Pearl Sutton wrote:What WOULD be safe then? I can't afford (and don't want) a premade powerwall, I will need something of this ilk at some point, what IS safe? I read your post and all the unsafe things, what is left? What is a good answer? You have a LOT more experience than me, what do I need to look at?
So perhaps I'm a bit more safety oriented, but I work in the field (obviously), so I'm going to lean on only releasing things which are safe. There are generally three types of controls one considers when putting a complex LiIon system together:
1. Ensure the incoming cells are "as safe as can be reasonably assured." This is done by a) only purchasing from a reputable manufacturer and b) assessing the health of each and every cell, even though they're brand new, before they go in a product. There are a couple of ways this can be done, such as AC impedance checks, self-discharge rate, etc., but all require you to know what the results for that cell are supposed to be.
2. Ensuring your assembly is well protected, monitored, and features various safety checks and disconnects. Things like low voltage cut off (that's more to protect the cells; not so much a safety thing), temperature monitoring (and possibly control; note that Tesla and Chevy and basically almost every EV manufacturer out there save Nissan has liquid cooled/heated battery packs), current interrupters (usually an integral part of the cell design), PTC (again, usually designed into the cell already), brick level voltage monitoring (meaning monitoring the voltage and as well as charge and self-discharge rate of each parallel string of cells), and I'm probably forgetting a few, other than having your charge and discharge controller (part of your BMS) take all this into account.
3. A system design that accounts for what will happen WHEN a cell goes into thermal runaway. Because it happens, even if you do everything right, there is a very very small, but not zero chance that any given cell may have an internal defect that was not caught in cell formation, incoming checks, and even the first few years of life. So if that cell DOES go into thermal runaway, what is your propagation protection to keep it from setting off its neighbor, and then that one setting off its neighbor, and getting a chain reaction. Sometimes it happens, but you'd be surprised at how a well-designed module or pack can have multiple cells be thermally initiated and NOT translate into the entire battery catching fire. Think of this as planning for the worst case scenario. Go search the internet for a video of a teardown to the cell level of a Tesla Powerwall, or Model 3. [Edit: forget the powerwall videos, all the ones I just checked are from older Gen1; just go straight to the Munro work on the Model 3.] Then ask yourself "what the heck is all that stuff in between all the cells?" Answer: protection.
Honestly, what _I_ would do if I were going to do a home build is either salvage an entire pack from some EV that has hit end of life, or use deep cycle lead acid. Seriously. I would build a small module (say, maybe 9 cells in a 3x3 config max), but after that, you would not catch me trying to assemble a larger home build out of Li-Ion cells. Ask yourself why the TSA won't let you check batteries. Terrorists? Riiiight. If the general public knew the number of Li-Ion fires that occur on planes each year, they wouldn't complain about the packing rules so much. Yeah, it's usually less than 10, but it's always more than 1. [Edit: it's worse than I recall... "According to FAA records, there were 46 incidents with lithium-ion batteries on aircraft in 2017 -- up from 31 in 2016." Remember these are commercial batteries made by real companies.]
If after all that you still REALLY want to do it, and you don't want to buy something already assembled at the base level from a place like BatterySpace.com (which is still very ghetto, but better than a home-build), then just understand the level of risk. I'm not saying don't do it, but if you're going to do it, figure out what your #3 is. If/when something goes wrong, ensure it won't harm a person, or cause significant financial harm. Then have at it.