Using Amp Meter

Discussion in 'The Projects Forum' started by radio1ne, Aug 1, 2009.

  1. radio1ne

    Thread Starter New Member

    Jul 18, 2009
    Not a life shattering question, but one I'd like to pursue. Thanks for all your time.
    I would like to know if its possible to use a digital amp meter (not a multi-meter) to read the actual amp supply of an auto battery at all times. Both in a charging state (running engine) and a neutral state (ignition off). My "war wagon" (retired cop) has numerous accessories running off direct 12V DC and several thru an inverter. Voltage is always in a constant flux.
    Would it be possible to wire an amp meter into a circuit that would show available amps on the battery and the current loss as each accessory is turned on ? Thanks........Chief C Richard Jenkinson, ret.
  2. jj_alukkas

    Distinguished Member

    Jan 8, 2009
    Yes, you can wire one but will have to power the ammeter from a different power source other than your car' s battery. A DC-DC converter would be perfect.
  3. someonesdad

    Senior Member

    Jul 7, 2009
    This is a case where I'd recommend an older-style analog meter. You'd want to find the kind that reads both positive and negative current. They were always available on cars when I learned to drive and they're much more useful than a voltmeter. You can buy an el cheapo from Harbor Freight for $6. They don't say how it works, but it shouldn't be too hard to make it read whatever scale you want.

    The general principle of operation is that you'll insert a shunt resistance in series with the cable that supplies the car's power (excluding the heavy wire running to the starter). The shunt resistance will depend on what the largest current in the line will be you're going to measure (go here for details).

    Personally, I'd use a meter with a 100 μA to 1 mA movement. With a bit of searching, you can find both on ebay. Depending on the current you want to measure and the meter you have, you may also be able to make your own shunt.

    Then you connect two wires across the shunt that measure the voltage drop from the current and run them to the connections on the meter. You'll likely need a resistor in series with the meter and a couple of diodes across it to protect against accidental overvoltages. Once it's connected, it should work for a long time with no hassles. The convention was always that positive current meant the battery was being charged and negative current meant that it was being discharged.

    Personally, I'd stay away from the digital instrumentation in a car unless you're willing to invest in a DC-DC power supply to isolate it from the car's electrical supply.

    Added via edit: if you know what you're doing or have an electrically-knowledgeable friend to help, you can also make a current sensor using a Hall Effect sensor. This has the nice advantage that no wires need to be cut.
    Last edited: Aug 1, 2009
  4. russ_hensel

    Distinguished Member

    Jan 11, 2009
    Starting currents can be very high ( say 100 amp ). Two 50 wat headlamps are almost 10 amps. Most multimeters max out at 20 camps. Wiring to the starter must be very heavy. If the fuse blows in the meter everything in the car goes out. You can do this but sweat the details.