If that's the case, I'm a little bit surpised that you drive a Leaf and are working on replacement Leaf battery packs.
Oh, don't get me wrong, I'm not replacing Leaf packs with Leaf cells... I'm replacing them with Panasonics
Without TMS though.
If you have the time....are the BMW/Panasonic/etc more prone to fires? More expensive? Absolutely require TMS? What is your opinion on why Nissan went with the chemistry they did?
Conjecture ahead, because as much as I am sort of a broad-knowledge engineer in this field, I'm not an expert in these things and don't actually design battery packs, I just apply them.
Fire risk depends on three things: 1) the prevalence of circumstances that lead to fire, 2) the extent of such a fire and 3) the consequences of that fire. All lithium ion batteries bar a rare few types have an inherent failure mode called thermal runaway. When the cathode material gets to about 130-150C at any point, an exothermic chemical reaction will spontaneously occur. That will generally boil the electrolyte, melt or vaporize other internal materials and cause the cell to bulge and rupture. Heat from the battery will first infect and spread thermal runaway within the cell, and depending on the thermal capacity of its surroundings it may also jump to neighboring cells, engulfing the entire pack.
The Leaf battery is basically the perfect storm here; it's got a relatively fire-prone chemistry. Not only are LMO/LCO chemistries relatively fire-prone; they have a large amount of flammable electrolyte. The cells are also not very well pressure-sealed (they're polymer pouch types in a compression frame) and have fairly low thermal capacity. The back row of cells is a LOT of cells packed right next to each other. A fire in one cell will quickly spread to the whole stack, basically setting the entire pack on fire. There are only 2 (or 4 in the new packs) thermal sensors and the pack isn't actively cooled.
Now, have I scared you yet?
The Yuasa and Panasonic packs (that's basically what we're talking about when talking about the other brands) are all some variation of NCA chemistry (nickel-cobalt lithium-ion). The Panasonics are *incredibly* low impedance, meaning they are using really thick conductor plates internally (aluminum/copper touching the active material) and low electrolyte volume. This increases thermal capacity and reduces the amount of flammable material, as well as simply making the battery more efficient (although heavier). They're all cooled to some extent (although some of this cooling is token) and they're in individual compression frames, meaning bulging cells can't cause other modules to get crushed. Add to that the fact that NCA is for the purposes of fire safety more stable (nickel is actually less stable, but the reactions that cause thermal runaway occur at higher temperatures), and you should have a battery system that just has to be much more fire-safe. I don't have statistics and there is always a way to fuck this up (e.g. some packs have the pyrofuse unnecessarily close to the actual cells, I'm sure they did their engineering right but I think it's scary), but this has to be an order of magnitude safer than the Leaf mk1/1.5.
All that said, battery fires are super-rare and much safer to be in than gas or diesel fires. There's something like 200 000 fuel fire accidents in the US a year. Even on a per-vehicle basis, battery fires are more rare and less deadly.
But if you really want the pinnacle of battery safety, you have to hand it to Tesla. Their coolant runs beside almost every cell, the thermal capacity of the matrix is tuned to never allow thermal runaway to infect neighboring cells. And each cell is fused individually. You can't do better than Tesla. They also have the most awesome BMSes and TMSes. Pure engineering porn. Overkill, though. i'd say this is all a bit of wasteful engineering, making their cars unnecessarily expensive.
And what are the odds that a different, longer-lived chemistry could find its way into Leaf-compatible replacement packs?
I'm going to try and either make a business out of it or tutorialize it. Oh, or fail (that's a very real possibility)
I love my Leaf but it is just a city car for me and by city, I really mean commuter and errand car. That is still >80% of my trips (probably normal for most in US) but IMHO EV's need to get better before more folks consider them acceptable as ICE replacements. I'm a little surprised that batteries this much better are available and Nissan is not using them. There must be a legitimate reason for their decision.
I'm very torn on the Leaf's design. If I give Nissan the benefit of the doubt, they must have really had a hard time finding a long-term, high-volume supplier. AESC, their battery supplier, is a pleb-tier shitty supplier now, but you have to keep in mind the Leaf was in development in the mid-00's. Batteries just weren't what they are now. NCA didn't exist, they HAD to use some doped form of LMO or LCO, and the latter can't provide the required current. They clearly wanted a sporty car, because the car is built like an absolute tank, with provisions for anti-roll and anti-dive, a sway bar and articulated suspension. I think the car was originally designed to be much less city commuter and much more fun car. It still is, really. Compare it to the sub-50kW Zoe and iMiev of the time, or the sub-30kW Wheego, Th!nk City, etc. But to get 80kW out of a 24kWh battery (almost 4C!), LMO was the only way to go. Or NiMH...
But... if I look at the car now, it's not well-designed at all. As an electric car with a small battery, you NEED to work on efficiency. Norway is one of the biggest markets for the Leaf, and it's fucking cold there. They should AT LEAST have put a heat pump in the first iteration. They should have had battery heating/cooling as standard using the general coolant loop. They should really have increased the battery to 30kWh immediately - they had plenty of room under the car. They shouldn't have had the hump in the back as an afterthought. They should have included the 6.6kW charger. They should have fucking known to put a light in the charging bay. They should have worked on the aerodynamics - the Leaf is one of the least efficient electric cars.
The entire car seems half-assed - not in its finish quality, but in its fitness for purpose. Why bother making an EV from the ground up if it's so bad at being an EV? Seriously, somebody had their head firmly stuck in their ass when designing this. Still the most successful EV...
Is Nissan delinquent here? Were they too soon, was the technology not ready? Bit of both I guess. I'd love to talk to their engineers to find out more.