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THERM CIRCUIT - USB LiIon/LiPoly charger - v1.2
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THERM CIRCUIT - USB LiIon/LiPoly charger - v1.2

by AJ1150 on Wed Apr 07, 2021 2:58 pm

USB LiIon/LiPoly charger - v1.2
PRODUCT ID: 259

I am adding this charging circuit to a custom PCB to allow for LiPo charging and LiPo/USB power. After reviewing Adafruit's files and reading the entire data sheet on the MCP73833 I don't quite understand what is going on with the Therm circuit. The following is written as a note on the Adafruit schematic;

"No Thermistor
RT1 = 0; RT2 = NC THERM=10K
10K NTC Thermistor
RT1 = TBD RT2 = TBD"

The datasheet shows a formula for calculating the Therm setup - but I'm trying to figure out how Adafruit produces their board.

Thank you!
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Re: THERM CIRCUIT - USB LiIon/LiPoly charger - v1.2

by adafruit_support_mike on Wed Apr 07, 2021 10:17 pm

When you charge a lithium cell near its maximum limit, it heats up. If the temperature gets too high, the polymer/electrolyte will start to break down and release hydrogen. That makes the cell expand, and creates the chance that the cell will catch fire.

Because of that, most LiPo chargers have an input that can read a thermistor and throttle the charging current if the cell temperature gets too high.

The thermistor has to be built into the cell to monitor its temperature, so our board just has a 10k resistor as a dummy connection to that pin. If you want to use a LiPo with a built-in thermistor, you can solder the wires from the thermistor into the PCB and cut the trace to the 10k surface mount resistor on the board.

The Solar Charger operates below the maximum charging current of most LiPos, so it won't produce enough heat to make a thermistor necessary.

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Re: THERM CIRCUIT - USB LiIon/LiPoly charger - v1.2

by AJ1150 on Thu Apr 08, 2021 8:52 am

Thank you for the response. So if I were to use a battery similar to the standard LiPo batteries Adafruit sells, with the silver wrapping and yellow tape with a 2 pin JST...

such as this one:
http://cdn.sparkfun.com/datasheets/Prot ... 1.0ver.pdf

Since those batteries don't have an external connection for the Thermistor then I would not connect a Thermistor or RT1/RT2 to the Therm pin on the MCP73833? I would just simply connect one single 10k Ohm resistor from ground to the Therm pin - as the schematic attached....

Thank you!
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Re: THERM CIRCUIT - USB LiIon/LiPoly charger - v1.2

by AJ1150 on Thu Apr 08, 2021 11:17 am

Also - I was planning to charge a ~110mAh battery at 40mA; which is over the standard charge rate of 0.2C(22mA) but obviously way less than 1C. Shouldn't be any problems with that? Even if I were to use a smaller 70mAh battery?

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Re: THERM CIRCUIT - USB LiIon/LiPoly charger - v1.2

by keantmp on Thu Apr 08, 2021 4:09 pm

Hi Aj, any update on this yet now?

Regards,
Diana G Brown

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Re: THERM CIRCUIT - USB LiIon/LiPoly charger - v1.2

by AJ1150 on Fri Apr 09, 2021 8:59 am

**EDIT: JUST NOTICED THE ACTIVE LED IS BACKWARDS**

I just posted this a couple of days ago, with my last post being a few hours before yours. No update per se but I have gone through and modified the circuit to build directly into my custom PCB. As far as I know it should work - I just need to double check the resistors on the LED's, since one of the three has different forward current and voltage drop.

From Mike's response - it seems that without a Thermistor you are to just connect a 10k Ohm resistor in series from Therm to ground as my schematic shows.

Also FYI I had another question about this board, which was does the power on LED (PG) draw any current with just the battery connected. After reading the data sheet (several times) it appears the mcp73833 remains off until there is a voltage at pins 1 & 2 greater than the battery voltage at pins 9 & 10; therefore not drawing and current from the battery.

I have it set to charge at 40mA by a 25k Ohm resistor - which I hope will be okay, I plan to use a very small LiPo (40mAh < LiPo < ~150mAh)
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Please be positive and constructive with your questions and comments.