This has come up enough in the REV Control system forum that it warrants it’s own thread (mostly to help keep that thread on topic)
I’d like to note up front that I work hands-on with Li-ion batteries daily as a Li-ion battery engineer. I’ve had to go through trainings on how to handle them when things go wrong, and let’s just say it’s been put to use. I hope that offers up more credentials than your typical random CD poster who can use google search.
So Li-ion is awesome as an energy source, it has pretty much the best energy/weight ratio of any battery, and is therefore also one of the most catastrophic when all that energy goes where it shouldn’t. Li-ion is used in TONS of consumer electronics and power tools, and thanks to a lot of engineering and a bunch of safety features implemented, it’s rare that these devices see catastrophic failure.
As far as applying this technology as a power source for FIRST, there are a couple of fatal issues, not all of which I’ve seen discussed and I’d like to bring attention to:
Li-ion is super expensive, especially compared to SLA. Just look at the cost of power tool batteries compared to an FRC SLA battery, something big enough to handle FRC loads would be hundreds of dollars a pop. I’ve seen the argument that Li-ion will last longer, and that’s true, kinda. Li-ion cells can handle 600-1000 cycles (cycles meaning fully discharged and then charged again) while SLA varies a lot more, let’s say between 300-1200. Now that number varies depending on a number of abuse factors, which I will get into, but generally speaking, Li-ion will last longer than our usual SLA. When Li-ion gets pushed to it’s limits, it will degrade it’s cycle life, typically when it gets too hot. Given the kind of loads FRC would put on this pack, it’s reasonable to assume it would not last as long as a tool battery might, and even those only have an expected lifespan of a few years.
SLA is super tolerant of stupidity. There’s a reason it’s used in vehicles and child ride on cars, they can tolerate temperature swings really well and they handle big amp loads much better than pretty much any other chemistry. They are great to give a powerful option to those just getting started with powering things, and they can handle being dropped from 10ft in the air surprisingly well (looking at you 2013/2017)
Li-ion gets hot, and that can be scary. When Li-ion start dealing with big amp loads, it starts getting hot fast, for reference most/all power tools typically have overheat protection to protect the user. The cells used would need to not only stay cool enough to handle a full match, but they would need to cool down fast enough to be used again later that day (and they can be a total pain to cool). Worst case scenario is thermal runaway, which can result in fire/explosion of the pack if something insanely dumb happens to it, and even if we assume whoever makes this pack puts in all the excellent engineering into adding safety features, I would not be surprised if bots died on the field cause their battery overheated (especially if it happened to be mounted near the compressor or some other warm robot component)
Cleaning up after an issue with SLA is way easier and safer than Li-ion. The only thing you should really do if a Li-ion pack catches on fire or starts getting hot is to chuck it in a bucket of freshwater or get more than 20ft away from it (not exactly ideal for FRC). SLA very rarely catches fire, and if it does spill it can be neutralized with common household ingredients and reasonably handled by students or mentors.
Transit of batteries becomes an even BIGGER pain than it already is. You cannot put a Li-ion pack on a plane that’s over 100Wh without explicit permission from airlines. Granted this is already an issue for our international friends, but dealing with shipping Li-ion is a much bigger pain than SLA
I’m not even going to go into detail about how everyone would need the funding for new batteries/chargers, we all know this would be disastrous for low/mid resource teams.
I say these things because I’ve put a lot of thought into how FIRST could implement a better battery, I would absolutely love to not have to deal with SLA anymore, genuinely. But given the context of the users and the use case in this situation, I don’t really think it’s feasible for Li-ion to be used in FRC as the robot battery. I would love to hear from others on the subject matter, but at least I hope I summarized the key points of this for those new to the topic