While I do agree that old school LiPo tech is a bit volatile, it is not a sure thing to burn. Something has to go wrong. I have some 15 year old LiPo batteries that I got long before LiPo was popular for RC hobby. I was the first to run them at my local RC car track, and they freaked people out, no one wanted to be near them. Whenever I ran, they kept 5 gallon buckets of sand around the track. Well, I still have those packs, and over a dozen more I have bought over the years. I have never has a single LiPo cell catch fire without being physically damaged. And even 2 packs that got split open and were leaking never caught on fire. I had cells swell up, some lost capacity, and others lost current output, and a few get very hot while running or charging, but not one burned.
I intentionally drove a nail through a fully charged but weak cell, and it did flair up, make a lot of nasty smoke, and a small flame became visible. That was only a single 5 amp hour cell, so yes, I can imaging a large pack being able to make quite a fire and a lot of heat, but again, something needs to go seriously wrong. Over charging is the most common cause of cells bursting and burning. And if there are flaws in the separator, then an overcharge is lighting the fuse. This is why GM reduced the maximum charge to 90%. That greatly reduces the internal stress on the cells. Very high capacity cells tend to use a thinner separator so they can stuff in more layers to get more watt hours. Both Sony and Samsung had problems when they released new higher capacity cells for laptops and phones, and now it looks like LG suffered this fate as well with the new gen EV cells. In a car, they are subjected to a very harsh use case. Not only do they see very high current charge and discharge rates, they also get shook around, bounced, and subjected to severe heating and cooling.
Many people (including me) are using repurposed EV batteries for solar energy storage. I have 36 KWHs of LG Chevy Bolt cells in my garage. My cells are from 2019 and 2020, Gen 2.1 and 2.2 versions. I am taking some precautions. They are all inside of steel cabinets. They can be quickly unplugged and rolled out of the garage. They are n a concrete floor, and the wall behind them has a layer of cement board over drywall. But the cells just sit there. I am not charging them at only 3 to 5 amps per cell, they are rated at 60 amps by LG. And in discharge, they max out at less than 12 amps per cell compared to the LG rating of 180 amps per cell. I am not worried about them at all. But I still treat them with care and monitor them constantly. If they ever do start to get warm at all, they will be rolled out and carefully inspected. With my low currents, the cells run as cold as the concrete slab of my garage. I will get an alarm if any cell ever gets to 60 C.
So why did the Bluetti catch fire?
That is a very good question. My first guess is that the solar charge controller failed and the cells went over voltage. But with that much damage, that may be impossible to ever prove. My inverter charger stores the battery voltage and current graphs so I can look back at what is going on. No such luck on the little Bluetti. With the solar panels plugged in, but no loads, that does leave a potential for a bad situation. A good solar charge controller should have no problem dropping it's output to zero, as long as the solar panel open circuit voltage does not go too high, but even a small leakage current with no load on the system can lead to voltage creep. When LiPo cells are stored and not being used, the basic rule is to run them down to 50% charge so that any potential voltage creep should never hit too high or too low of a voltage. My second bank of Chevy Bolt cells were very close to fully charged when I got them, and it did keep me pretty nervous until I was finally able to wire them up and pull some current to bring them down a bit. The whole time they were not connected, I would check the voltages from time to time, and those cells didn't drop off at all. I kept them out on my back porch on a concrete slab. All of these small power bank units are all in a race to give the most capacity. This has led to them charging the cells right to the top to be able to show that maximum capacity. Now that my cells are in service, I will not be pushing them past about 85% full ever again.
I sure hope they do find the true cause of this, and that if it is a design issue on the Bluetti, I hope it is properly fixed and the old units are recalled and updated or replaced. I also hope they use their own insurance to help out the original poster to get his cabin rebuilt. The design of the unit should eliminate the possibility for the user to cause anything like this to happen. It should have internal protection so even too much solar, or an improper load should just cause a shut down, not a fire, ever.