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Comment by leggomylibro | Hacker News Reader

18650 cells are great!

But be sure to protect them, and be aware that most commercial li-ion chargers and off-the-shelf protection circuits are designed to maximize short-term battery life. They'll charge to 4.2V, and many won't cut off until 2.5V.

Also know what to do if they short and catch fire; have an appropriate extinguisher and a plan for quickly getting failed cells outside and onto an inert surface like concrete.

If you want to make a 'powerwall' whose batteries could last you for a significant amount of time (e.g. decades), you'll also probably want to design your own charging and undervoltage protection circuits.

There's some good information here: http://batteryuniversity.com/learn/article/how_to_prolong_li...

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> have an appropriate extinguisher and a plan for quickly getting failed cells outside and onto an inert surface like concrete.

There are two problems:

a) There is no appropriate extinguisher when you have a thermal runaway on 20 kWh of chemical energy.

b) This isn't some shielded, extinguisher-nearby, one-time experiment, when you, for example, solder something off and then while yelling "see, I'm still alive!" run outside and throw it out. This is something that is intended to be running 24/7 at your home (and maybe mounted on wooden wall like the pictures in the link...). Unless you post guard duty shifts around the clock, there simply may be no person to perform the plan of getting failed cells onto concrete.

I see this as super-dangerous, especially when dealing with cells from many different vendors scavenged from thrown-out laptops.

That's fair, and you'd definitely want to balance each cell and think hard about where to put it, but these batteries are popular for a reason; they're a potentially versatile and effective power source with charge density that is currently very hard to beat, and I'm happy to see people trying to make information about doing it safely more widely available.

And the extinguisher would be for things around where the batteries used to be, in a catastrophic failure.

On A) a diyer could solve this the "nuclear" way by storing the batteries underground or submerged in a deep tank of fluid.

Of course, then you run into cooling and maintenance access issues.

That's quite an understatement. Venting superheated steam is no walk in the park, either.

Well it certainly wouldn't be superheated steam and it's not like it would be a ton of steam in the grand scheme of things. It takes about 2500 kJ of energy to boil a liter of room temperature water. Someone else mentioned 20 kWh of batteries which in a catastrophic failure I'm assuming the cells might release a little more than that amount of energy but for the sake of argument since I don't know exactly what percent to tack onto that we'll just look at the nominal capacity of the cells. 20 kWh is 72 MJ so that's only enough to boil a bit less than 30 liters of water. Submerging a burning lithium ion pack in something like a bathtub full of water is a great way to absorb all of that energy.

>There is no appropriate extinguisher when you have a thermal runaway on 20 kWh of chemical energy.

Yes there is, it's called a hose. But you're right, that 20 kWh isn't just going to up and disappear. I think it's realistically possible to do this with a modicum of safety even for a hobbyist but it needs to be passively safe. It can't rely on cells being inherently balanced with respect to one another, it needs to have some serious thought about what to do with that 20 kWh if the worst occurs but that's not an insurmountable task. Sprinkler heads are cheap and even just a garden hose will put out 25 gallons a minute. A very slight risk of water damage can be acceptable IMHO, I wouldn't have trouble sleeping with something cobbled together sitting in my garage so long as I planned for the worst along the way when piecing it together.

Depending on chemistry, 18650 cells are quite dangerous. Tesla at least puts them in a metal box and will warn you when they’re about to catch fire.

Well sure, something like a portable phone power bank is probably a better place to start than a battery for your house, but they're very useful things to learn about.

re: Tesla - and they have individual fuses on every cell. And they have liquid cooling for every cell. And they have a rugged enclosure that has been designed to fail in a somewhat safe way while (briefly) containing the fire.

Have you seen docs about their batteries ?

yes, they made all their patents public awhile back. also: https://www.google.com/search?q=tesla+battery+teardown&oq=te...

aren't all patents required to be public?

derp, I meant [quasi-]"public domain" and therefore, in some legal sense, safe for engineers to read without fear of getting sued. https://www.tesla.com/blog/all-our-patent-are-belong-you

Somehow I never considered batteries as part of their IP :)

Thanks

not a doc but a good writeup with pictures of failure testing the 'powerpack' product: https://electrek.co/2016/12/19/tesla-fire-powerpack-test-saf...

I sure as shit would not want 1/10th of that in my garage ever.

Thanks

Not arguing that you shouldn't pay close attention to safety considerations, but many of those optimizations are necessary for Tesla because they have constraints that would be easy to relax in a DIY scenario: very low volume/high density, the possibility of a collision, consistent proximity to humans, etc. I don't think it's fair to suggest that you need to be individually liquid cooling every cell to ensure safety.

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