What does a kilowatt hour look like? • Hacker News Board

Re: What does a kilowatt hour look like?

#121

Post by ralfd » Fri, Sep 23, 2016, 10:41 AM UTC

jrockway wrote: post #57

spdustin wrote: post #29

Given how popular (and damn near impossible to discern) unsafe and unprotected lithium-ion batteries are, the idea of making a homegrown power wall terrifies me in ways I can't even begin to contemplate. An 18650 battery, if combusted, can produce hydrofluoric acid with just water vapor in the air (or the water in your lungs) if they use lithium hexafluorophosphate as the electrolyte. Hydrofluoric acid can and will c…

You have to realize that 1kWh is a lot of energy. Imagine being hit (in a world without an atmosphere) by a 60 pound suitcase falling out of an airliner flying at 45,000 feet. Same amount of energy as that battery pack. (Alternatively: the most powerful air conditioner you can buy running full blast for 30 minutes. Incidentally, that battery pack has as much energy as 100mL of gasoline, which is why gasoline-powered…

Wow, 100 ml of gasoline could propel a suitcase into the stratosphere? That seems so counterintuitive.

Re: What does a kilowatt hour look like?

#122

Post by wruza » Fri, Sep 23, 2016, 10:51 AM UTC

adrianN wrote: post #104

bigiain wrote: post #26

Earlier quoted context omitted.

25 million kWh as mass is about 1g (E=mc^2) so 1kWh is 40 nanograms or about 1/150th of a grain of sugar. (handwaving away a method for 100% efficient conversion of mass to energy... But solving that would be a _really_ good thing...)

Just drop it in a black hole and collect the Hawking radiation.

I wonder how much energy it will take to push matter into already steaming blackhole.

Re: What does a kilowatt hour look like?

#123

Post by chrismcb » Fri, Sep 23, 2016, 10:59 AM UTC

ralfd wrote: post #121

jrockway wrote: post #57

Earlier quoted context omitted.

You have to realize that 1kWh is a lot of energy. Imagine being hit (in a world without an atmosphere) by a 60 pound suitcase falling out of an airliner flying at 45,000 feet. Same amount of energy as that battery pack. (Alternatively: the most powerful air conditioner you can buy running full blast for 30 minutes. Incidentally, that battery pack has as much energy as 100mL of gasoline, which is why gasoline-powered…

Wow, 100 ml of gasoline could propel a suitcase into the stratosphere? That seems so counterintuitive.

Only if there is no atmosphere.

Re: What does a kilowatt hour look like?

#124

Post by userbinator » Fri, Sep 23, 2016, 11:01 AM UTC

spdustin wrote: post #87

userbinator wrote: post #80

Earlier quoted context omitted.

I think your concerns are somewhat alarmist and tend to be a bit scaremongering. You may want to read an actual study about just how much HF is present in the fumes of burning cells. In this one, near the end they burned a whole laptop pack and analysed the gases that were emitted: http://www.brandforsk.se/MediaBinaryLoader.axd?MediaArchive_... The relevant datapoint is on page 61: "As seen the yields of HF is much l…

The PF6 ions in the electrolyte of some Li-ion batteries (not all use the same chemistry) will readily combine with hydrogen in the air to form HF if the battery is combusting. Cylindrical Li-ion cells, like those part of an 18650 battery, literally hold their chemistry under pressure, and a thermal runaway will normally cause the vents in the cell to release the pressure. If that's happening due to a short (which ca…

Cylindrical Li-ion cells, like those part of an 18650 battery, literally hold their chemistry under pressure, and a thermal runaway will normally cause the vents in the cell to release the pressure.

Unlike what you seem to be implying, lion cells are NOT "under pressure" in normal use. They are at atmospheric pressure. The vent is specifically so in case of abnormal conditions, it allows any pressure to escape instead of causing the cell to explode.

result of the release of pressure combined with the reactive nature of now exposed lithium

In a lion cell, there is no "exposed lithium" in the metallic, highly-reactive sense. It's a solution of lithium ions in an electrolyte, whose only real hazard is its flammability.

I believe I know the flashlight forums you refer to, and the incidents of HF poisoning too. Those were lithium primary cells, which actually contain significant quantities of metallic lithium and are definitely more reactive than lion. Even the MSDS you refer to says "in spite of their name, these batteries do not contain any lithium metal".

The MSDS is also the easiest way to scare yourself into thinking everything is highly dangerous. For example, look at the ones for sodium chloride. By the way, I would not trust that one so much as it gives a boiling point for ethyl acetate, a liquid at room temperature, of -84C. Similarly, dimethyl carbonate is listed as boiling at +4C, when that's its melting point. Perhaps that's where your impression that the cells are normally under pressure came from?

Likewise, I recommend looking at the data on the electrolyte solvents:

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

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

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

Not totally harmless, but not insanely toxic either.

Re: What does a kilowatt hour look like?

#125

Post by majewsky » Fri, Sep 23, 2016, 11:12 AM UTC

jrockway wrote: post #57

spdustin wrote: post #29

Given how popular (and damn near impossible to discern) unsafe and unprotected lithium-ion batteries are, the idea of making a homegrown power wall terrifies me in ways I can't even begin to contemplate. An 18650 battery, if combusted, can produce hydrofluoric acid with just water vapor in the air (or the water in your lungs) if they use lithium hexafluorophosphate as the electrolyte. Hydrofluoric acid can and will c…

You have to realize that 1kWh is a lot of energy. Imagine being hit (in a world without an atmosphere) by a 60 pound suitcase falling out of an airliner flying at 45,000 feet. Same amount of energy as that battery pack. (Alternatively: the most powerful air conditioner you can buy running full blast for 30 minutes. Incidentally, that battery pack has as much energy as 100mL of gasoline, which is why gasoline-powered…

> 60 pound suitcase falling out of an airliner flying at 45,000 feet. Same amount of energy as that battery pack

You did not account for the terminal velocity of the suitcase. It cannot accelerate indefinitely because of the air resistance.

If I assume that the suitcase will not break the sonic barrier (which seems reasonable), I get an upper bound on its kinetic energy of

  0.5 * 60 pounds * (1 mach)^2 = 0.41528548 kilowatt hours

according to units(1).

Re: What does a kilowatt hour look like?

#126

Post by Koshkin » Fri, Sep 23, 2016, 11:24 AM UTC

daurnimator wrote: post #77

Koshkin wrote: post #67

Earlier quoted context omitted.

> Newton-meter I do not know why (and I do not have a strong opinion on this), but somehow this (relatively recent) tradition of naming units after people feels weird to me. I know we are already used to these units, but imagine the unit of mass was called Einstein instead of kilogram.

The Newton unit has been around since 1948. But that was hardly the first term to be named after someone: Watt has been used since 1882, Farad since 1881, Fahrenheit since 1776...

Note that the name of the unit is 'degrees Fahrenheit', but I agree - this could be something that started the tradition, which became stronger with new discoveries in physics and chemistry in 1800s.

Re: What does a kilowatt hour look like?

#127

Post by jahnu » Fri, Sep 23, 2016, 11:32 AM UTC

majewsky wrote: post #125

jrockway wrote: post #57

Earlier quoted context omitted.

You have to realize that 1kWh is a lot of energy. Imagine being hit (in a world without an atmosphere) by a 60 pound suitcase falling out of an airliner flying at 45,000 feet. Same amount of energy as that battery pack. (Alternatively: the most powerful air conditioner you can buy running full blast for 30 minutes. Incidentally, that battery pack has as much energy as 100mL of gasoline, which is why gasoline-powered…

> 60 pound suitcase falling out of an airliner flying at 45,000 feet. Same amount of energy as that battery pack You did not account for the terminal velocity of the suitcase. It cannot accelerate indefinitely because of the air resistance. If I assume that the suitcase will not break the sonic barrier (which seems reasonable), I get an upper bound on its kinetic energy of 0.5 * 60 pounds * (1 mach)^2 = 0.41528548 ki…

> (in a world without an atmosphere)

Re: What does a kilowatt hour look like?

#128

Post by Koshkin » Fri, Sep 23, 2016, 11:35 AM UTC

nitrogen wrote: post #73

Koshkin wrote: post #67

Earlier quoted context omitted.

> Newton-meter I do not know why (and I do not have a strong opinion on this), but somehow this (relatively recent) tradition of naming units after people feels weird to me. I know we are already used to these units, but imagine the unit of mass was called Einstein instead of kilogram.

How recent is the trend? Most of the units I know are named for people: Joule, Watt, Volt, Ampere, Newton, Angstrom, Ohm, ...

I am sure Newton didn't name the unit of force after himself, so I would roughly place the invention of such possibility around early/mid nineteenth century, that is, about 200 years ago, which is when the research in such areas as electricity and magnetism, thermodynamics etc. was in full swing.

Re: What does a kilowatt hour look like?

#129

Post by gravypod » Fri, Sep 23, 2016, 11:45 AM UTC

keenerd wrote: post #120

asteli wrote: post #46

Earlier quoted context omitted.

Building a pack from random cells like this is pretty much a cardinal sin with Li-Ion pack design. Cell resistance and state of health are carefully matched during pack manufacture to ensure cells charge and discharge evenly. Li-ion cells are pretty sensitive, so on top of that, large packs have battery management systems (BMS) to balance cell voltages and monitor for faults and damaged cells. Attempting to charge a…

Charging the "pack" shown is impossible. There are no wires connecting any of the batteries. He made a case that can hold a bunch of cells. Not a battery pack.

That's ever beginning of a larger project I assume.

Re: What does a kilowatt hour look like?

#130

Post by gravypod » Fri, Sep 23, 2016, 11:47 AM UTC

qoobaa wrote: post #110

userbinator wrote: post #69

Earlier quoted context omitted.

Even just drawing in parallel from batteries like this is a fire hazard. I think you've confused parallel with series; in parallel, the batteries will naturally balance themselves to the same voltage and discharge proportionally to their capacity. It's the series configurations that are riskier and need extra balancing circuitry. Also, I'm pretty sure the bulk of fires are from the much more fragile pouch "lipo" cell…

I think @gravypod actually meant parallel connection, it might be quite dangerous in fact - you don't even need to draw anything from the battery to mess things up. If you connect a 100 cells in parallel and one cell dies creating a short in the circuit, the rest of cells will discharge through the shorted one. When you short a single 18650 cell, it outputs about 6-7 A of current on average. Multiplying by 100 gives…

Yes that was mentioned in one of the maaaany EEVBlog videos iv watch that these fail closed sometimes as well as the recommendation that "you should never buy lipo cells without a built-in controller" comeing from a guy far smarter then I about electronics design. If he is afraid of it, it's for a good reason.