[tomtranter]

That's cool. I'd love to be able to do that but my hacking skills are somewhat confined to the digital domain. What's going to be really interesting in the next few years is the number of batteries coming out of cars which could be re-purposed for grid storage or back-up domestic power like your set-up here. Typically an ev battery is determined to be at end-of-life when it's reached 80% of original capacity. However, the capacity is also dependent on how fast you try to cycle it and over what range of SoC. The bigger the SoC window and the faster the cycling the more stress put on the battery and the larger the losses. Taking batteries out of cars and putting them in boxes, cycling them slowly and within a smaller SoC window means you can get a lot more life out of them.

[silisili]

Thomas Massie, the engineer and arguably overqualified US representative, did just this and made a series of videos about it. I have no idea if this is the original channel, but it has the updates - part 1 linked below.

https://m.youtube.com/watch?v=qpPYkqpe-Ms

[tomtranter]

Fantastic! I will now enter the rabbit hole

[tra3]

> I'd love to be able to do that but my hacking skills are somewhat confined to the digital domain.

Same for the most part. I'm venturing into the real world, as it were. Have to have a goal in mind that interests you and then pieces come together.

> batteries coming out of cars

So far I've restricted myself to ~12v batteries because I don't fully understand the safety procedures required for high voltage applications. Eventually it's something I want to get into as well.

[ridgeguy]

To add to mschuster's great details below: remove all jewelry, watches, etc. when working with sources (even low voltage ones) that can provide very high current. In this instance, think of 12v lead-acid batteries that can deliver hundreds of amps into a dead short.

If the dead short path happens to be through your wedding ring, your finger can be deeply burned before your neuromuscular reflex can break the circuit.

[foobarian]

I found this out fidgeting with a 18650 cell in my pocket. (It was a salvaged cell without a plastic wrapper so all it took is bridging a few mm gap between the middle and the outer shell.)

[mschuster91]

> So far I've restricted myself to ~12v batteries because I don't fully understand the safety procedures required for high voltage applications. Eventually it's something I want to get into as well.

To add a bit of context for you and anyone willing to experiment with batteries:

First, the danger isn't just the voltage. Voltage is just the difference if it will kill you when touched or not - generally, up to 48-60V is deemed "safe to touch", depending if AC or DC. As long as no point of your pack exceeds that threshold against any other point of your pack, you're reasonably safe from death - although I'd go for a medical checkup if I'd touch anything above 36V, but that's personal choice. If the current path goes across your heart (e.g. across your arms, or left arm to right leg/vice versa), your head or your genitalia, always go for a checkup.

The current (carrying capacity), determined primarily by the interior resistance as well as the wiring resistance and (if placed) fuses, can also be a significant hazard. Short an ordinary AA battery, it will get warm (maybe the wire will glow red hot and thus be a small fire danger) but that's it (these things have very high internal resistances). Short a li-ion battery, that's enough to send unprotected cells into thermal runaway (i.e. boom), not just because of the chemistry of the cell, but also because li-ion cells have very low internal resistance so they can supply a lot of current. Short a pack of li-ion cells or a car starter battery? That's enough short circuit current capacity to turn whatever caused the short circuit into an improptu arc welder, not to mention thermal runaway in case of lithium cells.

Now, for some recommendations:

Always fuse off cells or packs as close to the batteries as possible. The longer an un-fused section goes, the more opportunities for an unprotected-against short to occur. Fuses have not just different current capacities (i.e. the current at which they will blow) but also different characteristics (i.e. how fast they'll respond to a given amount of overcurrent). Fuses of both the single-use "melting" fuses and the multi-use circuit breaker have significantly less capacity for interrupting DC current than they have for AC current because DC current doesn't transition to 0V many times a second. Select your fuse(s) to match appropriately!

Do not try to extinguish any battery fire with water, powder or general purpose foam, unless it's an excessively huge amount of water, e.g. a bathtub, small pond or more (and I'd only throw a burning battery in a pond with fish if there isn't any alternative, because the byproducts will probably kill the fish). This risks making the fire much much worse, or turning it into an explosion. CO2 isn't harmful, but it's useless. Your best bet are dedicated fire extinguishers for metal fires (here in Germany, "Class D"), or in a pinch, sand - the point is primarily to drain the burning battery of thermal energy to stop the runaway.

Whenever you are working with batteries, or if you're smoking with e-cigarettes/vapes and charging them, keep a bucket of sand nearby for a first/immediate response to a developing fire.

Never expose a lithium cell to strong heat, e.g. a soldering iron. This can and will send the cell into thermal runaway. Use sockets or, if you absolutely have to make a pack, a spot welder.

Always design battery packs with adequate protection: charge/discharge current, overvoltage (including current spikes, e.g. from motors that undergo external power input or from coils being turned off!), undervoltage, temperature (best: per cell!) and pack voltage/balance. If you can, protect it against ripple load both from charging (=bad chargers) and from intended usage, both are bad.

Leave cells "room" to breathe and to absorb external shock, unless you want to end up like Samsung's last infamous Note series.

If possible, design your battery pack to have some extra voltage headroom - don't (routinely) discharge it to whatever is the minimum operating voltage, don't charge it right up to the maximum voltage. General best practice to ensure longer life is 20% on both ends. The sort-of exception are lead-acid batteries in low-power (!!!) solar powered applications, they'll just turn excess current from the panel to heat.

Design your battery pack in a way that allows for safe disconnection under load - e.g. by using a mechanical, shorter "pilot contact" that triggers a MOSFET or dedicated DC relay. Otherwise, the user may pull it under full load and you'll get arcing. That is just as valid for general high-current electric connectors - if you have CEE sockets for example, go for the more expensive ones with a dedicated internal relay.

[1] https://en.wikipedia.org/wiki/Extra-low_voltage#Regulations

[mschuster91]

Oh, and to add another design hint. Closely related to "always fuse off cells or packs as close to the batteries as possible" - make sure that you cover bus bars, terminals, wire solders and the likes wherever possible. The classic in small form factor is Kapton tape or heat shrink tubing, for anything larger go for voltage-rated plexiglass.

And for heavens sake if you're in small form factor and you got a battery... please just don't go and solder the ruddy wires onto the PCB directly. Use any cheap-ass connector you like.

Many a thing and occasionally even a life got destroyed by someone accidentally dropping a screw or a tool onto a live busbar. A wrench shorting out even a "plain" 230V circuit but right at the exit poles of a megawatt scale transformer makes for quite the firework.

[ansgri]

Thanks for the great overview! This is why I'll never want to deal with HVDC: all this, with increased risk of arcing, and "conventional" electric shock, no thanks

[tuatoru]

Great safety advice! Ninety percent of practical engineering is thinking about what can go wrong.

[mschuster91]

If people would do this in IT as well the world would be a safer place... unfortunately, people can imagine a house burning down in a fire much better than they can imagine the fallout of an IT security issue.

[tra3]

Appreciate your advice above!

So much of learning is trult internalizing the information. Specifically, I thought I understood that low voltage poses little risk of electrocution but it never clicked until now that low voltage + high current can ignite things. I've been very careful to fuse everything as you suggest, but my bus bars are exposed. A 12v 100amp short would be nasty.

> If people would do this in IT as well the world would be a safer place.

I'm of 2 minds here. The engineer in me wants everything as robust as possible, but that comes with trade offs, right? Do I want my cancer radiation treatments or flight software to go through rigorous checks? Absolutely. Does my web store need the same kind of rigour? Probably not.

[foobarian]

You would probably enjoy seeing what happens, thanks to our industrious Youtuber friends who took the risk so we don't have to! [1]

[1] https://www.youtube.com/watch?v=ywaTX-nLm6Y

[mschuster91]

> I've been very careful to fuse everything as you suggest, but my bus bars are exposed.

Wrap as much of them in heatshrink tube as you can, that should be the easiest.

> Do I want my cancer radiation treatments or flight software to go through rigorous checks? Absolutely. Does my web store need the same kind of rigour? Probably not.

Hmmyeah, but getting your web store breached can still have serious financial implications.

[maxerickson]

It's likely enough there will be commercial products aimed at doing this with widely used modules. If there are, they will probably be cheaper than doing it from scratch.

[tomtranter]

Hopefully. The thing that makes most sense is two-way charge points but for whatever reason these aren't that common

[pjc50]

Hmm. It sounds nice, and people will definitely do it in resource-constrained setups, but I suspect for mass production use nobody wants to touch a EOL pack - all your cost savings are wiped out by the first fire.

Not to mention the technological evolution. By the time battery packs manufactured today are EOL, maybe we'll have high cycle life solid state sodium batteries coming off the production lines.

[tuatoru]

> my hacking skills are somewhat confined to the digital domain

Texas Instruments have the INA219 and INA226 I2C high-side current sensors for DC up to 36V. I just know of those because of Arduino. There are many others too.

Over 36V or so, that you'll want a Hall Effect sensor suitable for your current ranges and an ADC.