Comment by dtx1 | Hacker News Reader

Comment by dtx1 | Hacker News Reader

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)

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.

Bonus Points for an UPS that will keep them charged at 100% permanently.

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.

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.

Fact: All Battery powered devices should have easily user replaceable batteries

Ahhh but that sweet incompatibility driven mark-up...

Don't worry, the market will self-regulate. Any day now! :)

It's happening:

https://www.cordless-alliance-system.com/

and

https://www.powerforall-alliance.com/en/

Of course there had to be multiple efforts...

ayayay, that reminds me to check the battery of my ~2008 Bosch :D

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.

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.

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.

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.

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.

Victron is good stuff, seconded. Used extensively in marine applications, I've yet to see a unit fail that wasn't purposefully abused.

> 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.

They didn't say you couldn't learn, they said don't fuck around. That seems like pretty reasonable advice to me.

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]".

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.

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.

> 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

> 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.

An essential part of that is actually understanding it and knowing what your limits are.

When you do extensive research, understand all the risks involved, and seek help to review your work, it is definitely possible to do projects like these - even as a hobbyist. The problem is that this looks to be closer to "hold my beer while I grab whatever cheap module I come across on AliExpress".

I design electronics for a living, and I personally wouldn't want to touch a project like this with a ten-foot pole. The risks are just not worth it. There are plenty of other fun things to do which aren't going to burn down my house.

It's extremely helpful! It helps keep the author of TFA safe.

The "you don't" is leaning on the idea that if you actually know what you're doing, an internet comment saying you don't isn't going to dissuade you. If you aren't confident enough to ignore an unequivicol "you don't understand this", then you definitely don't know enough to mess around with this. (The converse, of course, isn't true. There's always going to be someone who is still overconfident enough to ignore it even without having enough knowledge).

Reply on news.ycombinator.com