> They also could stop price-gouging the consumers on the packs, which is one of the reasons people want to repair them to begin with.
This is a huge piece of it, stop ripping us off! This has been going on for decades -- from NiMH battery packs for power tools, to laptop batteries.
I assume a big difference in the danger aspect with the bike batteries is their size/capacity? Less to go wrong with a 2AH cordless drill battery, than a 20AH bike battery?
Cordless drills usually (but definitely not always) use LIFePO4 so should be a little bit safer. Here too: be careful of cheap stuff.
But a 50 cell 36V 625Wh e-bike battery is an entirely different beast and when it goes you're going to be very sorry if you are nearby. Individual cells that go are already very impressive, a large pack is going to very effectively spoil your day (if not your life).
What happens exactly when a 625 Wh battery pack "goes"? Does it explode? Does it burn fiercely for several minutes?
625 Wh is the energy equivalent of a few ounces of gasoline. That's certainly enough to do some damage, especially if it gets misted to an explosive air/fuel ratio, but not something that most people would be afraid to handle.
That's precisely what is so deceptive. Compared to a lead acid battery of equivalent capacity the dangers are far higher (though to be splashed with acid can ruin your life quite effectively as well).
What will happen - usually - is this: one cell will go bad, into thermal runaway. Maybe it had a bad interconnect which vibration fixed and now the rest of the cells in parallel will dump a few hundred amps into it or maybe the pack was subject to a fall, who knows. But let's assume that one cell only got damaged. This cell is in close proximity to the rest of the cells in the pack but typically they'll have between two and six closely spaced neighbors. Those in turn will start to go up after which there is a geometric expansion until the whole pack is gassing out and possibly already burning.
Elapsed time from first symptoms to violent explosion/fire is on the order of 10 to 20 seconds at best. And once it gets going your best chance of stopping it is to immerse the whole thing in a very large volume of water or to bury it in a large amount of sand. The vapors are not going to be pleasant to deal with, anything combustible that it can come into contact with will likely burn.
So naively I'd think that fusing the cell interconnects so that "a few hundred amps" cannot pass would be a way to deal with that, but I guess if it were that simple it would be done.
This is done in some packs, but this introduces a new risk: now, once a fuse is blown the pack will be asymmetrical and that in turn will put more strain on the remaining cells in that particular block of parallel cells. So likely that is a stay of execution rather than a solution. Another problem is that cells can develop higher internal resistance than their brothers causing them to overheat. In general, pack construction is a very delicate interplay of mechanical stability, thermal stability and electrical properties. Get anything wrong and it translates into risk, and it isn't always obvious that you've done something wrong.
Even the big manufacturers mess up. That's a pretty good sign that this isn't an easy problem.
So I think I'm agreeing with you these large battery packs sound almost too dangerous to be sold as consumer products. Are there any other examples of consumer/household products that can fail so dangerously and that have not been recalled or banned?
There are things such as space heaters, heating pads, kitchen appliances, etc. that have been known to start fires, but at least if you're in the room with them you can probably react and extinguish them if you have water or a fire extinguisher handy.
I don't know anything that comes close to this. Just recently someone gifted one of my children a battery powered toy, I checked it and decided that I couldn't tell the difference between that toy and a purposeful attempt at arson...
The big thing that stands out for me is that the perception of the risks (impact / likelihood) is entirely unlike the real risk. For most other things those are much more in line. And that's why people get caught out.
Nobody thinks twice about leaving their e-bike charging in the kitchen overnight, or to take it into an elevator or in an extreme case even a bedroom (1 room studio, no storage space). Stairwells are another such spot. I charge mine outside under an overhang, and only do so when I know it isn't going to be anywhere near freezing.
This is a huge piece of it, stop ripping us off! This has been going on for decades -- from NiMH battery packs for power tools, to laptop batteries.
I assume a big difference in the danger aspect with the bike batteries is their size/capacity? Less to go wrong with a 2AH cordless drill battery, than a 20AH bike battery?