Temperature reading off - calibration possible?

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MartinB
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Temperature reading off - calibration possible?

Post by MartinB » Sat Jun 18, 2016 9:32 am

Hi everyone,

I am new to the ultrasmartcharger and would appreciate any help.

I noticed, that the temperature reading from one of my chargers is off by 15°C (reads 39°C instead of actual 24°C).
The reading for the second charger is off as well, but not so much (like 3°C or so).

I wonder what's causing this. Is there a way to calibrate the sensors?
The delta T seems to be plausible, so I guess the sensors itself are working properly.

Best regards
Martin

Edit: I am using the Rev. 3 (HW Version 3.01 FW Version 1.000 Compiled Jun 28 2015)

Mark
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Re: Temperature reading off - calibration possible?

Post by Mark » Sat Jun 18, 2016 10:38 am

Hi Martin,

I'm guessing that you're seeing the same error on both channels of that charger which is reading 15 degrees high?

I'm curious as to what's actually causing that - if both channels are giving the same error, the sensors are probably fine - it might be the voltage regulator which is putting out an incorrect voltage and that would cause errors with the ADC readings...

There isn't any system built in for calibrating the temperature readings - one way to fix it would be to adjust the constants in the source code and recompile and upload it to that charger. To have it reasonably accurate though, we would really need to know what is causing the error - otherwise we don't know whether it's an offset error or a scaling error, or possibly both - simply adjusting the offset would fix the reading at that temperature, but wouldn't necessary correct it fully at different temperatures.

BTW, if I recall correctly, the temperature sensors are specified to be accurate to within +/- 1 degree Celsius, so even the other charger being out by 3 degrees is rather strange - I haven't noticed any significant errors like that with any of the chargers that I've got...

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MartinB
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Re: Temperature reading off - calibration possible?

Post by MartinB » Sat Jun 18, 2016 11:25 am

Hi Mark,

to illustrate things, I made a screenshot of my current charging process:
screenshot.png
screenshot.png (42.17 KiB) Viewed 7190 times
The attached photo shows the setup. Charger on the left is on COM5, charger on the right is on COM6.
I insterted cells in bay 2 (idle), just to get a temperature reading.
I measured the actual temperature using an IR thermometer.
Ambient temp. is around 25°C at my desk. Temp. of cells beeing charged is a little higher (25,9°C).

I would say reading of charger at COM5 is acceptable. That of the one at COM6 is likely to cause trouble for cells actually getting hot.
The offset is elevated for both bays, as you have guessed.

Please let me know, if you need any additional information!

Best regards
Martin
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IMG_7907.png
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Re: Temperature reading off - calibration possible?

Post by Mark » Sun Jun 19, 2016 12:56 am

Are you able to do another test when the ambient temperature is either significantly higher or lower - that should make it possible to tell whether it's just an offset error, or if there is also a scaling error as well.

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Re: Temperature reading off - calibration possible?

Post by MartinB » Mon Jun 20, 2016 11:09 pm

Sorry, that it took me so long to reply. But finally I found time to test at another temperature:
image001.png
image001.png (57.75 KiB) Viewed 7185 times
I used my convection oven at 50°C.
image003.png
image003.png (57.18 KiB) Viewed 7185 times
Measured again using my IR Thermometer. Temperature was distributed evenly for both pcb's (50,3°C).
Screenshot.png
Screenshot.png (40.96 KiB) Viewed 7185 times
Screenshot of USCManager.

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Re: Temperature reading off - calibration possible?

Post by Mark » Mon Jun 20, 2016 11:15 pm

OK, that's quite a big error - around 36 to 37 degrees!

I'll have to work out the adjustments required and get back to you.

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MartinB
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Re: Temperature reading off - calibration possible?

Post by MartinB » Mon Jun 20, 2016 11:20 pm

Finally I put the data into Excel. Gave me this:
TempPlot.png
TempPlot.png (24.49 KiB) Viewed 7184 times
I think it is save to say, that we do not have a fixed offset error here.

I attached the .xls worksheet as well.

Please let me know if I can do anything else to help to figure this out.

Best regards
Martin
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Temperature-Test.xlsx
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Mark
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Re: Temperature reading off - calibration possible?

Post by Mark » Mon Jun 20, 2016 11:47 pm

Do you have access to a multimeter perhaps?

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Re: Temperature reading off - calibration possible?

Post by MartinB » Tue Jun 21, 2016 4:45 am

Yes, of course. What measurement do you need?

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Re: Temperature reading off - calibration possible?

Post by Mark » Tue Jun 21, 2016 4:49 am

If you remove the micro SD card from one of the chargers, you will find the pads for the ISP header underneath.

With the charger facing so that the ISP header is closest to you, can you measure the voltage between the leftmost 2 pads. If you can do that and report the voltages on both of your chargers, that might help to pinpoint the source of the error.

If you're not clear on which pins I'm referring to above, let me know and I'll make an image showing the 2 that I'm referring to.

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