help measuring power in excel

Discussion in 'General Electronics Chat' started by toffee_pie, Sep 16, 2014.

  1. toffee_pie

    Thread Starter Active Member

    Oct 31, 2009
    Hi all,

    I want to do some measurement/data logging and quite keen to plot some power graphs (Wh/Ah) for battery tests. I have a volt meter than I can use for Current/Voltage logging and a current load sink for constant current. I also have thermo-couples and can log battery temperature too.

    So, I am wondering how I can do plots of power (x axis Wh or Ah, y axis voltage) bearing in mind i cant log current and voltage at the same time. Is there any way around this other than doing one plot for current and the other for voltage and super imposing them or the like.?

    It seems I need to integrate the time scale to get power plots but not sure how i can do this in excel.

    any thoughts?

  2. MrCarlos

    Active Member

    Jan 2, 2010
  3. shteii01

    AAC Fanatic!

    Feb 19, 2010
    You can log current and voltage "at the same time". If you measure voltage, then measure current 10 milliseconds later, nobody is going to care that it took you 10 milliseconds to switch from one meausrement to another.

    I have used this:
    To get data from uC to Excel, then you can plot and do all kinds of perverted Excel things.
  4. toffee_pie

    Thread Starter Active Member

    Oct 31, 2009
    I dont have an arduino, i should have enough equipment as it is but want to check for sure. !

    I have the agilent 34461A meter to do my logging (V,I) and a B&K load to discharge the cells. I am still not sure how I can extract my results and get meaningful graphs, indeed I also have voltage losses to deal with due to the internal resistances of the cell and attached test equipment.

    Can I get around with just measuring current and use a series resistor to calculate a voltage drop?, this should give me power and i could convert to Ah/Wh, probably need to integrate also over the time scale? :$

    thanks for the replies.
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
    I use a 1000W locomotive braking resistor to discharge a large lead-acid battery bank. Unfortunately, the resistance or the large resistor varies with its temperature, so I had to log both voltage across the resistor, and current through the resistor using two different A/D channels to get accurate plots of WattHours. Computing a running integral of the product of V*A over time is a trivial calculation in Excel or in an Arduino.
  6. wayneh


    Sep 9, 2010
    You have two very different issues; 1) getting the data into Excel and 2) manipulating the data. As Mike notes, if you're skilled in Excel then any plotting you want to do is "trivial". If you need help in Excel, there are lots of sources to help you with that.

    Getting the data to Excel is called data acquisition and you need hardware to accomplish that. There are many, many choices. I use a device called a Labjack U3HV, which I am very impressed with. It's about ~$120 and has many powerful features. I have used it successfully to collect data and feed live plots in Excel. It's probably huge overkill for your particular task but maybe you would find it useful for future work also. It would spare you from learning about Arduinos and such, but maybe that learning would also be useful to you. Like I said, choices.