Read ThingSpeak Data and Predict Battery Discharge Time with Linear Fit

This example shows how to read battery data from a ThingSpeak™ channel and analyze the data to determine the remaining battery life. Use a linear fit to predict the date that the battery will fail, and then write the remaining time in days to another ThingSpeak Channel. You read data for a 12 V battery connected to a microprocessor reporting its voltage to ThingSpeak every half hour. Then use regression to predict the day and time when the battery will fail.

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.