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We have run into a requirement where we need to terminate the charging of our Lithium Polymer battery when the battery temperature exceeds the manufacturer rated temperature.

My immediate reaction is to wire in a thermocouple to the battery pack and run that into the MCU input which would then might toggle some sort of MOSFET charging cutoff switch. Obviously, this would be pretty disruptive to an already very space constrained design.

Are there any cleaner solutions to meet this requirement?

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    \$\begingroup\$ Go and search for a "battery protection IC", there are plenty of those that include an option to use a thermistor (temperature dependent resistor) to monitor the battery temperature. A thermocouple is too cumbersome as these generate minute voltages which have to be amplified. Also an MCU is overkill. A temperature cutoff can be made using a thermistor, a couple of resistors, a MOSFET and an opamp. \$\endgroup\$ Commented Sep 21, 2020 at 14:16

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When I've designed battery packs we've always used thermistors to monitor the centre of the pack of cells.

Thermistors are easier and often cheaper than thermocouples to get hold of. Thermocouples are tricky things to measure, as you need to start doing cold junction compensation when you have a junction of different metals. Then you are measuring down the the uV range. Tricky do to cheaply and easily.

My BMS designs use a thermistor (NTC) plugged into a standard microcontroller's ADC. The micro controller then turns the charging FET on or off. This is the cheapest way to we found to make charging li-ion packs safe.

We have also done it with various Battery monitoring ICs, which often have a dedicated FET output (different ones have high side or low side drivers for charging and discharging set ups) and dedicated NTC (thermistor) inputs. So it's all there ready for you, just need to add the FET and NTC.

You talk about it as though it's a nice to have, if you plan to sell this product in the EU or US, you'll need more protection than just this (I believe the UL regulation for the US basically means you need to have two separate FETs which can isolate the pack during charging). Charging a li-ion pack is a dangerous thing: they can explode if handled badly. Please make sure you have proper control of the charging and discharging of the pack.

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Thermocouples are inappropriate since they (to a first approximation) measure temperature differences- meaning you would definitely need a second temperature sensor not in the battery pack. The voltages are also rather small so they require care to get good readings.

From what I've seen, the industry standard for batteries is a cheap precision 10K NTC thermistor.

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