Read Data from ThingSpeak Channel

Start by storing channel and date information in variables, and then use thingSpeakRead to read the data. Channel 592680 shows the scaled measurement of voltage from a 12 V battery. Use the DateRange name-value pair to use a specific selection of data.

batteryChannelID = 592680;
startDate = datetime('Oct 20, 2018');
endDate = datetime('Oct 23, 2018');
batteryData = thingSpeakRead(batteryChannelID,'DateRange',[startDate endDate],'Outputformat','Timetable');

Convert the Data for Fitting and Plot

The channel stores raw data from the device. Convert the analog-to-digital converter (ADC) measurement to voltage using the experimentally determined conversion factor 14.6324. Then use scatter to generate a plot.

myVoltage = 14.6324 * batteryData.Voltage;
scatter(batteryData.Timestamps,myVoltage,'b');
ylabel('Voltage (V)');
hold on

Fit the Data

The timetable datetime format is useful for reading and plotting. To fit the data, the datetime needs to be in numeric format. Use datenum to convert the timestamps into a number of days, and subtract the starting number to keep the values low. Use polyfit to perform linear regression on the data, and polyval to evaluate the fit at the existing time values. Add the fit line to the previous plot.

battTimes = datenum(batteryData.Timestamps);
battTimes= battTimes-battTimes(1);
myFit=polyfit(battTimes,myVoltage,1);
fitLine=polyval(myFit,battTimes);
plot(batteryData.Timestamps,fitLine,'r--');

Predict Discharge Time

The battery should not be discharged below 10.4 V. Find the number of days until the fit line will intersect with this voltage.

endDays = (10.4-myFit(2))/myFit(1)

endDays = 13.1573

There are just over 13 days until the battery dies.

Write Prediction to ThingSpeak

The thingSpeakWrite function writes the result to a ThingSpeak channel. Return the output from thingSpeakWrite to ensure a successful write operation. Change the writeChannelID and writeAPIKey to write to your own channel.

writeChannelID = 17504;
writeAPIKey='23ZLGOBBU9TWHG2H';
result = thingSpeakWrite(writeChannelID,round(endDays,4),'WriteKey',writeAPIKey)

result = struct with fields:
         Field1: '13.1573'
         Field2: []
         Field3: []
         Field4: []
         Field5: []
         Field6: []
         Field7: []
         Field8: []
       Latitude: []
      Longitude: []
      ChannelID: 17504
        Created: 03-Jun-2019 15:24:43
    LastEntryID: 50018
       Altitude: []

The result shows the successful write operation and reports the data that was written.