If you have the space for it, just put it outdoors 5' from anything flammable and you're good to go. This is not a hobby for folks in apartments unfortunately.

I've had one of my DIY ebike batteries short and fail spectacularly at near full charge and was able to push it with a broom out of the garage into the driveway before any damage was done. Now I have a bench with wheels that I can take into the driveway for initial testing.

Current building regulations in the UK are moving towards outside storage of batteries now because of NMC's having a high tendency to combust. The battery management systems do detect a lot of issues, thermal and shorts but at some point I think government is going to force these to be on the outside of houses.

The right answer is LifePO4 for home storage, does not combust and has good enough density.

Shameless plug: we're building a repairable e-bike battery where you can use your own cells at https://infinite-battery.com

We worked on a very sturdy casing, with some specific features to release pressure and limit the fire event propagating cell to cell, you can check it here https://www.youtube.com/watch?v=v0NXXfCA2CY

Ideally build it in away from your house, as others have said, but in terms of actual safety systems:

-get a high quality BMS from a reputable source, it should supports current limits and thermal probes - configure current limits with as much overhead as possible, the less you drive them, the cooler they'll stay - make sure you have sufficient thermal probes inside key points in the pack(s) and that they're configured in the BMS to cut draw - add thermal fuses as well, knowing where to put these is important, too - house the packs so to minimize fire risk and cascading issues, especially if space is not a concern

A runaway NMC battery is notoriously difficult to extinguish, so the best thing to do is to not do it and investigate LFP prismatic cells instead.

Many people building home storage batteries use a shed a few meters away from their home.

Doesn't matter. If you try any of these techniques your insurer will still void your home insurance if your house catches fire.

Proper cooling seems to be the primary thing to do.

https://www.sciencedirect.com/science/article/pii/S259017452...

Proper Home Storage systems are pretty safe:

https://www.bves.de/en/2024/12/17/study-home-batteries-fire-...

There are special containers for transport of (even damaged) lithium batteries, which don't look overly bulky:

https://www.zarges.com/en/solutions/transport-and-storage-of...

Small outbuilding. Concrete pad. Cinderblock walls. Sheet metal roof. Safe distance from anything important. Typical cost is $3000-$4000. Farms often have little buildings like this.

(There are pictures of such buildings online. Search is returning awful LLM-generated garbage landing pages, so I don't have a link.)

I think there’s not much you can do other than placing it very far away from the house. Those big packs just have way too much energy.

If it reaches thermal runaway... there's not much you can do. There's simply too much potential energy in the pack.

https://www.reddit.com/r/SweatyPalms/comments/1gbryd1/factor...

Throw it in a massive body of water is about as good as you can do until the energy is depleted. (Also, making a human perform the puncture to initiate a violent chemical reaction is WILD. Do better)

https://www.reddit.com/r/interestingasfuck/comments/1nuo0fj/...

For real dev work... essentially special sheds with massive ventilation to handle thermal runaways, rated by amount of kwh.

Distance, so a separate building far enough away from anything you want to keep that it won't carry over. Cabling underground but not in a duct. Space the cells a bit and ensure good airflow between them. A single runaway cell is how it usually starts and that in turn usually first shows up as increased internal resistance. Avoid mixing capacities, brands, different internal resistance values, cells that self-discharge, cells that have any sign of physical abuse or damage. (The guy in the video totally misses the internal resistance angle, as well as the self discharge which takes a long time to test and he already spent two months or more on his power bank.) Weld, do not solder your connections. Assemble cell groups first and then do QA on the cell groups as if they are larger capacity cells, and monitor them with a FLIR for any sign of cells misbehaving, especially under larger cell group charge/discharge currents. If a cell in the center is more than a few degrees warmer than those on the outside that's a serious concern.

Other prevention measures: strict inbound QA on the cells.

That's a lot of work, to the point that if you value your time you are better off buying a factory made pack of a reputable because they will almost certainly do a better job than you will on the safety aspect.

Getting Lithium-Ion packs right (especially larger ones) requires more up-front funds in terms of gear (especially testing gear for large volumes of cells is quite pricey) as well. The only reason I would do another (big) pack is if the form factor or capacity I want is not available at all.

The brief flash of that 'rechargeable powerbank' in the video has so much wrong with it that it isn't even funny and that's before looking inside the pack. All of those crossing wires, brrr. And those modules in the linked video don't look much better. Oh, and he's got one cell group in there with fewer cells (13:26) so that pack will unbalance immediately during charge or discharge. Effectively the whole pack has as much capacity as that one smaller cell group times the number of cell groups. The lack of integrated balancing wires is another puzzle for me, crossing balancing wires is a major headache when building larger packs, you want the very best grade of wiring for that with vibration resistant insulation and some kind of wire guide to ensure the wires can't move or cross. The whole BMS setup looks like an afterthought, rather than that it was designed in.

Oh, no interlock on the two separate breakers for the inverter (configured to work in island mode) and the house power. Wait until someone engages both. You need a transfer switch there, not two separate breakers.

His remark that it all looks 'super dodgy' and that 'I do not recommend anybody ever builds a pack like this' is an interesting one: if you are aware of all that, why do such a crap job in the first place? If you are going to go through that much work you might as well do a proper job.

Meanwhile, this is really just an ad for JLPCB. Running around the house and the workshop at everything working is a bit cringe, it is as if that's just padding to extend the video runtime.