car battery current monitor

Discussion in 'The Projects Forum' started by brendonshaw1703, Jan 17, 2015.

  1. brendonshaw1703

    Thread Starter New Member

    Jun 7, 2014
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    I want to measure the total current consumption in my car, I want to add additional electrical items and worried I will have a flat battery. I also want to log the current, so thinking ardunio would be ideal to show the data with rtc to provide time stamp.

    The question is how to measure the current, I have been looking at shunts and hall effect sensors. Both seem to do the job, but when the vehicle current drops to 30mA at key off this is a very small current to measure and so many factors could affect it such as cable length to the sensor. The range will be 30amp's to 10mA with +- 1mA accuracy.

    I have been looking at amplifiers, but there is so much information about monitoring temperature with the shunt and protecting the hall effect sensor from external factors which could be present in the engine bay.

    I think either option would be hard to build as the accuracy of measurement at low currents would cause a very small signal which would be subject to noise and the ADC would need a large range to provide a very small resolution. I am thinking of using a teensy board with a 16-bit adc and is very small, so I can mount everything on the battery cable.

    Any help would be appreciated, I seem to go around in circles to understand if this is possible.
     
  2. tcmtech

    Well-Known Member

    Nov 4, 2013
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    If you want an accurate point to measure power loading on your vehicle going off the alternators output lead is the best point to work with. That will tell you exactly how much total amperage your system is using at any point in time when the engine is running.

    As far as hall effect sensors go you must be looking at the wrong types. There are countless commercial application grade types that are simple three wire devices (+ - Signal out) that will stand up to worst conditions you can ever imagine and will give accurate readings from MA to many tens of amps if you choose the right one.

    Nowas far as idle standby power draw from the battery when the engine is off they you would want to read the line that connects the vehicles electrical system to the positive side of the battery and not just from the heavy lead coming of the battery that also supplies power to the starter.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    16-bits would give a theoretical 1/2 mA resolution for a 30A maximum current but, as you note, achieving this in an automotive underhood environment is a challenge. Having the ADC near the measurement point will certainly help. The ADC digital output then can be routed to the processor inside the passenger compartment.

    A Hall-effect device has the advantage of no requiring the addition of a break in the wire to measure the current. Of course it may require more calibration than a resistive shunt to get the accuracy you want.
     
  4. brendonshaw1703

    Thread Starter New Member

    Jun 7, 2014
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    Thanks for the quick response and it sounds like the hall effect sensor will do the job. I was wondering could anyone recommend a suitable hall effect device?
     
  5. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Google is your all knowing omnipotent friend. :rolleyes:

    Start with 'electronics parts supply' for a general search and be prepared to be overwhelmed.
     
  6. bwilliams60

    Active Member

    Nov 18, 2012
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    First of all, what kind of devices are you referring to? Most devices hooked up to a vehicle's electrical system are designed to shut off when the key is turned to the off position or go to "sleep" after a short period of time, thereby creating very little draw. If done properly, no additional electronic gadgets should be necessary. Do these gadgets turn off when power is removed? Do they need to stay powered up?
    Secondly, if they draw excessive power, you may need to change the alternator output to a higher level and then you may have to change the size of the wiring to accommodate the larger alternator.
    Lastly, an alternator does not tell you how much power a vehicle is using all the time, only when it is running and that includes overcoming battery internal resistance. When shut off, the alternator is out of play. The only true measurement point is on the cable going to or coming from the battery and it would need to be a single lead for proper measurement. A typical OEM rating for parasitic draw is around 50mA. You should measure your draw first and then decide how to proceed.
     
  7. #12

    Expert

    Nov 30, 2010
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    That sounds a bit pessimistic...which is exactly what I was feeling.
    I had a 1948 Ford pickup truck that was equipped with everything I wanted except an air conditioner.
    Today, it seems that 12 or 15 microprocessors are not enough. Let's add another one!
    Not.
     
  8. #12

    Expert

    Nov 30, 2010
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    Then there is this part. Automotive modifications are not allowed on this website, and I think that if there are so many that you're concerned about leaving the battery dead, that is more than a monitoring circuit.
     
  9. tcmtech

    Well-Known Member

    Nov 4, 2013
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    I dont think that adding a hall effect current sensor over any stock wiring would count as a modification being none on the actual wiring is being modified in any way.

    As long as he says everything is either being powered off the vehicles cigarette/power ports of any of the factory supplied aux power points with stock wiring harness adapters and fuses he's good to go by our terms and conditions being none of the stock vehicles wiring is being modified or used in a modified manor. ;)
     
  10. #12

    Expert

    Nov 30, 2010
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    Thanks for explaining the Terms of Service and the way they are usually interpreted on this site.
     
  11. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Finding loopholes in rules is sort of my hobby.

    Mostly it's just to annoy the rule makers but hey that still counts as a hobby in my book. :D
     
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