Operating temperatures for off-grid batteries

What battery chemistry?
Lithium-Ion ?

Yes, probably. Thinking about going with a single Tesla Model S battery module. My understanding is that you don't need to worry about hazardous off-gassing, venting, fires, etc with these sealed batteries.

Is the Yurt in a (permanent) location, or are you moving from place to place? For a fixed location you might dig a hole in the ground for a waterproof battery box (plastic box) with a lid covered by blue board insulation (extruded polystyrene). My batteries in an unheated outbuilding are wrapped with just blue board and seem to hold at about 53 F during the winter months near Silver City, NM at about 7,000 ft.

Yes, it's in a permanent location and we're not planning on moving it in the near future. I like the hole in the ground idea, unless it ends up having to be too deep where it will be warm enough and make it difficult to reach for service/maintenance.

I'm surprised that an unheated outbuilding stays that warm. How big is the outbuilding?

As suggested earlier, I would consider an insulated box. Some of the ways to keep batteries from freezing is to use a heating pad or heating wires in the bottom of the box. There may be challenges with that depending on your source of additional electricity.
Since you are using a Tesla module I assume you will have a 24 volt system. I believe a BMS is essential to prevent over charge or discharge. It will not prevent the battery from freezing but many do have temperature sensors that can disconnect if the batteries get to0 cold to safely charge or discharge.

The outbuilding is a generator building is about (12 ft x 12 ft) with two huge holes in it, a screened area on one wall overhead, and a fixed louver above the concrete floor. The blue board wrap is all that is used around the FLA L16 batteries. The top is simply a a loose sheet of 1-inch blue board with scraps of blue board to allow for spacing the batteries gasses to escape. The heavy juniper canopy in the winter does not allow for much sunshine to heat the building.

One thing I have a problem with in other areas of the homestead are rodents, they like to chew on wires. They killed my car once and I had to have it hauled to Town for repairs. So you might want to consider conduit or wire looms to protect from squirrels and chipmunks and others. Skunks, porcupines, and raccoons do not seem to be a problem at my location. I have bears here, but in the 6.5 years I have been out here, they have not destroyed anything. I do not leave trash outside and generally haul it weekly to Town.

Typically June is the hottest month, hovering near 90F, but the shade of the building keeps the sun off of the batteries, And typically they do not exceed 95 F in the summer. They do get hotter if I do not open up the blueboard wrap, I have seen about 104 F once, because I was late in opening up the blue board.

Yeah, there's a bit of a chicken and egg problem here. Need power to get the power. Do you happen to have a link to a product you might know about, for the heating pads?

Yes, I'd go with 24V and add a BMS. Seems pretty straight forward, as long as you're following a well designed plan, which I will. Basically, I'm gonna treat the yurt like an RV and use RV style appliances, preferably propane wherever possible (fridge, freezer, small cooktop, small heater). I'm assuming all of these might need some kind of power to start and run, so that's where the solar comes in to save the day. But I don't want to run full electrical appliances if I can avoid it. And you can clearly avoid it in many cases.

NOOOO ! Tesla auto batteries do catch fire and burn. Even with their massive thermal management system. Do Not Think for an instant, that they are simple and easy.
Li Batteries CANNOT be charged below the frost point. They need an electrical power warming blanket for cold weather.. Below 40F, they start getting "sleepy".

My advice to you, is a pair of Lead Acid (LA) 6V 200ah golf cart batteries, wired in series, which creates a 12V 200ah battery. Either flooded, or sealed AGM. Both can be placed in a large plastic tote., with proper bottom support. Lead acid batteries will not freeze if fully charged. (well, not in the lower 48 states)
Any LA battery sitting below 80% full will quickly sulphate in a couple days, so freezing should not be a worry. AGM style LA batteries can be frozen and survive when thawed before charging.

Li batteries have little self heating, lead acid batteries have about 10% charge converted to heat, so they will stay warm enough .
Many people ruin their first set of batteries, a cheap set ($200) of golf cart batteries is easier to deal with, then a $5000 battery that dies from being mis-configured.

Some "experts" here promote their favorite battery style, forgetting that neophytes have a lot to learn about batteries. And if you are not living in a place full time, what makes more sense, a $200 or $5000 battery ?

Freeze point Pb battery.jpg

Duly noted. I appreciate the wisdom of your experience and comments.

Good point. Sounds like a good idea to get started small and inexpensively and work up to something more complex, if warranted. This is our first off-grid experience so the less we have to worry and think about the better.

Right. So about my question, can you use the battery to warm the battery? It seems like you could.
And while I'm thinking about it, how good are these BMS/charge controller/overcurrent protection systems at doing their job? Seems like if they're properly wired and configured they do a great job. But are there problems or pitfalls to using them?

I didn't get the idea for the Tesla battery from this forum. But it did come off the internet!