USB / DC / Solar Lithium Ion/Polymer charger Thermistor

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corinthian
 
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USB / DC / Solar Lithium Ion/Polymer charger Thermistor

Post by corinthian »

I've built a USB / DC / Solar Lithium Ion/Polymer charger, and as directed in the tutorial (http://learn.adafruit.com/usb-dc-and-so ... he-charger), I've soldered the 10k Thermistor, and removed the surface resistor. To test the thermistor, I put it in the freezer, and the "power good" and "charge done" LED lights up, supposedly indicating that the charging has stopped (also seen in this forum post: http://forums.adafruit.com/viewtopic.ph ... or#p115465).

When I put the thermistor in hot water (over 50 degrees C), I don't get the same LEDs lighting up. Is the thermistor not working? I've tested the resistance of the thermistor, and it reads about 9Kohm, which is about normal. I'm not sure how else to troubleshoot this.

Any help would be appreciated. Thanks.

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adafruit_support_bill
 
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Re: USB / DC / Solar Lithium Ion/Polymer charger Thermistor

Post by adafruit_support_bill »

The spec sheet mentions factory preset voltage thresholds, but does not specify the associated temperatures:
The MCP73871 device compares the
voltage at the THERM pin to factory set thresholds of
1.24V and 0.25V, typically. Once a voltage outside the
thresholds is detected during a charge cycle, the
MCP73871 device immediately suspends the charge
cycle
60C is a common upper limit recommended by manufacturers.

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corinthian
 
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Re: USB / DC / Solar Lithium Ion/Polymer charger Thermistor

Post by corinthian »

I tested this in temperatures up to around 70 degrees C, and I still didn't get the LED readings. Sorry to be nitpicking, but this will be in a weather station project unattended, and I just don't want to have the battery catch fire, etc.

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Re: USB / DC / Solar Lithium Ion/Polymer charger Thermistor

Post by adafruit_support_bill »

I tested this in temperatures up to around 70 degrees C
What is the voltage on pin 5 when you do that?

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corinthian
 
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Re: USB / DC / Solar Lithium Ion/Polymer charger Thermistor

Post by corinthian »

Sorry, I figured it out. In each case where I was taking a reading, there wasn't an ample amount of solar charge occurring. When the board is receiving ample solar charge, the LEDs flash. Under these parameters, the thermistor accurately indicated a ceasing of charging when temperatures exceeded the 0 - 50 degree C range.

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Re: USB / DC / Solar Lithium Ion/Polymer charger Thermistor

Post by adafruit_support_bill »

Good diagnostic work!

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