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Frozen LiFePO4 batteries

umdumchuck

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Nov 19, 2020
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I understand that you shouldn't or can't charge a LiFePO4 battery below -20 C.
Does it hurt the battery if it drops below freezing, if it's disconnected? Does it degrade the battery?
I plan on using a BMS with a temperature sensor.
My system will be in an unheated cabin, while not in use.
 
Charging below freezing causes plating of the lithium ions onto the anode. This is a permanent loss of the lithium ions used to transport charge. Causes severe capacity loss.
 
I understand that you shouldn't or can't charge a LiFePO4 battery below -20 C.
Actually, you should not charge below 0C

Does it hurt the battery if it drops below freezing, if it's disconnected? Does it degrade the battery?
I plan on using a BMS with a temperature sensor.
My system will be in an unheated cabin, while not in use.

I have researched this a lot and have not found a definitive answer. However, it seems that LiFePO can be stored down to about -20 C.

In my cabin, I have a well insulated storage box inside the well insulated cabin. In the battery box I have a heat pad hooked up to a 0F mechanical thermostat and and directly to a small 20W solar panel (no battery and no charge controller in the circuit) Over the winter, the cells are disconnected from everything (included the BMS)
 
Thank you. I'll do the same. Since I already have a 12 volt Lead Acid 400 watt system, I'll keep it for winter use with a battery warmer. My initial plan was to set my BMS to disconnect well above the danger point. Now, I think I'll do what you do, disconnect the LiFEPO4 batteries. Thank you for your help. How big is your system and what BMS do you use?
 
It is a 24V 200aH 1p8s battery bank with an ANT BMS. It uses a 3000W inverter.

Edit: Corrected the Inverter size.
 
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Thank you. I'll do the same. Since I already have a 12 volt Lead Acid 400 watt system, I'll keep it for winter use with a battery warmer. My initial plan was to set my BMS to disconnect well above the danger point. Now, I think I'll do what you do, disconnect the LiFEPO4 batteries. Thank you for your help. How big is your system and what BMS do you use?

BTW: My installation is a little unique because my charge source is a micro-hydro generator that is shut down in the winter. Consequently, any drain on the battery over the winter could result in a sever undercharge. Since the self-discharge rate for LiFePO4 is so low, (~2%/month) I can let them set for several months without worry about undercharge. However, I have to worry about cold storage..... thus the simple heater and small solar panel.
 
I am newish here, had an issue not sure what it was, but I am here now.

I am building a teardrop camper using 2 of the amp outdoors 60 ah lifepo4 batteries I saw here. To prevent them charging when its cold I bought one of the cheap 12 volt thermostats and wired it to 2 12 volt bee hive heating pads one under each battery. I also hooked the output to a 30amp relay that interrupts the line from my charger. I have the sensor taped to one of the batteries, between the two of them, I have it set to come on at 4 C and turn off at 8 C. The nice thing about this setup is I have it on a switch, when the switch is off it will just charge nomally, so I just need to turn it on when it gets cold out. It has been keeping my batteries around 5 to 7 C.

Yes I know I am using battery power to run the relay and the heating pads but I will only use it during charging so not a great loss.
 

I saw your heater design and it was really helpful. Thanks for posting it.

I'm thinking of doing something similar with one of these RV water tank heaters wrapped around a metal battery box. The pad has built in thermostat control with an operating range of 45-68 degF, and it's rated for 5.8A at 13.5V, for 78W output.

I am planning to connect it either to the load output of the MPPT or to 12V battery, and I'm not sure if it makes a difference. I was thinking that the MPPT load output might allow the heating pad to run off of the PV even if the battery is in a low temperature state, and to run off of the battery if there is no sun. However I'm not sure that the MPPT load output will stay live when the battery is at low temperature.

Is there a standard low temperature behavior for MPPT's, or is it manufacturer specific?

Am I right in thinking that this kind of small load is what the load output on the MPPT is for?
 
I am not seeing much benefit to using the load output. My thought is to put as little stress on a costly SCC as possible.

Then I guess the answer is just to run it off the battery. Maybe I'll find a way to manually reconnect it to run it off the panels directly in the (unlikely?) event that the battery is below its discharging temperature or fully discharged below its charging temperature.
 
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