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Cheapest way to detect 0°C (+/-5°C) to limit lipo charge current when <0°C
AloyseTech:
Hi,
I'm designing a device running on a Lipo battery. The device temperature can be from -30 to +50°C. I need to ensure the charge current of the lipo is limited to 15mA or less when the temperature is below 0°C, but it is mandatory to have an higher charging current (50mA at least) when the temperature is higher than 0°C.
The cost of the solution is very important and must be very low. The charger is a HX4054A (cheap charger similar to MCP73831) : ~ $0.02 including attached passives.
Is there a better option than using a cheap analog comparator (~$0.05)? Maybe the good thermistor could be used as the Rprog directly, but it would require a higher beta coefficient than standard 3000-5000, right?
jbb:
I would suggest getting a charger with a temperature sensor channel built in. Might cost a few more cents, but will be smaller and likely more reliable. And provide high temperature shutoff too, which I highly recommend.
How is the battery attached? A battery with a 3 wire (+, - and NTC) lead can lose accuracy due to voltage drop in the -be lead. This is usually only a factor at high temperatures.
If the NTC is on your PCB near the LiPo, I suggest doing some lab testing to make sure the NTC temperature is close to the LiPo temperature. Heat flows along PCBs and through case plastics can be surprising.
Maybe you could use a temperature switch IC? You might be able to find a cheap one with a preset 0C trip point and open drain output or something.
AloyseTech:
Using a most advanced charger IC with temperature management is possible, but I've not been able to find a cheap solution.
I forgot to mention that it is mandatory that the battery charges below 0°C, the charger must not shutoff. Most charger with temperature management will simply shutoff if not between 0°C and 125°C.
My battery pack is a 2 wires lipo, the temperature sensor must be on the PCB. Lab measurement to initially trim the system is possible but a per-product calibration must be avoided.
Also, I already have an over current protection by design.
Do you have some cheap temperature switch reference ? I could not find any really cheap one...
xani:
resistor/thermistor divider driving a transistor working as cheap comparator ? You probably won't find anything cheaper unless it is a specialized chip
AloyseTech:
--- Quote from: xani on December 11, 2019, 01:37:48 pm ---resistor/thermistor divider driving a transistor working as cheap comparator ? You probably won't find anything cheaper unless it is a specialized chip
--- End quote ---
You're probably right. The issue with this solution is that I have to find a transistor with a well characterized threshold. Would you have some part number in mind?
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