Options for running LiFePO in cold weather (-25c)

[jailcee]

I have my solar system with LFP 24v battery at an of grid cabin that, in the winter, can get to -25c during cold snaps. I'm using an overkill BMS so I have low temp cutoff but my question is how have others solved the issue of wanting the system to keep running in low temps?

While the place warms up relatively quickly when I'm there, it could go a couple of weeks without heat and at cold temps. I've seen Will's tip of using RV tank hearing pads, which at first seems like an ideal solution, but every pads I find seem to have at least one review where the pad sorted out and could have started a fire so I'm less keen.

One option is to simply only have it charging when I'm there and the battery is warmed up but that's hardly ideal with solar. I'd rather the system charge itself when I'm away.

Anyone already solved this for themselves?

 

[FilterGuy]

I have my solar system with LFP 24v battery at an of grid cabin that, in the winter, can get to -25c during cold snaps. I'm using an overkill BMS so I have low temp cutoff but my question is how have others solved the issue of wanting the system to keep running in low temps?

While the place warms up relatively quickly when I'm there, it could go a couple of weeks without heat and at cold temps. I've seen Will's tip of using RV tank hearing pads, which at first seems like an ideal solution, but every pads I find seem to have at least one review where the pad sorted out and could have started a fire so I'm less keen.

One option is to simply only have it charging when I'm there and the battery is warmed up but that's hardly ideal with solar. I'd rather the system charge itself when I'm away.

Anyone already solved this for themselves?

LiFePO4 has a very low self discharge so I completely disconnect the cells when away. I even disconnect the balance harness from the BMS.

The Cells are stored in a very well insulated box (~R30) and I have this to keep the temp up:

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

 

[DJSmiley]

A simple heater will do the job and keep the cells from freezing, thus allowing charge.

If the box is properly isolated, the power draw won't be massive, and any available solar should be able to keep up with the additional powerusage of the heater.

You could also include a solar radiance meter: If it detects enough light radiation and temp is too low, enable the heater.
Advantage: Heater is powered by the sun (it's only on when there is sun).
Disadvantage: It takes a while for the cells to reach their temperature to allow charging. Placement of the temperature sensor is critital, since the outside of the cells are faster up to temp than the full cells 'deep inside'.

I would go for the fist option, since that will give the cells time to normalize the temperature even in the core.

You might look into LiFeYPo4, that is more suitable for low temps, but that only makes sense when getting new cells.

 

[HRTKD]

If the cells aren't kept at a temperature above 32° F and allowed to follow ambient temperatures, it could take a very long time to bring the cells back up to the point where they can be charged. Charging below 32° F is not recommended. Discharging below 32° F is OK, but I wouldn't do it at a high rate.

I keep my cells between 35° F and 45° F so they can be charged whenever the sun comes up and the PV becomes active.

 

[jailcee]

I agree that if cells are allowed to get to very low temperatures at their core (like -20) it would take a long time to warm them back up just with ambient in order to charge them.
I also agree that a system that can draw power from the batteries themselves to keep them heated above at least 2c, in a well insulated enclosure, is the right solution.
My biggest concern is with anything purposefully creating heat through resistive current is a component failure that causes a runaway heating, leading to a fire. I'm looking at FilterGuys's plan and I could put something like that together. I'd want the best, least likely to fail components here, not the cheapest I can find on Ali. Any recommendations on where to source that?

 

[HRTKD]

I purchased pad heaters for my cells/battery. My approach can be found in this thread:

LiFePO4 Battery Warmer Install

This is my install of a custom set of battery warmers from Ultra Heat. I had contacted them through email asking about a warming profile for LiFePO4 batteries. They responded with a design to my specifications, including warming pads that fit my batteries exactly. While I initiated the...

diysolarforum.com