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Amp-Hour Meter for Solar Batteries

Discussion in 'Electronic Design' started by Marky, Dec 16, 2007.

  1. Marky

    Marky Guest

    Hi All,
    I am designing an Amp-Hour meter to log the current going into my
    solar batteries, which is charged by a PWM regulator. The regulator is
    pulsed at 100- 150hz. In my design I am using a shunt resistor to
    measure the current and a micro calculate and display the Amp-hour
    value. I need the current reading to be =<1% accurate, so what sample
    frequency I need to set the adc to get this result? How many adc bits
    I need ? Do I need to convert the sample voltage to rms voltage (e.g
    V* ((duty cycle)^-2)) to calculate the Amp -hour?

    Parameters
    Max charge current = 20Amp
    Shunt Resistor 0.005Ohm
    signal conditioning = op-amp with gain of 20
    Adc ref voltage = 3.3V

    Thanks

    marcus
     
  2. <1% ?

    Most all meters, scopes are 2% so forget about even a 1% accuracy.

    Cheers
     
  3. MooseFET

    MooseFET Guest

    You are using this on DC I asssume.

    20 * 0.005 = 0.1V

    You need a low offset voltage op-amp to amplify this up to a better
    voltage for the ADC's input. Since this is DC, you don't need much
    bandwidth. If you want the most accurate results you can get, look at
    chopper stablized op-amps.

    Next you need to limit the band width of what you put into the ADC to
    be much less than 1/2 the rate the ADC is sampling at. Again since
    this is DC, you don't need to provide much bandwidth. The cut off
    frequency of the filter will be set by how large of capacitors and
    resistors are practical. You want to use either plastic or NPO/COG
    capacitors.

    Once you have figured out the filter's bandwidth, multiply that by,
    lets say, 10 and you have the minimum sample rate.

    Chances are you will want to use a 12 bit ADC. The accuracy you need
    comes from its offset voltage and its linearity not simply from the
    number of bits.

    Assuming you have a micro reading the ADC, you can further improve the
    readings by averaging the ADC output over 0.1 second periods. This
    filters out most of the AC hum you may have picked up.
     
  4. something like this maybe?
    http://focus.ti.com/docs/prod/folders/print/bq2013h.html


    martin
     
  5. If you want to buy one instead of designing one from scratch, see:
    Why such accuracy?
    Well 1% is 100 "samples" over the range. Good form is to have +/- 1
    LSB accuracy, so that's 200 "samples". 256 bits is 8 bits of
    resolution.

    The 100-150Hz will result in some calculation problems, so it's
    probably best to just integrate the detected current across your shunt
    with a big cazapitor, and deal with slow changing DC instead of
    rapidly pulsing DC.
    Full scale voltage = 20A * 0.005 ohms = 0.1V
    A gain of 20 will give you 0 to 2 volts to feed to the A/D.
    No clue what you're using for an A/D converter but whatever you select
    will need to work with a 0 to 2V input range.
     
  6. Marky

    Marky Guest

    No reason, I thought it was a good starting point.

    I will give this a try.
    Interesting, I will order a freebie sample from TI to play with.

    I have plenty of 10k and 200K resistors in my tool box.

    Thanks guys, your help is appreciated!!
     
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