starter.
  
  The Voltmeter is connected between a live feed and earth. As the voltmeter is high impedance it does not draw much current and smaller cable is required.
  
  Chris
  
  Update
  
  There have been several articles regarding this over the years - see technical directory:
  
  p18 jun99 No.228
  p78 Oct 02 No.268
  p50 Mar 03 No.273
  
  

  • Volt Meter or Ampmeter

    Posted By : Van Hamlin, 08-Aug-2006, 01:44pm

    Plenty of stuff on the message board...
    
    
    
    
    http://www.tssc.org.uk/messageboard.asp?board=vite...
    
    
    http://www.tssc.org.uk/messageboard.asp?board=gt6&...
    
    Van
    

    • Volt Meter or Ampmeter

      Posted By : John Davies, 08-Aug-2006, 02:08pm

      Chris, Van,
      
      I barely know the difference between volts and amps, but surely in the 21st there is another way to measure the current , apart from letting it all flow through a dash mounted meter with a socking great cable attached to it?
      
      Some sort of sensor on the battery cable, with a teensy weensy little wire to the dash gauge?
      
      A bit of Googling revealed that the words "Hall effect" and "Clamp meter" seem to provide suitable references, eg: http://en.wikipedia.org/wiki/Clamp_meter
      
      Would these work in the way I describe?
      
      John

      • Volt Meter or Ampmeter

        Posted By : Jon Wolfe, 08-Aug-2006, 02:39pm

        Fitting an ammeter is a pain as you need to set it with the correct shunt otherwise it'll misread - you can also damage the alternator if you get it wrong. It also means you have high current supply running through the dashboard which is bad and partly why modern cars do not have them. They were really for when cars had a dynamo. A voltmeter is much better because its much easier to fit, safer and will give you nearly the same info that an ammeter does.

        • Volt Meter or Ampmeter

          Posted By : Danial Phoenix, 08-Aug-2006, 03:11pm

          i think both are ok but nither tell you how much usable power is left in your batery, tels you if theres les than 12v but that usualy to late as car wont start.
          
          i have a batery gauge that tels me how charged the batery is, 1 red light flashing for flat, 1 red for useless, 3 yellow not much u can do and wont start a car, and 4 green for startable and usable, when all light its full, then as you use it without puting charge back in, you can watch it go down till you see the last green light and you start car, and watch it charge the battery. perfect.
          
          use to have it when i cused a rediculous sized sound sytem in my old car, and just as invaluble on a classic which had a dynamo.

          • Volt Meter or Ampmeter

            Posted By : Van Hamlin, 08-Aug-2006, 03:32pm

            The battery gauge with LEDs is just 4 lights that come on at different voltages. There is no wizardry here, and it doesn't tell you anything you can't work out from a voltmeter. It just makes it easier.
            
            The ammeter shunt does not have to be near the ammeter, as long as the resistance of the wires between the shunt and the ammeter is low compared to the resistance of the ammeter. You could put the ammeter shunt next to the solenoid.
            
            The hall effect transformers work well and you could use the output of one to power a gauge. But the ones I have seen need a + and -15V supply. On + and -12V they still work quite well, but the max is reduced a bit. Trouble is you don't have a + and - supply in the car.
            
            Van
            

            • Volt Meter or Ampmeter

              Posted By : Danial Phoenix, 08-Aug-2006, 03:40pm

              , the one i have doesent just have 4 light and does show battery state. desighned for road sweepers and fork lifts, ect to keep a check on the battery power left to stop overusage beond starting the vehicle.

        • Volt Meter or Ampmeter

          Posted By : Chris Povey, 08-Aug-2006, 03:31pm

          I have not had to use a shunt for any automotive ammeter that I have used.
          
          The instrument will be shunted internally. The current passing through the instrument is a function of the voltage dropped accross it so if it is designed fo a 12v automotive system it should need no further setting up. (Simple ohms law)
          
          I cannot see how the alternator could become damaged if you did play around with a shunt - even if you short cicuit the ammeter it is only the same as it not being there at all. (I guess open circuiting it may not be so good but the voltage control in the alternator should cope with this situation)
          
          Any one of the cables behind the dash is capable of carrying enough current to cause damage.
          
          Chris

      • Volt Meter or Ampmeter

        Posted By : Chris Povey, 08-Aug-2006, 03:45pm

        John,
        
        you are quite correct there are more sophisticated ways of measuring the current with devices such as on the link you posted. I have not seen these for automotive (in car) applications.
        
        However I have seen complete electronic dash boards with data logging included so I am sure there are many very clever ways of obtaining data relating to any measureable parameter on a car. I am quite sure that there are no 50amp cables connected to these units!!
        
        Chris