I really agree with you. The idea of a homebuilt powerwall sounds really cool. But it's not something I would come close to working with.

Awhile ago I did a some volunteer work at a radio station. One of my tasks was to check for arcing. That involved going into the transformer room, disabling the safety systems, unracking some of the electrical components, powering the system on, turning off the lights and then looking for flashes from electrical arcs. It also involved going out at night with night vision goggles to check for arcing on the antenna. If we saw anything we knew what needed to be fixed. Safety was important there, but I developed a healthy respect for electricity. After working there I'm a little freaked out about the idea of dealing with high voltage systems.

High voltage, sure, but outside of an inverter that you can/should buy prepackaged none of this needs to be high voltage. The system in the embedded video has 14 sets of cells in series, meaning 40-60 volts.

Danger of electrocution is not particularly high, but can easy start a serious fire with such batteries. The energy capacity with create a seriously hot arc if shorted, and there is a potential for thermal runaway of the battery cells.

I once saw two marine lead-acid batteries explode and catch fire in the boat next to mine when the owner had dropped something big enough to short circuit and not disintegrate itself. Instead a fat copper cable exploded and then probably both batteries boiled, exploded and then everything caught fire. It took about 3 seconds.

Luckily he avoided the acid spray, but he had to empty 4 large fire extinguishers from the nearby boats to put out the fire and cool down the remains of the batteries enough so they did not immediately catch fire again.

I have a huge respect for what a huge solar array and a big bank of batteries can do after seeing what a relatively puny battery could actually do.

A standard AC system can't deliver even a percentage of that current.

I had something similar happen to me in a Datsun 180B once.

Driving home in the rain, and the tail light wires were submerged in water that was pooling in the trunk and shorting out. As I put my foot on the brakes - on the freeway in the pouring rain - the fusible link in the engine bay decided to just catch on fire instead of being a fusible link. The battery quickly boiled and I had a sudden and catastrophic engine bay fire on a crowded road.

A car battery has enough amps to literally use as a welder (two jumper leads, an arc welding rod and a 550CCA car battery is enough to stick most things together in an emergency :P ), and is more than enough to create a massive fire.

Yesterday I was putzing around with these exact batteries (swapping some old tooling that had great build quality but were NiCad junk). Those 18650s pack a punch as current sources. I had an ammeter hooked up and we're talking easily in the 10s of amps for a brushed DC motor of your standard cordless Milwaukee from the 1990s.

I did the same to fix one of those $40 dollar Swiffers (i.e., we're not really talking about powering industrial steel chop-saws powering through I-beams). It drew ~2.5 amps at no-load, no problem.

OSHA[1]:

"17-90mA" : Death is possible.

"90mA+" : Death is likely.

And remember breakers blow only when the sub-circuit's current exceeds the threshold (generally ~15amps in the US) so you can spec a standard gauge of wiring sufficient to consistently draw that current, not overheat and burn your house down. It's (generally) not checking to see if the current sunk = current sourced. You need GFCIs("RCD"s in other locales) to actually get that functionality.

The rule of thumb I've heard is 50mA passing through your heart is where the v-fib dangers begin. The real risk begins at as low as 20mA, because thats when your hands lose their muscular control. You can't let go of the source, and your heart enters v-fib (~100 mA) and you get oxygen starvation and brain death[3]. You have tales of people using one hand to support themselves on a grounded breaker box, using the other hand to just have a look-see, and not even realizing they're in v-fib, feeling a little 'off', sitting down for a bit to catch their breath and found dead 20 minutes later next to the box.

Your body basically acts as a resistor in parallel with the circuit. Here's the ASME's resistance model of an adult males' body[2]. As you can see, you're body is definitely not a 10meg resistor. If you're working in damp, humid, or hot (i.e., you're perspiring through your hands, and that Na+ is just looking for a donor electron!), the risk increases. Whether you're working on a 460v3ph 15 horsepower Hardinge lathe or a Swiffer, the best piece of advice I ever got was "have a healthy respect (and fear) for your tools". Use isolation transformers, current limiters, and CAT-rated gear and read a sufficient amount of information to inform yourself in advance of the potential risks[4].

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[1] http://web.archive.org/web/20130428070054/http://www.osha.go...

[2] http://risk.asmedigitalcollection.asme.org/article.aspx?arti...

[3] Brain death I've always heard is ~5 minutes for an average adult male with a decent pulmonary system and respiration capacity-- less if you're a couch potato with less lung VO2 capacity, more if you're Lance Armstrong. Obviously not a physician.

[4] I was googling to pick find a diagram to depict the resistance model of a human and stumbled across Allaboutcircuit's safety text (chapter 3, for those who are wondering). It's pretty good for "general" electronics. Obviously if you're working with things like tubes ('valves'), transformers, and other subsets, you're going to want to read about the safety precautions you want to take for those specific areas as well.

How many watts are we dealing with here? What's the actual power output of a 1 second long direct short? I'm pretty sure if you shorted that 40-60 volts across a hotdog it would explode. Even if we disregard if that voltage can stop a heart (which it can, 50v is minimum), the amount of power you are dealing with is the amount of power that causes things to explode.

I wouldn't go anywhere near a diy power bank without proper knowledge and training.

With batteries (and even small battery packs) there are two main safety issues: (1) chemical combustion (due to mechanical damage or stress or electrical stress i.e. over-charging) (2) Burns and fires caused by arcing.

For (2) the exact configuration basically does not matter, because generally speaking all battery packs capable of storing a large amount of energy are also able to release a large amount of energy in short time. So if you cause a short you'll always get a nice arc. Note that most kinds of protections are way too slow to suppress arcing, so you can always burn yourself. Arc-fault detection is more difficult in DC systems as well.

(1) is mostly a matter of doing things right. This means active temperature monitoring and measures (cooling, venting, shut-off) for larger packs. Independent over-voltage and over-current monitoring. And of course the little things, like wiring things up the right way and placing sense wires at the proper locations. For example, this is a simple thing of doing things wrong:

    +  Charger  -
    |           |
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |           |
    +   Sense   -

Designing these kinds of circuits correctly requires a lot more experience and detail knowledge than one may think at first. (Besides miniaturization this is another reason why one-chip battery management chips are so popular in the industry).

——————

I did a few things with a few types of batteries (lithium ion and lead) and these little cells can cause an impressive amount of uh "disturbance" in short order. I admit I am almost as careful (in a different way, obviously) with them as when handling high-voltage things like the HV oscillator in an old 'scope.

>

    +  Charger  -
    |           |
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |           |
    +   Sense   -

What is being sensed here and why is it wrong? If voltage is being sensed and the busses are fat chunks of copper, this seems entirely proper. What am I missing?

EDIT: Wait, I think I know. You also need independent current measurement at each cell to detect internal shorts. And probably a way to automatically isolate such cells.

"Charger" would of course also be for discharge. "Sense" can either be charge sense, but that's not very common (due to CCCV charging), or monitoring. In either case, in this way of wiring things up the uppermost cell has a lower resistance connection to the charge/discharge port compared to the lowermost cell. So that uppermost cell will handle higher currents and overall will degrade more quickly. (This frequently applies when connecting "identical" parts in parallel in power electronics, e.g. capacitor banks).

If the voltage feedback (assuming there is any) is taken from the lower side, then a reduced voltage (by U=Rbus×Ibus) will be measured, which is lower than the voltage present at the cell terminals. Siblings mentioned that you want protection against internal cell shorts, using fuse wires or similar.

A better configuration would look like

    +  Charger
    |
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
                |
       Charger  -

As you can see the total length of wire/bus between the pack's poles and each cell is the same length, so has the same resistance.

If each cell is connected to the bus bars with fuse wire (like in the Tesla battery pack for instance) this connection diagram seems valid. Same way it is connected in the car/powerwall packs that tesla makes at least.

Short detection < bingo.

Technically it depends on the cell chemistry and how they fail.

High voltage is 1000V AC or 1500V dc

https://en.wikipedia.org/wiki/High_voltage

If you're an electrician, yes.

In casual conversation it can mean close to mains voltage or higher.

That article itself quotes a bunch of wildly different standards.

I guess anything that can hurt enough to consider medical help is high voltage.

So that would be six volts or something like that? ;)

Yeah i have read enough scary stories about people dealing with 18650 cells (effectively AA sized lithium batteries used in high powered flashlights etc), that trying to build a powerbank from used laptop batteries with enough capacity to handle domestic appliances is not something i would undertake.

I have a sneaky suspicion that their insurance isn't going to cover the fire this may cause. These are lithium batteries, with who knows what for quality control, being used by people with who knows what for education. This could be a tragedy in the making.

The idea that insurance won't cover fires from unpermitted, DIY stuff is mostly urban legend. Most homeowners' policies are what are called "all hazards" policies. They cover any kind of damage for any reason, with explicit exclusions like war, earthquake, flood. Outright arson / fraud by the policyholder would also obviously be excluded.

Banks would object to policies that excluded unpermitted work, since it's relatively common and this type of exclusion would leave them with lots of exposure.

So, they may decide to drop you like a hot potato afterward, but a fire from your homebrew powerwall would likely be covered. Read your policy fine print to be sure.

They're lithium-ion batteries, which contain very little elemental lithium and do not result in lithium fires (which are very difficult to put out). True lithium batteries (the kind that can cause lithium fires) are not rechargeable.

http://batteryuniversity.com/learn/article/safety_concerns_w...

[edited to add link]

The batteries being talked about are 18650s, which is mentioned pretty clearly in the article and apparent from every single picture of batteries in it. This makes sense, because the vast majority of laptops sold across the world use a set of 18650s as their batteries. These are also the form factor of batteries being used in current Tesla powerwalls (though I believe they will also switch to the 21700 form factor as production of that ramps up).

18650s are just as safe as any other battery chemistry in everyday use, but they're a high energy lithium cell. In particular, if you do bad things to them, they may light on fire. It's not that hard to take appropriate precautions to avoid that, and reasonably sized lithium-ion battery fires can generally be put out with a standard househould fire extinguisher.

It also appears the community is collaborating to spread good information about powercell design to avoid/manage fires inside the units, in the unlikely event a cell starts a fire. If the container for the powercell is designed correctly, it can contain a complete unit burndown: https://electrek.co/2016/12/19/tesla-fire-powerpack-test-saf...

In short, you're overestimating the risks and underestimating the level of safety attainable with these products.

> In short, you're overestimating the risks and underestimating the level of safety attainable with these products.

No. When it comes to things that might produce fire inside your house, there is no such thing as overestimation. You cannot be over protective when it comes to fire safety.

Source: I was a firefighter for +7 years.

"You cannot be over protective when it comes to fire safety."

Ridiculous. There are obvious examples of overestimation of risk, why not prohibit electricity in homes entirely for example? Should I get rid of my kitchen stove?

This kind of absolute rhetoric is banal, obviously absurd, and is not helping anyone.

Effective safety measures involve accurately quantifying and balancing risk.

In the article it says they don't put them in their house. They put them in a shed or other detached building.

> there is no such thing as overestimation. You cannot be over protective when it comes to fire safety.

Have you ensured your house is built exclusively of fire resistant materials, and avoided buying modern furniture with inflammable fabric covering and interior padding? Have you installed a Halon system in your home and all physically adjoining structures?

What an absurd statement.

Question: isn't it easier to use one of those fancy arc detector breakers instead of visually inspecting the whole equipment?

I'm not too familiar with those, but it looks like they just shut down the circuit. We were looking for arcing to pinpoint where the problem was. It was a short wave radio station, so it had a large antenna. Arcing in the antenna was probably caused by a broken wire. That could be fixed by climbing up and replacing it.

You're right. You want to detect and pinpoint, not just cut off the current. Silly me.

Not in a radio station. Those typically trigger from RF.

While we're at it, what's reddit electronics subreddit say about this:

https://www.reddit.com/r/AskElectronics/wiki/beginners

tl;dr; do not attempt anything with powerelectricity without safety precautions

ps: there are many electric safety pdf issued from various US states such as https://www.dir.ca.gov/dosh/dosh_publications/Electrical_Saf...

DIY projects always get glorified like this. As if people figured out this secret trick to bypass consumer capitalism. When the whole idea of capitalism is about not having to invest a ton of effort into learning a new craft every time we want to use a new product/service. We pay companies to master that craft for us with money we earn from focusing on our own individual skills.

It'd be more accurate to sell it as a rewarding experience to do as a hobby rather than a clever cost-saving commercial solution.

If money and ROI is the primary focus then most specialized knowledge could be better utilized in other ways than simply building a product once for yourself in your spare time.

That's the early on naive version of distribution of effort and mastery. The market evolves into a very average product quality and all kinds of tricks (focus on shallow quality, brand name versus longevity, repairability...) to improve benefits, which is the core signal in capitalistic feedback loops.

It's also good to do stuff on your own. I have to admit, I am too dogmatic about not buying new things. It's only good if you have time to invest it's true. But after a threshold it's of great value. I remember the fear of electronics and household appliances, it's now gone. Capitalism turned optimization into blindness.

Amen to what you just said.

Years of effort and learning have culminated with these projects that they've undertaken and executed.

We should NOT undervalue that.