4 ms·

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 max

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_lithium_based_batteries http://batteryuniversity.com/learn/article/how_to_prolong_li...

amluto 9y ago
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.
- leggomylibro 9y ago
  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.
- jpm_sd 9y ago
  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.
  - agumonkey 9y ago
    Have you seen docs about their batteries ?
    - jpm_sd 9y ago
      yes, they made all their patents public awhile back. also: https://www.google.com/search?q=tesla+battery+teardown&oq=tesla+battery+teardown https://www.google.com/search?q=tesla+battery+teardown&oq=te...
      - gruez 9y ago
        aren't all patents required to be public?
        jpm_sd 9y ago
        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 https://www.tesla.com/blog/all-our-patent-are-belong-you
      - agumonkey 9y ago
        Somehow I never considered batteries as part of their IP :) Thanks
    - cagenut 9y ago
      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-safety/ 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.
      - agumonkey 9y ago
        Thanks
  - vamin 9y ago
    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.
throwaway1X2 9y ago
> 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.
- leggomylibro 9y ago
  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.
- smileysteve 9y ago
  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.
  - Piskvorrr 9y ago
    That's quite an understatement. Venting superheated steam is no walk in the park, either.
    - MertsA 9y ago
      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.

amluto 9y ago

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.

leggomylibro 9y ago
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.
jpm_sd 9y ago
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.
- agumonkey 9y ago
  Have you seen docs about their batteries ?
  - jpm_sd 9y ago
    yes, they made all their patents public awhile back. also: https://www.google.com/search?q=tesla+battery+teardown&oq=tesla+battery+teardown https://www.google.com/search?q=tesla+battery+teardown&oq=te...
    - gruez 9y ago
      aren't all patents required to be public?
      - jpm_sd 9y ago
        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 https://www.tesla.com/blog/all-our-patent-are-belong-you
    - agumonkey 9y ago
      Somehow I never considered batteries as part of their IP :) Thanks
  - cagenut 9y ago
    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-safety/ 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.
    - agumonkey 9y ago
      Thanks
- vamin 9y ago
  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.

throwaway1X2 9y ago

> 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.

leggomylibro 9y ago
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.
smileysteve 9y ago
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.
- Piskvorrr 9y ago
  That's quite an understatement. Venting superheated steam is no walk in the park, either.
  - MertsA 9y ago
    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.