13 ms·

Second Life UPS Mark II

78 points by proxysna 3y ago

dtx1 3y ago
Please don't fuck around with 18650s if you don't know what you are doing. And unfortunately the author doesn't. The main fuse is a necessity and the thermal fuses are a neat idea but ESPECIALLY when using repurposed cells, cell level fuses are a must and they must be calibrated to account for the higher internal resistance of reused cells. The author also only briefly mentions the necessity of matching cells and measuring internal resistance. This is not something to casually brush over. Also, for UPS usage, most 18650s are not the right kind of chemistry. They don't like to sit at high charge state for extended periods of time. Use LifePo cells for that. They are much more forgiving and significantly less likely to murder you in your sleep. One 18650 going into thermal runaway is bad (ask me how I know...). A pack of 18650s you causally fuck with can easily burn down your home. Wanna see some results of that? https://endless-sphere.com/sphere/threads/dogman-dans-e-bike-burns-his-house.87975/ https://endless-sphere.com/sphere/threads/dogman-dans-e-bike... TL;DR: high energy density batteries can cause high energy rapid unplanned discharges. Please don't fuck around with batteries unless you know what you are doing (you don't)
- m463 3y ago
  I wish we could have UPS systems that could take large bring-your-own 12v size LiFeP04 batteries. The ones with terminals that look like car batteries.
  - dtx1 3y ago
    Fact: All Battery powered devices should have easily user replaceable batteries
    - jacquesm 3y ago
      Ahhh but that sweet incompatibility driven mark-up...
    - black_puppydog 3y ago
      Don't worry, the market will self-regulate. Any day now! :)
      - jacquesm 3y ago
        It's happening: https://www.cordless-alliance-system.com/ https://www.cordless-alliance-system.com/ and https://www.powerforall-alliance.com/en/ https://www.powerforall-alliance.com/en/ Of course there had to be multiple efforts...
        black_puppydog 3y ago
        ayayay, that reminds me to check the battery of my ~2008 Bosch :D
        jacquesm 3y ago
        Yes, you should. But, even if the construction of the Bosch packs is not up to their usual standards (especially the pannier mounted packs are terrible from a water ingestion POV) the BMS is top notch and it tends to shut down the battery if it detects any serious problems, far better than most el-cheapo BMS. Look for any traces of moisture on the boards and the cells and if you spot any it is probably safer to discard the pack or have it rebuilt. The seals need very careful attention, less so in the frame (vertically mounted) packs. The powertubes are much better from an engineering point of view.
  - jjeaff 3y ago
    Most any UPS that has a user replaceable battery can be swapped to 12v lifepo4 batteries. they won't likely fit, but I took the two 12v lead acid batteries out of my UPS and extended the wires out the back and to a new shelf that holds two 12v lfp4 batteries. You should get 12v lifepo4s that have built in BMS to protect them from over/under charge. Mine works great and I added a lot more watt hours to it. The only thing that won't work well is the battery level percentage indicator since lfp4 don't read the same as lead acid.
  - massifist 3y ago
    I have an older (cheapo) UPS that I just replaced the battery on, which is probably over 15 years old now. It was much easier than I thought. Coincidentally, right after I replaced the battery and left it to charge overnight, it saved my workstation from a power outage [events which aren't too common in my location] the very next day. It was quite satisfying to be able to save my work and shut down properly. The old battery had stopped working long before replacement. So I guess the UPS was no better than a surge protector at that point.
  - aaronax 3y ago
    These exist, just look for inverter/chargers. Tripp-Lite APS750 as an example though may not be LiFePO4 compatible. I use one as a sump pump power backup. Various Victron products as another example that are actually LiFePO4 compatible: MultiPlus, Quattro, or others depending on what capabilities you need.
  - SigmundA 3y ago
    They have them, they are called inverter-chargers. I recommend Victron or Magnum for quality with UPS grade switchover. I keep wanting to do a Victron 3000 12v for a critical loads in my house (fridge, lights, outlets) to use with some spare 6V lead acids I have I took out of my RV when I switched it to LFP. In the future I could switch the batteries out for LFP if I wanted. If building from scratch I would probably do a 48v rack style LFP with 48v inverter instead again probably Victron, wish they would make a rack mount version.
    - jacquesm 3y ago
      Victron is good stuff, seconded. Used extensively in marine applications, I've yet to see a unit fail that wasn't purposefully abused.
- jstanley 3y ago
  > they must be calibrated to account for the higher internal resistance of reused cells How can the cell tell whether it is "reused" or not? If it is true that the cell-level fuses must be calibrated to account for the fact that the cells are reused, would it not also be true that fuses must be recalibrated after some time even if the cells remain in their original application? > Please don't fuck around with batteries unless you know what you are doing (you don't) The "you don't" part isn't helpful, because it encourages a worldview in which technology is magic that can not be understood or harnessed.
  - xyzzy_plugh 3y ago
    They didn't say you couldn't learn, they said don't fuck around. That seems like pretty reasonable advice to me.
    - jstanley 3y ago
      I wasn't objecting to "don't fuck around with batteries", I was objecting to the unequivocal "you don't [know what you're doing]".
      - dtx1 3y ago
        I wrote it that way because the hacker ethos here makes a lot of people assume they know more than they actually do about all things tech. It's easy to learn how batteries work and how to make big packs of cells but the practical and institutional knowledge required to do it safely is not something one should just "hack" into. That's why.
      - jacquesm 3y ago
        I totally second that wording and I do know what I'm doing. This setup is only safe in a structure that doesn't have people or other stuff in it and that you are not going to be upset about if it burns down.
  - dtx1 3y ago
    > If it is true that the cell-level fuses must be calibrated to account for the fact that the cells are reused, would it not also be true that fuses must be recalibrated after some time even if the cells remain in their original application? If you are being conservative, the fuse is already well below any peak ouput a cell might give you so aging cells are considered. Also in large packs, cells should (in theory) age roughly equally. But If you match a LG HG2 and a Sony Conyon and both fuse them for 50A and then wait a few years of usage your gonna have a bad day
- deleted 3y ago
  [deleted]
- jacquesm 3y ago
  This is spot on and if you don't understand why then you are out of your depth. If the author reads this: decommission your project, please and safely destroy the cells. If you really want to do this project drop me an email and I'll help you as much as possible to do this safely (and with a different chemistry). Matching cells is hard and cells that have been through a different life cycle, especially cells that have been turned off for safety reasons won't stay matched for long. That is because their internal make-up will be different and so they will respond differently to charge/discharge cycles with one or more of the cells taking the other ones with them. And please use a FLIR on the pack to determine if there are cells that are already 'hot' compared to the rest. That's a sure sign that something bad is brewing. All it takes is one degraded cell and this project is a fire waiting to happen.
  - dtx1 3y ago
    Bonus Points for an UPS that will keep them charged at 100% permanently.
    - jacquesm 3y ago
      Yes, that's why LiIon simply is the wrong choice for this project. But I can see why they did that: they are readily available, much more so than LiFePo4.
crote 3y ago
From a technical perspective it's an impressive project, but geez that's a lot of cells being packed together! Especially with them being no-name refurbished bargain-bin cells and the very minimal literally-dollar-store-quality protection added, I'd be scared shitless about this catching fire sooner or later. If any one of those 96 cells goes bad you've suddenly got a basically unstoppable inferno in your home. I mean, you do you, but I sure wouldn't be happy if my neighbour was doing projects like these in their garage!
- Beijinger 3y ago
  I guess it depends how far the garage is away from my home.
- londons_explore 3y ago
  Mount it in a waterproof box outdoors, and just expect that sooner or later it will catch fire, but when it does nothing will be damaged. That's what I did with my DIY solar batteries - and so far they haven't caught fire.
  - jacquesm 3y ago
    You still need thermal management if the temps can go near or below zero.
    - londons_explore 3y ago
      Depends if you care about longevity...
      - jacquesm 3y ago
        True, but charging while too cold can also damage the cells in ways that are not immediately visible and that in turn might trigger thermal runaway rather than just cell capacity degradation. That's because the cells on the outside of the pack will likely be much later to the party than the ones on the inside during charging. But any good BMS will simply refuse to charge as long as the pack is too cold.
jacquesm 3y ago
This project is a good reminder I really should finish that LiIon article that I've had on the back burner ever since I built the long range e-bike. Some observations just from the pictures: - corners of the nickel strips are not rounded, chances of cutting through the insulation of the cells, and in the high res pictures you can see some places where that might already be happening. - welds are all over the place, from nice penetration to very shallow, possibly welds with high resistance, possibly burn through of the interconnects. - physical construction of the pack is dangerous, no protection against impact if the whole construct is dropped. If it ever is dropped I would recommend immediately placing it in a fireproof container that is not indoors. And don't transport it in elevators or vehicles. - no insulation rings between the tops of the batteries and the metal strips! That's really asking for trouble. You can see more than one spot on the pictures where the metal strip is directly in contact with the plastic and the welding process has likely further reduced the safety margin. The plastic sheathing of the cells is not enough protection for this kind of construction. Any insulation failure there means that you now have shorted an unfused pod and that pretty much guarantees a fire. - balancing wires cross and pass over parts of the pack at a different voltage, a short of a balance wire can set off a thermal runaway. - in general, wire management needs to be substantially improved if this is ever to become safe. Way too many wires that carry wildly different voltages cross and recross, especially close to soldering joints that might cause issues because the wires tend to shift inside the insulation if they're under tension while soldering. I'm both impressed and horrified, this is the most clear illustration ever that I've seen for 'knows enough to be dangerous'. Laying out the sense/balance wires on a pack is a lot of work if you want to do it safely and even the pros (looking at you, Bosch) get this wrong. The result is that if ever one of the sense wires shorts out that it will essentially become a fuse and the energy imparted to the cells may just be enough to set off a thermal runaway. Wire management on the project here is terrible.
- spacecadet 3y ago
  Nice comment. I was scrolling and going "ooh looks like a bomb", "oooooh", "oh yes thats basically a bomb".
  - jacquesm 3y ago
black_puppydog 3y ago
A few people here do mention LiFePO4 batteries as a safer alternative. As a total battery-noob, honest question: assuming the author had indeed used LiFePO4 for the build, with standard (-ish?) over- and under-charge protection. How much safer would this be? It's still a relatively big battery pack (read: chemical energy) sitting in someone's broom cupboard, isn't it?
- jacquesm 3y ago
  A lot because they suffer less from thermal runaway and are more resistant to physical damage. That's why the better tool brands use that chemistry too and why you see car manufacturers switching to them. I think LiIon was a huge mistake and that as various pieces of gear that have them built in ages that we'll see more and more issues with them. They're everywhere: cameras, laptops etc, stuff that has a service life of 10 years, sometimes more and as they age they become more dangerous, especially when people re-purpose them. I've rebuilt a couple of packs for brand name cameras and e-bikes and it is very interesting to take apart the old packs to see what you find inside. More than one had the start of a fire in them, evaporated wiring, scorch marks and so on. I'm pretty sure those are the good ones ;)
  - wongarsu 3y ago
    Dense energy storage is almost inherently dangerous. LiIon are great for applications where you actually need that density. But the scale of those use cases has made it ubiquitous and cheap, leading to LiIon to be used in many cases where the density isn't needed at all and is just a pointless risk.
- droopyEyelids 3y ago
  The concept to research here is thermal runaway. There is a lot of chemical energy in wood, too, right?
  - NegativeLatency 3y ago
    Yes but generally if I’m rough with a piece of wood it doesn’t start a fire.
mhaberl 3y ago
If anyone is considering attempting this, please ensure you first educate yourself about thermal runaway. Using a collection of old lithium batteries like this can be extremely dangerous, and without exaggeration, it can kill you. When (yes when, it will happen) one battery overheats and catches fire, it will cause the others to do the same, leading to a significant fire. During combustion, these batteries also emit gases, so put this into a box and you risk an explosion on top of the fire.
- jacquesm 3y ago
  > please ensure you first educate yourself about thermal runaway YES!. That's why you should monitor a setup like this with a FLIR from all sides during the first month of operation or so, if no batteries ever run hot during charge/discharge cycle you may be good to go. But better keep an eye on that balance, especially in packs that are sitting at high voltage for a long time (like this one does) because you won't know how the pack behaves during discharge and recharge and that is likely when any problems will be most apparent. Thermal runaway can happen very quickly once a certain threshold has been passed. If you really want to do a project like this with Lithium Ion and you are not space constrained you would do better to have a bit less capacity and charge the batteries to 3.6 ... 3.8V or so. That would work much better in the long run.
bob1029 3y ago
If I were going to do a DIY UPS, I would try to use lead acid batteries designed for this exact purpose, and install them in a room per the manufacturer specifications for ventilation and relevant charging interlocks. Not sure why we would want to chase miniaturization as a design criteria for something like this application (I.e. "rack mount"). Lithium ion is accessible and dense, but hilariously unsafe as you scale up in a DIY residential setting. I won't even get official, first party lithium ion backup power installed in my home out of fears it could cause a catastrophic fire. I keep a 2kwh lipo pack on a concrete slab behind my house for emergencies. If you are storing these things in your home, you need to do some napkin math regarding what 2000 watts for an hour could do to your home if released over a much shorter timeframe.
- geek_at 3y ago
  I have been in a few "DIY Powerwall" groups which usually source the cells from old laptop batteries and this community is very aware of the fire danger so they come up with hilarous solutions for their home racks like having plastic bags full of sand above the batteries so when they catch fire the plastic melts and the Lithium cells get covered in sand. Most people do try to have the cells on the side of - or inside outdoor metal sheds to minimize the fire damage
  - bob1029 3y ago
    I think the entire premise is cursed. That is to say, if you are finding yourself with a need to store more than maybe one KWh of energy in a residential setting, you might need to reevaluate your overall architecture. A generator, while not ideal ecologically, is a perfect abstraction in engineering terms. Especially generators connected to pipelines. Those are the real win in safety - the energy is only received on site the exact moment it is required. Some trade offs with resilience but you don't have to go that far. On site diesel and LPG have proven safe and reliable as well. For me the batteries are just about transferring the load. If you need run time, you should find yourself a generator. For non emergency, non-UPS scenarios, sure you could maybe plan an off grid setup. But I think we'd be mixing use cases a little bit with this angle. Beyond a certain point, I don't think there exists a safe way to store certain embodiments of energy on site. Sandbags or no.

78 points by proxysna 3y ago

dtx1 3y ago

Please don't fuck around with 18650s if you don't know what you are doing. And unfortunately the author doesn't. The main fuse is a necessity and the thermal fuses are a neat idea but ESPECIALLY when using repurposed cells, cell level fuses are a must and they must be calibrated to account for the higher internal resistance of reused cells. The author also only briefly mentions the necessity of matching cells and measuring internal resistance. This is not something to casually brush over. Also, for UPS usage, most 18650s are not the right kind of chemistry. They don't like to sit at high charge state for extended periods of time. Use LifePo cells for that. They are much more forgiving and significantly less likely to murder you in your sleep. One 18650 going into thermal runaway is bad (ask me how I know...). A pack of 18650s you causally fuck with can easily burn down your home. Wanna see some results of that? https://endless-sphere.com/sphere/threads/dogman-dans-e-bike-burns-his-house.87975/ https://endless-sphere.com/sphere/threads/dogman-dans-e-bike... TL;DR: high energy density batteries can cause high energy rapid unplanned discharges. Please don't fuck around with batteries unless you know what you are doing (you don't)

m463 3y ago
I wish we could have UPS systems that could take large bring-your-own 12v size LiFeP04 batteries. The ones with terminals that look like car batteries.
- dtx1 3y ago
  Fact: All Battery powered devices should have easily user replaceable batteries
  - jacquesm 3y ago
    Ahhh but that sweet incompatibility driven mark-up...
  - black_puppydog 3y ago
    Don't worry, the market will self-regulate. Any day now! :)
    - jacquesm 3y ago
      It's happening: https://www.cordless-alliance-system.com/ https://www.cordless-alliance-system.com/ and https://www.powerforall-alliance.com/en/ https://www.powerforall-alliance.com/en/ Of course there had to be multiple efforts...
      - black_puppydog 3y ago
        ayayay, that reminds me to check the battery of my ~2008 Bosch :D
        jacquesm 3y ago
        Yes, you should. But, even if the construction of the Bosch packs is not up to their usual standards (especially the pannier mounted packs are terrible from a water ingestion POV) the BMS is top notch and it tends to shut down the battery if it detects any serious problems, far better than most el-cheapo BMS. Look for any traces of moisture on the boards and the cells and if you spot any it is probably safer to discard the pack or have it rebuilt. The seals need very careful attention, less so in the frame (vertically mounted) packs. The powertubes are much better from an engineering point of view.
- jjeaff 3y ago
  Most any UPS that has a user replaceable battery can be swapped to 12v lifepo4 batteries. they won't likely fit, but I took the two 12v lead acid batteries out of my UPS and extended the wires out the back and to a new shelf that holds two 12v lfp4 batteries. You should get 12v lifepo4s that have built in BMS to protect them from over/under charge. Mine works great and I added a lot more watt hours to it. The only thing that won't work well is the battery level percentage indicator since lfp4 don't read the same as lead acid.
- massifist 3y ago
  I have an older (cheapo) UPS that I just replaced the battery on, which is probably over 15 years old now. It was much easier than I thought. Coincidentally, right after I replaced the battery and left it to charge overnight, it saved my workstation from a power outage [events which aren't too common in my location] the very next day. It was quite satisfying to be able to save my work and shut down properly. The old battery had stopped working long before replacement. So I guess the UPS was no better than a surge protector at that point.
- aaronax 3y ago
  These exist, just look for inverter/chargers. Tripp-Lite APS750 as an example though may not be LiFePO4 compatible. I use one as a sump pump power backup. Various Victron products as another example that are actually LiFePO4 compatible: MultiPlus, Quattro, or others depending on what capabilities you need.
- SigmundA 3y ago
  They have them, they are called inverter-chargers. I recommend Victron or Magnum for quality with UPS grade switchover. I keep wanting to do a Victron 3000 12v for a critical loads in my house (fridge, lights, outlets) to use with some spare 6V lead acids I have I took out of my RV when I switched it to LFP. In the future I could switch the batteries out for LFP if I wanted. If building from scratch I would probably do a 48v rack style LFP with 48v inverter instead again probably Victron, wish they would make a rack mount version.
  - jacquesm 3y ago
    Victron is good stuff, seconded. Used extensively in marine applications, I've yet to see a unit fail that wasn't purposefully abused.
jstanley 3y ago
> they must be calibrated to account for the higher internal resistance of reused cells How can the cell tell whether it is "reused" or not? If it is true that the cell-level fuses must be calibrated to account for the fact that the cells are reused, would it not also be true that fuses must be recalibrated after some time even if the cells remain in their original application? > Please don't fuck around with batteries unless you know what you are doing (you don't) The "you don't" part isn't helpful, because it encourages a worldview in which technology is magic that can not be understood or harnessed.
- xyzzy_plugh 3y ago
  They didn't say you couldn't learn, they said don't fuck around. That seems like pretty reasonable advice to me.
  - jstanley 3y ago
    I wasn't objecting to "don't fuck around with batteries", I was objecting to the unequivocal "you don't [know what you're doing]".
    - dtx1 3y ago
      I wrote it that way because the hacker ethos here makes a lot of people assume they know more than they actually do about all things tech. It's easy to learn how batteries work and how to make big packs of cells but the practical and institutional knowledge required to do it safely is not something one should just "hack" into. That's why.
    - jacquesm 3y ago
      I totally second that wording and I do know what I'm doing. This setup is only safe in a structure that doesn't have people or other stuff in it and that you are not going to be upset about if it burns down.
- dtx1 3y ago
  > If it is true that the cell-level fuses must be calibrated to account for the fact that the cells are reused, would it not also be true that fuses must be recalibrated after some time even if the cells remain in their original application? If you are being conservative, the fuse is already well below any peak ouput a cell might give you so aging cells are considered. Also in large packs, cells should (in theory) age roughly equally. But If you match a LG HG2 and a Sony Conyon and both fuse them for 50A and then wait a few years of usage your gonna have a bad day
deleted 3y ago
[deleted]
jacquesm 3y ago
This is spot on and if you don't understand why then you are out of your depth. If the author reads this: decommission your project, please and safely destroy the cells. If you really want to do this project drop me an email and I'll help you as much as possible to do this safely (and with a different chemistry). Matching cells is hard and cells that have been through a different life cycle, especially cells that have been turned off for safety reasons won't stay matched for long. That is because their internal make-up will be different and so they will respond differently to charge/discharge cycles with one or more of the cells taking the other ones with them. And please use a FLIR on the pack to determine if there are cells that are already 'hot' compared to the rest. That's a sure sign that something bad is brewing. All it takes is one degraded cell and this project is a fire waiting to happen.
- dtx1 3y ago
  Bonus Points for an UPS that will keep them charged at 100% permanently.
  - jacquesm 3y ago
    Yes, that's why LiIon simply is the wrong choice for this project. But I can see why they did that: they are readily available, much more so than LiFePo4.

crote 3y ago

From a technical perspective it's an impressive project, but geez that's a lot of cells being packed together! Especially with them being no-name refurbished bargain-bin cells and the very minimal literally-dollar-store-quality protection added, I'd be scared shitless about this catching fire sooner or later. If any one of those 96 cells goes bad you've suddenly got a basically unstoppable inferno in your home. I mean, you do you, but I sure wouldn't be happy if my neighbour was doing projects like these in their garage!

Beijinger 3y ago
I guess it depends how far the garage is away from my home.
londons_explore 3y ago
Mount it in a waterproof box outdoors, and just expect that sooner or later it will catch fire, but when it does nothing will be damaged. That's what I did with my DIY solar batteries - and so far they haven't caught fire.
- jacquesm 3y ago
  You still need thermal management if the temps can go near or below zero.
  - londons_explore 3y ago
    Depends if you care about longevity...
    - jacquesm 3y ago
      True, but charging while too cold can also damage the cells in ways that are not immediately visible and that in turn might trigger thermal runaway rather than just cell capacity degradation. That's because the cells on the outside of the pack will likely be much later to the party than the ones on the inside during charging. But any good BMS will simply refuse to charge as long as the pack is too cold.

jacquesm 3y ago

This project is a good reminder I really should finish that LiIon article that I've had on the back burner ever since I built the long range e-bike. Some observations just from the pictures: - corners of the nickel strips are not rounded, chances of cutting through the insulation of the cells, and in the high res pictures you can see some places where that might already be happening. - welds are all over the place, from nice penetration to very shallow, possibly welds with high resistance, possibly burn through of the interconnects. - physical construction of the pack is dangerous, no protection against impact if the whole construct is dropped. If it ever is dropped I would recommend immediately placing it in a fireproof container that is not indoors. And don't transport it in elevators or vehicles. - no insulation rings between the tops of the batteries and the metal strips! That's really asking for trouble. You can see more than one spot on the pictures where the metal strip is directly in contact with the plastic and the welding process has likely further reduced the safety margin. The plastic sheathing of the cells is not enough protection for this kind of construction. Any insulation failure there means that you now have shorted an unfused pod and that pretty much guarantees a fire. - balancing wires cross and pass over parts of the pack at a different voltage, a short of a balance wire can set off a thermal runaway. - in general, wire management needs to be substantially improved if this is ever to become safe. Way too many wires that carry wildly different voltages cross and recross, especially close to soldering joints that might cause issues because the wires tend to shift inside the insulation if they're under tension while soldering. I'm both impressed and horrified, this is the most clear illustration ever that I've seen for 'knows enough to be dangerous'. Laying out the sense/balance wires on a pack is a lot of work if you want to do it safely and even the pros (looking at you, Bosch) get this wrong. The result is that if ever one of the sense wires shorts out that it will essentially become a fuse and the energy imparted to the cells may just be enough to set off a thermal runaway. Wire management on the project here is terrible.

spacecadet 3y ago
Nice comment. I was scrolling and going "ooh looks like a bomb", "oooooh", "oh yes thats basically a bomb".
- jacquesm 3y ago

black_puppydog 3y ago

A few people here do mention LiFePO4 batteries as a safer alternative. As a total battery-noob, honest question: assuming the author had indeed used LiFePO4 for the build, with standard (-ish?) over- and under-charge protection. How much safer would this be? It's still a relatively big battery pack (read: chemical energy) sitting in someone's broom cupboard, isn't it?

jacquesm 3y ago
A lot because they suffer less from thermal runaway and are more resistant to physical damage. That's why the better tool brands use that chemistry too and why you see car manufacturers switching to them. I think LiIon was a huge mistake and that as various pieces of gear that have them built in ages that we'll see more and more issues with them. They're everywhere: cameras, laptops etc, stuff that has a service life of 10 years, sometimes more and as they age they become more dangerous, especially when people re-purpose them. I've rebuilt a couple of packs for brand name cameras and e-bikes and it is very interesting to take apart the old packs to see what you find inside. More than one had the start of a fire in them, evaporated wiring, scorch marks and so on. I'm pretty sure those are the good ones ;)
- wongarsu 3y ago
  Dense energy storage is almost inherently dangerous. LiIon are great for applications where you actually need that density. But the scale of those use cases has made it ubiquitous and cheap, leading to LiIon to be used in many cases where the density isn't needed at all and is just a pointless risk.
droopyEyelids 3y ago
The concept to research here is thermal runaway. There is a lot of chemical energy in wood, too, right?
- NegativeLatency 3y ago
  Yes but generally if I’m rough with a piece of wood it doesn’t start a fire.

mhaberl 3y ago

If anyone is considering attempting this, please ensure you first educate yourself about thermal runaway. Using a collection of old lithium batteries like this can be extremely dangerous, and without exaggeration, it can kill you. When (yes when, it will happen) one battery overheats and catches fire, it will cause the others to do the same, leading to a significant fire. During combustion, these batteries also emit gases, so put this into a box and you risk an explosion on top of the fire.

jacquesm 3y ago
> please ensure you first educate yourself about thermal runaway YES!. That's why you should monitor a setup like this with a FLIR from all sides during the first month of operation or so, if no batteries ever run hot during charge/discharge cycle you may be good to go. But better keep an eye on that balance, especially in packs that are sitting at high voltage for a long time (like this one does) because you won't know how the pack behaves during discharge and recharge and that is likely when any problems will be most apparent. Thermal runaway can happen very quickly once a certain threshold has been passed. If you really want to do a project like this with Lithium Ion and you are not space constrained you would do better to have a bit less capacity and charge the batteries to 3.6 ... 3.8V or so. That would work much better in the long run.

bob1029 3y ago

If I were going to do a DIY UPS, I would try to use lead acid batteries designed for this exact purpose, and install them in a room per the manufacturer specifications for ventilation and relevant charging interlocks. Not sure why we would want to chase miniaturization as a design criteria for something like this application (I.e. "rack mount"). Lithium ion is accessible and dense, but hilariously unsafe as you scale up in a DIY residential setting. I won't even get official, first party lithium ion backup power installed in my home out of fears it could cause a catastrophic fire. I keep a 2kwh lipo pack on a concrete slab behind my house for emergencies. If you are storing these things in your home, you need to do some napkin math regarding what 2000 watts for an hour could do to your home if released over a much shorter timeframe.

geek_at 3y ago
I have been in a few "DIY Powerwall" groups which usually source the cells from old laptop batteries and this community is very aware of the fire danger so they come up with hilarous solutions for their home racks like having plastic bags full of sand above the batteries so when they catch fire the plastic melts and the Lithium cells get covered in sand. Most people do try to have the cells on the side of - or inside outdoor metal sheds to minimize the fire damage
- bob1029 3y ago
  I think the entire premise is cursed. That is to say, if you are finding yourself with a need to store more than maybe one KWh of energy in a residential setting, you might need to reevaluate your overall architecture. A generator, while not ideal ecologically, is a perfect abstraction in engineering terms. Especially generators connected to pipelines. Those are the real win in safety - the energy is only received on site the exact moment it is required. Some trade offs with resilience but you don't have to go that far. On site diesel and LPG have proven safe and reliable as well. For me the batteries are just about transferring the load. If you need run time, you should find yourself a generator. For non emergency, non-UPS scenarios, sure you could maybe plan an off grid setup. But I think we'd be mixing use cases a little bit with this angle. Beyond a certain point, I don't think there exists a safe way to store certain embodiments of energy on site. Sandbags or no.