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Homemade Low Temperature Disconnect for LiFePO4

Dutchman

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Mar 15, 2021
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I'm going to replace my kaput Pb (lead acid) batteries with LiFePO4 soon. One thing that was holding me back was finding a low temperature disconnect to prevent charging ( nd destroying) the batteries below 0 degrees C. The few commercial devices I found seemed pretty expensive and overkill ( I don't need Bluetooth on it thank you very much). Witha temperature sensor, 200A relay, an RC snubber circuit to suppress arcing, and a Zener diode and power resistor to regulate the temperature sensor power voltage this will do the job. And I spent $35 and that is enough to make two units with spare parts left over.
 

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If you have not bought the batteries yet, why not just buy ones with a BMS that has working low temperature cut off? More and more of even the low end models that have came on the market in the last 6 months seem to have them, and if you are building your own, just buy a BMS with this feature
 
If you have not bought the batteries yet, why not just buy ones with a BMS that has working low temperature cut off? More and more of even the low end models that have came on the market in the last 6 months seem to have them, and if you are building your own, just buy a BMS with this feature
So far all I have found that has low temperature cut off is the S OK battery and $1000 for 200 amp hours. Meanwhile I have found comparable batteries which Will gave a good review of for $650. But they have no low temperature disconnect
 
I just bought a pair of (irregular, mis-made, etc. ReBel 200AH batteries for $688,plus tax and $30 shipping) which has low temp cut off (short version the batch was built with a non-waterproof switch, and a 100AH, not a 150AH BMS), so they were discounted by about $170
 
I just bought a pair of (irregular, mis-made, etc. ReBel 200AH batteries for $688,plus tax and $30 shipping) which has low temp cut off (short version the batch was built with a non-waterproof switch, and a 100AH, not a 150AH BMS), so they were discounted by about $170
Im getting new 200AH 12v LiFePO4 for $650 Shipped plus tax. BMS has all but low tem protection.
 
Not that I entirely disagree with avoiding Bluetooth, but a Bluetooth bms does double duty as a charge monitor showing battery level. Can save you some coin overall since you don't necessarily need a shunt then.
 
Witha temperature sensor, 200A relay, an RC snubber circuit to suppress arcing, and a Zener diode and power resistor to regulate the temperature sensor power voltage this will do the job

This goes between the panels and the solar charge controller? Or is it between SCC and battery bank?
 
This goes between the panels and the solar charge controller? Or is it between SCC and battery bank?

Thinking aloud:

It would need to go between the panels and controller wouldn’t it. Disconnecting the SCC from the battery while having the panels connected can cause controllers that auto recognise battery voltage to misread it

In cold temperatures you only want the charging turned off. Next was to do that would be turn off the charge source only ie the panels

This would likely mean lower rated components too. PV tends to be low constant current compared to the spikey high current power demands of systems with inverters
 
Thinking aloud:

It would need to go between the panels and controller wouldn’t it. Disconnecting the SCC from the battery while having the panels connected can cause controllers that auto recognise battery voltage to misread it

In cold temperatures you only want the charging turned off. Next was to do that would be turn off the charge source only ie the panels

This would likely mean lower rated components too. PV tends to be low constant current compared to the spikey high current power demands of systems with inverters
There is a low temp discharge limit too you know.
 
I’m happy with my batteries from eBay, 299 shipped, plus tax, and has low temp disconnect… so… deal?
 
What would that be? Would it be expressed as a C value?
Usually -20 degrees Celsius or -4F.

After that you "can" discharge very slowly but that's not really much different than charging. You "can" charge really slowly at low temps but it's still not a great idea.
 
Will it be auto reset? If it is does, does it have adjustable hysteresis?
Not that I entirely disagree with avoiding Bluetooth, but a Bluetooth bms does double duty as a charge monitor showing battery level. Can save you some coin overall since you don't necessarily need a shunt then.
Sorry but qhat do you need a shunt for? Monitoring current?
 
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