Storage LFP during frost

[Nol]

I am busy to calculate an off grid system with PV panels in combination with 2 LFP batteries of 300Ah
It is for a vacationhouse without power in the north of Sweden, around Östersund
Because we are mostly there during the summer I am intent to disconnect the LFP and store them in the house during the winter.
But is it possible to store a LFP while it freesing around ? House is not heated during the winter and it can freese around -20°C
If not I have to take them back home or find an other solution.

 

[FilterGuy]

But is it possible to store a LFP while it freesing around ? House is not heated during the winter and it can freese around -20°C
If not I have to take them back home or find an other solution.

-20C is not too bad for storage. That is within the storage spec of some cells. Do you know what the storage spec on your cells is?

I had a very similar situation and built a well insulated battery box with this warmer:

Simple, Indipendent Battery Box Heater

To get the document, click on the orange button at the top of the page. This is a simple 20W battery Box heater that operates independently of the rest of your system. I use this for storing unused cells in a very cold climate, but it can be...

diysolarforum.com

Note: Be sure to disconnect the battery and BMS harness while you are away.

 

[Nol]

I don´t know any specs because I don´t have them yet, I am orienting what is possible
For my RV I have these but for my vacationhouse I think about 300Ah or bigger like these

Thanks for the link to the warmer which does not drain the battery, that is good.
But the PV is 24V - 20W and the warmer is 120V - 250W
Can you explain to me how it works ?

 

[FilterGuy]

But the PV is 24V - 20W and the warmer is 120V - 250W

Panel:
Rated maximum power: 20 watts; Voltage at Pmp : 35.2 volts; Current at Pmp : 0.57 amps
With the Pmp of 35.2V and Imp of .57A, the ideal resistance to make the panel operate at its peak is 35.2V/.57A = 61.75ohms

Heat Pad
120 Volts | 250 Watts
The heat pad draw 250W/120V=2.08A when plugged into 120V
That means its resistance is 120/2.08A = 57.6ohms

So... if you hook the pad directly to the solar panel, it will be just about right to get the full 20W out of the panel.
(It is only a 20W pad the way it is being used.... but in an insulated box, that is all you need or want.)

 

[FilterGuy]

I don´t know any specs because I don´t have them yet, I am orienting what is possible
For my RV I have these but for my vacationhouse I think about 300Ah or bigger like these

It is good that you are figuring everything out before you start buying. I see too many people that start buying before they have a complete design and then realize they bought the wrong thing.

When you decide what cells to buy, ask the vendor for storage temperature specs. You might find that you don't have a very big problem.

 

[Nol]

Thanks for the explanation which I understand, basic rules are comming back ?
I keep it on my mind ?

 

[upnorthandpersonal]

Also, research is showing that storing at freezing temperature slows calendar aging.

How to Find Happiness With LiFePO4 (Lithium-Ion) Batteries –...

By: Rob Beckers You have just sold your first-born into slavery, remortgaged the house, and bought yourself a lithium-ion battery! Now you want to know how to take care of your precious new purchase: How to best charge lithium-iron batteries, how to discharge them, and how to get the maximum...

www.solacity.com

 

[FilterGuy]

BTW: I just updated the resource for the Independent Battery Box heater.

Added a page showing the resistance calculations
Added a page showing an option to use resistors instead of a heat pad.

Simple, Indipendent Battery Box Heater

To get the document, click on the orange button at the top of the page. This is a simple 20W battery Box heater that operates independently of the rest of your system. I use this for storing unused cells in a very cold climate, but it can be...

diysolarforum.com

 

[Nol]

@upnorthandpersonal thanks for the notice.
I don´t understand this :
Storing them at low temperatures is certainly much better than storage at high temperatures.
Calendar aging slows down dramatically at low temperatures.
It say it is better to store them at low temperatures and later it say it slow down dramaticly at low temperatures
Should it be : Calendar aging slows down dramatically at high temperatures

@FilterGuy
Do I have to disconnect the BMS completely while I am away ?
I mean disconnect all cables from the cells ?

Resistors is a good option and cheaper ?so thanks for the update
I understand they need to be fixed on an aluminium plate and the plate can stand beside the LFP cells ?

 

[FilterGuy]

Do I have to disconnect the BMS completely while I am away ?
I mean disconnect all cables from the cells ?

When the balance harness is hooked up, the BMS is powered on and drawing a bit of power. If the cells are going to be unattended for a long time, this could drain them to a damaging level. Consequently, I disconnect the harness from the BMS.

I also disconnect the Main Positive from the rest of the system. This guarantees there is no parasitic loss from the rest of the system. (I have a big switch installed to do this.)

I do not disconnect the bus bars between cells.

I understand they need to be fixed on an aluminium plate and the plate can stand beside the LFP cells ?

With only 20W, the plate probably won't get very hot. However, I do not like to let the plate touch the cells..... just in case.

 

[upnorthandpersonal]

I don´t understand this :
Storing them at low temperatures is certainly much better than storage at high temperatures.
Calendar aging slows down dramatically at low temperatures.
It say it is better to store them at low temperatures and later it say it slow down dramaticly at low temperatures
Should it be : Calendar aging slows down dramatically at high temperatures

It slows down calendar aging at low temperatures. Calendar aging is damage that the battery suffers not from use, but just from age. Slowing down calendar aging is a good thing.

 

[Nol]

@FilterGuy
If I understand well then just unpluging the balance cables from the BMS is enough
My intension is to disconnect the main positive and negative from the rest of the system

@upnorthandpersonal , thanks I misunderstand it