from Hacker News

Second Life UPS Mark II

by proxysna on 12/6/23, 7:52 AM with 82 comments

  • by jacquesm on 12/6/23, 9:39 AM

    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.

  • by bob1029 on 12/6/23, 12:11 PM

    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.

  • by black_puppydog on 12/6/23, 10:02 AM

    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?

  • by dtx1 on 12/6/23, 8:47 AM

    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...

    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)

  • by crote on 12/6/23, 9:13 AM

    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!

  • by mhaberl on 12/6/23, 10:15 AM

    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.