JT
I'm planning on replacing my ANL fuses with Class T fuses when switching to lithium. The difference is mainly the Amperage Interrupt Capacity (AIC). The way I understand it is that some fuse types melt a fusible link, but there may be a problem if the amperage is so high that it still arcs across where the link was. In other words, it pops off for 100 amps but it will still arc if the current is 1,000 amps. The ANL can have that issue and some don't even provide the AIC data. The other issue is that at higher currents the ANL "ignition protection" might not work. The plastic casing pops open, sparking out molten metal. May or may not be an issue, but best to not take the chance.
The Class T fuse has several features that increases its AIC. The fusible link is a piece that falls away, leaving a larger gap. Plus, the area is filled with silica dust. The dust fills the gap and if there is any arching the dust melts to the electrodes, basically becoming a glass insulator. The casing is metal, so no melted plastic or shooting sparks. It has an AIC rating of 20,000 amps. That's a lot, but some of the dead short current tests for 100Ah LFP batteries have been several thousand amps. My dead short current for 2x280Ah LFP in parallel might be under the 6,000 amp range (for an ANL fuse), but since I don't know I'll spend a few bucks and get the better fuse.
The ABYC 7" rule can be extended to several feet if the cable is in a conduit. Most of my larger cables are already in conduits, but that tells me that even the 7" length could be in a conduit. No harm in going above and beyond ABYC. I have some fireproof flexible around and might as well use it up.
What happens when a BMS fails? That depends on the type of failure. If it is an overtemperature failure, the battery goes off line until the temperature sensor cools off. I don't know how long that might take. From the testing I've seen online, the sensor itself is almost instantaneous, but I don't know how long it would actually take a overheated cell to cool off. An overcurrent reset in my spec sheet said 30 seconds, but when top balancing my batteries (and goofing up) it seemed much faster. An undercurrent shutdown could be a different story, depending on how the battery is charged. Some shore chargers will not charge a battery if it is under a certain voltage. There are supposedly special chargers that somehow (maybe have a lower threshold?) can revive a seemingly dead LFP battery. I don't think it will matter with my setup because of having LFP in parallel. In general, a BMS shut down means that the battery will be trying to get back on line momentarily.
A BMS failure doesn't do anything. Lead-acid batteries don't even have them. Runaway current that melts things down means that the rest of your electrical system wasn't properly constructed/fused. It is more likely that a toilet will spontaneously catch on fire.
It is a lot of amps. I don't need that much. What I needed was redundancy so that:
1. I didn't have to worry about alternator issues if a battery shut down momentarily
2. I didn't have to buy some workaround gadget
3. I could "drop into" my existing electrical (except upgraded fuses)
4. I could charge at a higher rate
5. I had enough LFP batteries that my starter current was only a small amount of their rated current
Having the ability to anchor out for several days without a generator is just a bonus.