[lazide]

I’m sorry, that really doesn’t make any sense.

There is several missing factors here, and it’s a useless metric. Lithium ion can do ‘100 hours’ as well - based on discharge rate. It can also do 10 minutes - based on discharge rate. 100 hours is literally useless on it’s own because it doesn’t tell you anything concrete. 100 hours….. of what?

It looks like a classic science writing article where they left all the important units off, misunderstood the whole thing the audience was looking for, and made it all super confusing and pretty useless compared to an actual paper.

[azernik]

100 hours = (kWh/kg) / (kW/kg) - ie capacity divided by the most cost-effective discharge rate.

[ComputerGuru]

I agree but that’s a highly misleading number because there’s a conflation of scenarios. Capacity is one thing, useable capacity at the requested discharge rate is another. You can’t pick and choose which numbers to use.

[azernik]

The combination of duration class and capacity is useful, though. 1MWh of 10-hour storage gives you a very clear idea of usable energy.

[amelius]

What if the discharge rate versus cost plot is mostly flat?

[azernik]

Then typically you'll run it at a higher discharge rate, and has something cheaper with a higher capacity/discharge ratio.

[coder543]

> > You could do the same thing with Lithium Ion, it would just be cost prohibitive... 10x as expensive, supposedly.

> Lithium ion can do ‘100 hours’ as well

Did you reply to the wrong person? I already addressed your entire comment several different ways in my original comment, and you didn't address anything I wrote in mine, as far as I can tell. Maybe I'm not the best at explaining things?

> 100 hours... of what?

Cost effective energy storage solutions that can discharge for 100 hours at maximum operating output. "Cost effective" is absolutely the key factor. Lithium ion is not currently cost effective at such durations. It is cost effective at about 4 hours, but some sources will say anywhere from 2 to 8 hours.

If you aren't familiar with a particular industry, it's perfectly logical for that industry's shorthand to sound like total nonsense. You (and most of HN) are not the target audience for a statement like "100 hour battery". Grid scale energy storage operators are the target audience, and they understand what that means. It means slow batteries, which also implies cheap, because no one would buy slower battery technology if it weren't a lot cheaper. Form Energy explicitly declares the technology to be 10x cheaper. We'll see if that holds up in reality.

Someone unfamiliar with computer science who overhears a conversation about "garbage collection" would likely be very confused. "Computers don't emit garbage! RAM is reusable, how could bytes of RAM become garbage?" But, obviously garbage collection is a real thing with computers, even if it sounds like nonsense to someone outside the industry who just knows enough to know what RAM is.

Here's an article[0] that talks about long duration energy storage, and they even mention Form Energy. Relevant quote from the article:

> Lithium-ion batteries have absolutely dominated new storage construction in recent years. But they rarely can deliver their full power capacity for more than four hours — that’s what people mean when they say “discharge duration.” Batteries technically can go for longer, but it generally costs more than it’s worth in today’s market dynamics.

One more choice quote:

> Batteries cannot yet compete with gas plants in providing prolonged power for multiple days. But a cost-effective 24-hour duration storage system could handle longer demand peaks, and a 48-hour system could do even more.

It's all about cost effectiveness.

[0]: https://www.greentechmedia.com/articles/read/so-what-exactly...

[jazzyjackson]

Bystander here: I thought your first explanation was perfectly clear, battery nerds don't want to hear nothing but Watt Hours.

[lazide]

Huh, that is almost as ridiculous as using nameplate disk capacity units that sound the same, but are are different than the ones used and reported by the operating system.

In your first comment you seemed to be talking right by the person, but I guess not.

Thanks for the correction.

[neohaven]

That cost-effectiveness discussion was enlightening. I find you have a knack for explaining what people don't see, by the way.

Open question, which maybe you might have info about, I'm just curious if maybe anyone crossed something on this; I wonder if that characteristic is inherent to specific energy storage tech; for example in Li-Ion IMR/INR batteries, this might be in part because of the voltage sag and discharge characteristics? In other words, a battery that sagged less under load and had a flatter discharge curve would have a wider range of acceptable durations? For example : https://www.richtek.com/battery-management/img/battery-disch...

This graph gives a general idea of how battery performance degrades the more load you put on Li-Ion batteries, the kind in flashlights, vapes, etc. The ICR18650s of the world et al.. They are not great for energy storage, long term, frankly. The harder you discharge them, the more the voltage sags, requiring even harder discharge to maintain a proper power output, leading to more heat, and power loss in spiky demand conditions. Leaving them on but at low-power for a very long time seems to be more efficient, but by the time you hit that low of a power-delivery, they become less cost-effective, because you could do the same with a hydro plant, a reservoir, and pumps. Li-Ion batteries need management circuits, balanced charge, can be overcharged, can be drained 'till they turn into a brick. They're far from idealized batteries that discharge at all rates consistently at equal voltage all along their Wh rating. A Constant-Power drain on Li-Ion heats up and degrades way worse at low charge or low battery condition, etc.

In other words, kWh is never the be-all-end-all of energy storage. If you want long, sustained power in tiny sips, I'm pretty sure nothing beats these ZnO batteries for hearing aids that use oxygen (or is it air?) as fuel. What seems to really matter is the ability to push out kWh on a consistent basis is, as far as I can tell, and we have no really good solution for this?

[specialist]

Nicely done. Thank you.

Am total noob. Been learning from The Limiting Factor on youtube that many different Li-ion chemistries have different use cases, ideal applications, knock-on considerations.

Why Tesla intends to use different chemistries for car, cybertruck, semi, powerwall, etc. Why others are choosing different chemistries, like Apple and lithium-titanium, for their particular needs. (Such subtlety. Total nerdgasm. What a time to be alive.)

So your explanation makes perfect sense.

[warble]

Battery engineer here. 100 hours doesn't mean much by itself

[2Gkashmiri]

hey battery engineer. can you help me out with this stupid solar home battery confusion i have been having? 1 battery system is 12volt X 180AH =2160WH. (lets say lead acid) 2 battery system is 48Volt x 45AH=2160WH(Lets say this is lithium)

if i am using an inverter to convert DC to AC viz 220volt where my AC appliances run, does it matter on the battery side?

if i have a load, say a pc running 1 KWH, will these two battery systems run for 2 hours both (assuming discharge rate is same, i know i know, just calculating)

i am doing this because solar batteries are being sold at 48Volt x small AH to come up with KWH but at the same time lead acid are only sold at 12Volt but high AH. what if i buy 7AH 12Volt x 25 batteries to get same KWH?

does battery voltage or amperage matter when converting to AC? how does that work?

oh, BTW, i have a 5.2KWH solar array that is grid tied

[lazide]

Not the parent poster, but do have some familiarity here.

Higher battery voltage == less copper, smaller wires for the same wattage. Sometimes also a bit more efficiency too (less resistance, smaller absolute voltage drop moving current around).

Your units are a bit messed up load wise - is your computer running at 1 kWh per hour? For how long? (Yeah, that is confusing - but that would be the energy used to power a 220 volt, 4.5 amp load for an hour, every hour).

One of the confusing things with some of these units is kWh is a measure of energy, even though it has a time component in the name (kWh can be directly converted to joules). It isn’t ACTUALLY a measure of power (aka energy over time), even thought it implies it is because it has ‘hour’ in the name.

Power is energy over time.

So you have an absolute number for storage (your battery in this example can store 2.1kWh of energy - 2.1kWH is equivalent in energy to 2100 watts drawn out over an hour).

Your inverters, panels, chargers, etc. will be rated for a level of output POWER (energy over time) then will draw/convert that (say 4 or 8 kw) to/from the batteries. You can easily have an inverter that can draw down those batteries super fast (say will pull down 8kw continuous, so you’d get less than 15 minutes on that battery - if it doesn’t get damaged or explode), or one that draws power out slowly (say 200 watts max) to make it last 10 hours say.

Same with your chargers, etc.

You’re solar panels are almost certainly not rated in kWh, but kw- aka maximum instantaneous power they would be producing at any moment under ideal conditions, not aggregate energy they would be producing or storing over time. This is important because they don’t store power and actual energy production will depend on a lot of factors, like amount of time in the sun, etc.

[2Gkashmiri]

Thanks. How we see load is, the product advertises 1000 watts so for me that means 1kwh or one unit of electricity. Same for something that is 200 watts or 2000 watts.

I am aware of the solar thing. I have a grid tied system and I sell the energy to grid. I produce around 30 kWh a day which is good.

My problem is storage. If I want to store energy, should I go with 48volt 24 ah battery or 12volt 96ah ? As I said, there will be an inverter giving 220volt output so does battery voltage matter?

[ComputerGuru]

Again, I think you are making the same mistake in your first sentence. 1000W does not mean 1kwh or one unit of electricity. It is a measure of instantaneous (or maximum instantaneous, there’s no way your pc regularly runs at 1KW unless you’re mining) power consumption, the “now” voltage times the “now” amperage (times the power factor… maybe).

For storage, none of that matters. What you need to look at is the cheapest total capacity in kWh after accounting for conversion losses at your median discharge rate. Let’s say your median consumption is 450W, then you see the efficiency of your inverter with 12V vs 48V input to produce a sustained 220V at 450W. Maybe it’s the same efficiency for both 12 and 48v or maybe one is 85% and the other is 80% so you need to multiply the capacity by that amount to get capacity after inverter conversion losses to account for the only difference between the two setups.