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Haji

Banned
Location
India
tallgirl:
You extol Ah capacity compared to Wh capacity of a battery, though the Wh rating is more accurate than Ah rating and more modern.
One disadvantage of Ah rating is voltage rating also need to be specified: Same Ah rating may have different Wh ratings.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
The most accurate statement you can make is that it requires some amount of energy to store some number of amp-hours of charge ...

So I got it right - way back when I made the statement that precipitated all this - when I spoke of watt-hours needed to charge a battery. (Note that charge is a verb here, not a noun.)

And this whole diversion into the question of how to measure the charge (noun, not verb) once stored, really wasn't a response to the original topic.

Funny that the guy who changed the subject has been the only one not to post back about it.
 
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mivey

Senior Member
Appealing to NASA doesn't make it true.
Just stating facts. I was trying to find a source credible enough for you. Perhaps that was too lofty a goal since you don't even believe your own sources.

What you see in those half-cell reactions aren't watts. They are electrons -- those "xe-" thingies.

That means the reaction is in terms of AMPS, since an amp is a coulomb-per-second, so an amp-hour is a coulomb-per-second for 3600 seconds. You can't convert between coulombs per second and watts or watt-hours no matter how hard you might try.

The reason amp-hours work better -- and this is a "better" conversation, as well as a discussion about why kilowatt-hours are a horrible measure -- is because those electrons really do have to make their way through the circuit, even if the voltage is depressed.
A saying about forests and trees comes to mind. We use the battery as an energy storage device. Focusing on the amps and neglecting the watts is forgetting that to get the energy into the battery we have to have both amps and volts and that means watts.

Think about an energy storage system using water. You can focus on the molecules of water all you want. How many gallons we stored is only part of the useful information. We also consider which pump we use, which pipe we use, the height, etc. The main focus for this power application is energy. Pump efficiencies, the pipe characteristics, and many other things make a difference in the energy efficiency.

But to focus on electrons is like focusing on how many gallons of water we have. It really is only part of the story. For power loads we use watt-hour calculations and for current loads we make amp-hour calculations. Watt-hours and amp-hours both serve a purpose.

When you can get a half-cell reaction that's in watts or watt-hours, you get back to me.
How about when you charge the battery using amps without volts you get back to me.
 

tallgirl

Senior Member
Location
Great White North
Occupation
Controls Systems firmware engineer
How about when you charge the battery using amps without volts you get back to me.

People actually do that, every single day.

Answer this one question --

Which uses more energy to recharge a battery from 50% SoC: Recharging at C/5 or recharging at C/20?

If you still don't get it, answer this one --

Which is more amp-hours in a battery: A 100% SoC battery charged at C/5, or a 100% SoC battery charged at C/20?
 

tallgirl

Senior Member
Location
Great White North
Occupation
Controls Systems firmware engineer
tallgirl:
You extol Ah capacity compared to Wh capacity of a battery, though the Wh rating is more accurate than Ah rating and more modern.
One disadvantage of Ah rating is voltage rating also need to be specified: Same Ah rating may have different Wh ratings.

Batteries aren't sold, typically, with watt-hour ratings. Smaller batteries, such as ones for consumer devices (laptops, tablets, phones) may have a watt-hour rating, but a L-16 6 volt deep cycle battery is listed as having a set of amp-hour capacities at a variety of rates of discharge.
 

tallgirl

Senior Member
Location
Great White North
Occupation
Controls Systems firmware engineer
Except in EV usages?

Energy capacity, even in EVs, is entirely dependent on the discharge cycle as well as temperature.

As I mentioned, I have an EV. In the summer the charge is limited by high temperatures (ambient temperature here is often over 40C). In the winter the charge is limited by the low temperatures (ambient temperature here does get below -10C).
 

tallgirl

Senior Member
Location
Great White North
Occupation
Controls Systems firmware engineer
If you'd like to take a look at the complexities that are storage capacity, here's the data sheet from a Rolls S-460 6 volt L-16 format battery, like what is commonly found in off-grid and battery backed PV systems --

http://rollsbattery.com/public/specsheets/S-460.pdf

Pay careful attention to the capacities that are listed in the bottom half of the first page.
 

Besoeker

Senior Member
Location
UK
The most accurate statement you can make is that it requires some amount of energy to store some number of amp-hours of charge which can then produce some number of watt-hours worth of energy under some discharge-cycle-specific conditions.
Accurate?
Ah and Wh are quite different.
You could take one Plant? cell that would have more Ah capacity that a string of two dozen Yuasa Yucel VRLA units in series.
The VRLA string may be lower Ah but have very much greater energy storage.
And the purpose of batteries, whether primary or secondary, is to store energy to be on tap when you need it.
Energy. That's what it's about. That's the nitty gritty.

Yes, I am aware that the specific capacity is depends on discharge rate, ambient temperature, state of charge....
Mostly, in my experience, VRLA nominal capacity is specified at a 20 hour discharge rate at 30C. For Ah, energy density per volume, energy density per unit weight.
We do UPS systems. Generally for public buildings, cinemas, opera houses, posh hotels.....stuff like that.
The usual specified autonomy is three hours. Clearly, the batteries need to be selected for such duty.

Moving on...
From your extensive knowledge of batteries, which I don't in any way dispute, I have something you might be able to help me with.

This came into my email today from one of my guys who had been on a preventive maintenance visit:

From what I could see, there were 19-off SAFT SLP12 Batteries ? the nominal output from the unit was 24V dc.
The system requirement is 3 hours following a boost charge, with a load of 30A at 24Vdc (although I think the maintained load is nearer to 16A now).

The batteries are housed in a panel, which has dimensions 480mm wide x 540mm deep x 650mm high.

Please let me know your thoughts on our providing them with replacement batteries and if you need any more info in the meantime.....

The SAFT SLP12 is obsolete.
Can you suggest an alternative product/arrangement that would fulfil the performance requirements?
 

tallgirl

Senior Member
Location
Great White North
Occupation
Controls Systems firmware engineer
Accurate?
Ah and Wh are quite different.

Yes, they are very different.

The issue that I encounter with clients and others in the PV business has to do with the variability of available energy with respect to a given amp-hour capacity. My approach with clients is to get them to focus on amp-hours and the way in which amp-hour capacity is affected by demand, and temperature to a lesser extent.

For PV folks, the voltage is whatever the nominal voltage is -- I have a 48 volt system, which is common for grid-interactive battery backed systems (most of my grid-interactive clients have 48 volt systems as well), but I have plenty of clients with 24 volt systems. So, I have about 520Ah @ 48 volts nominal. I could do quick math and come up with about 25kWh. For spring and fall, that's a good number. One thing I focused on with my company's software was temperature compensating capacity. Right now the temperature compensated SoC is 90%, so I have less than if it were reasonable outside, instead of cold and miserable.

If I was working with someone who didn't want to have a protracted conversation about batteries, that's how I'd frame it -- something on the order of 5kWh worth of "regular cycling" (80% SoC) capacity and 12.5kWh for "OMG! The grid is down!" (50% SoC) once in a while capacity. I'd make sure a client understood that when the temperature-compensated SoC is 90% (or low 80s, as it was the other day), that they DO NOT have a 25kWh (or whatever) bank. In your line of business -- large scale UPS systems -- I imagine you design for high rates of discharge to low states of charge. I also suspect you don't have to deal with as much by way of temperature. I see batteries in parts of the world (I have clients around the world) that are close to freezing.

You could take one Plant? cell that would have more Ah capacity that a string of two dozen Yuasa Yucel VRLA units in series.
The VRLA string may be lower Ah but have very much greater energy storage.

Right, and if you have a 48 volt nominal system, or a 24 volt system, or even a 36 volt system, you have to have those amp-hours with the correct working voltages. "What are the volts?" is a very common question when I shift the focus to amp-hours, but the voltage has to be in the working range for the equipment. In the Great White North I have clients who use 2 volt cells so they can run "26 volt" systems -- 13 2.2 volt cells in series.

Moving on...
From your extensive knowledge of batteries, which I don't in any way dispute, I have something you might be able to help me with.

This came into my email today from one of my guys who had been on a preventive maintenance visit:

The SAFT SLP12 is obsolete.
Can you suggest an alternative product/arrangement that would fulfil the performance requirements?

I have done =some= design work for 8 to 12 hours of standby power, but nothing that short. Typically I work around 48 to 72 hours of autonomy. For on-grid folks, 24 to 36 tends to be the practical limit.

From a quick visit to Google, that looks to be a Lithium battery, which is outside what I've worked with which is FLA and VRLA. Anything I'd suggest would be much too large, much to heavy (I suspect) and involve lead.
 

Besoeker

Senior Member
Location
UK
Yes, they are very different.

The issue that I encounter with clients and others in the PV business has to do with the variability of available energy with respect to a given amp-hour capacity. My approach with clients is to get them to focus on amp-hours and the way in which amp-hour capacity is affected by demand, and temperature to a lesser extent.
But the bottom line for a battery system, it's raison d'?tre, is to store energy.
The number, arrangement, and type of cells will be determined by the application and the required energy to be stored.
That's it.
 

tallgirl

Senior Member
Location
Great White North
Occupation
Controls Systems firmware engineer
But the bottom line for a battery system, it's raison d'?tre, is to store energy.
The number, arrangement, and type of cells will be determined by the application and the required energy to be stored.
That's it.

The purpose of a table is to put things on top of. Not knowing how that's going to work out makes that a rather useless table.
 

Besoeker

Senior Member
Location
UK
The purpose of a table is to put things on top of. Not knowing how that's going to work out makes that a rather useless table.
Quite. The log tables I used at secondary school were superseded by slide rules, then electronic calculators.....now I mostly use a computer.
 

cowboyjwc

Moderator
Staff member
Location
Simi Valley, CA
The thing sounds like they are just sepreating the hydrogen from the water and making a fuel with it. The problem with that is the amount of electricity it takes for this process far exceeds the output of hydrogen.

Had a company here in town that was working on a process to seperate it so that they could sell it to the military so that they would always have a fuel source as long as they had water.

1. They blew up the building they were in.
2. They blew up the building they moved to and killed one of the sons.
3. They blew up the building they moved to again and maimed one of the sons and hurt one of the workers.

Not a very stable product to work with. Thats why the military made a bomb with it.
 

mivey

Senior Member
How about when you charge the battery using amps without volts you get back to me.
People actually do that, every single day.

Answer this one question --

Which uses more energy to recharge a battery from 50% SoC: Recharging at C/5 or recharging at C/20?

If you still don't get it, answer this one --

Which is more amp-hours in a battery: A 100% SoC battery charged at C/5, or a 100% SoC battery charged at C/20?

You only addressed one of my arguments. The rest is you re-living some argument that you had in the past that you just can't seem to get over.

Perhaps you should hunt that person down and discuss battery charge rates, charging inefficiencies, and whatever else it is you need so you can put that behind you. We know how batteries are charged and discharged and how they work so there is no reason to pretend anyone is arguing about it. No one is saying watt-hour counting is better at amp-hour counting than amp-hour counting. But neither can amp-hour counting replace watt-hour counting. They each serve a useful function in a battery management system.

Points in dispute are the accuracy of amp-hour counting alone and the importance of energy and power in battery applications. It also seems that you have recently presented the idea that you can exchange electrical energy using amps without volts.

In case you ever to decide to address just my arguments, I'll provide a current summary of my points so far:

1) Amp-hour counting and watt-hour counting both have to be supplemented to be accurate except for some specific conditions.

2) Energy storage capacity of a battery is important. The fact that we use watt-hour calculations for power loads supports that argument.

3) Both amp-hour and watt-hour operations provide useful functions.

4) Adding and subtracting battery energy can be and is done as part of some battery management systems.

5) Specific power is a function of battery chemistry and packaging.


As for the amps but no volts, we have one more point to cover:

We use the battery as an energy storage device and we exchange electrical energy and chemical energy. Since an electric current is not a flow of energy, you can't charge a battery using amps without also using volts. It was not done today or yesterday and it won't be done tomorrow.

The electrical energy flows to the battery in the electromagnetic field. The electric field and magnetic field are related to the voltage and current and it is the cross product of these fields that tells us the energy flow. Leave out one or the other and you have no exchange of electrical energy.

When charging or discharging a battery, we are moving charges between two points and that requires work (no super-conducting wire/battery stuff if you don't mind). The work per unit charge is the voltage difference. We measure the work needed by the potential difference. For our simple case, we can say voltage and potential are the same.

So, charging a battery using amps without volts has NOT been done every day: NEVER HAPPENED.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
So, charging a battery using amps without volts has NOT been done every day: NEVER HAPPENED.
Quite true. :)
On the other hand, you can charge a battery by applying a regulated voltage or by applying a regulated current. Alone or in various combinations, these two modes make up the charging profile of any battery charging system. What complex charge profile is "best" will depend heavily on the battery chemistry and construction and on the use case.
So, being generous with the language, one could say that you are charging the battery using "only volts" or using "only amps" if you are regulating only the one or the other. But that would be confusing. :)

BBOR0BB
 

tallgirl

Senior Member
Location
Great White North
Occupation
Controls Systems firmware engineer
Quite true. :)
On the other hand, you can charge a battery by applying a regulated voltage or by applying a regulated current. Alone or in various combinations, these two modes make up the charging profile of any battery charging system. What complex charge profile is "best" will depend heavily on the battery chemistry and construction and on the use case.
So, being generous with the language, one could say that you are charging the battery using "only volts" or using "only amps" if you are regulating only the one or the other. But that would be confusing. :)

BBOR0BB

It's not at all confusing. Constant-voltage and constant-current (or near constant-current) charging profiles exist and are widely used, particularly in PV (that's this forum ...) systems for 2 or 3 (or 4, if you count equalization) stage charge cycles.

My remark about not using volts is that voltage is reasonably irrelevant once the nominal voltage of the system is established. A 48 volt nominal system isn't some other nominal voltage. Fixating on power, as though energy = power * time is relevant to battery charging and discharging, is a mistake and I'm more than willing to allow people to make as many mistakes as they want.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
It's not at all confusing. Constant-voltage and constant-current (or near constant-current) charging profiles exist and are widely used, particularly in PV (that's this forum ...) systems for 2 or 3 (or 4, if you count equalization) stage charge cycles.

What I was saying was that calling either a constant-voltage stage or a single stage profile with constant-voltage a "volts only" profile and calling a constant-current stage an "amps only" stage would be confusing. My example is the earlier comment that "you cannot charge anything with amps only, you have to have volts too." :)
 

Besoeker

Senior Member
Location
UK
My remark about not using volts is that voltage is reasonably irrelevant once the nominal voltage of the system is established.
Established, yes. Irrelevant, no.

I'm more than willing to allow people to make as many mistakes as they want.
That same latitude is being extended to you.
The bottom line for a battery system, it's raison d'?tre, is to store energy. Volts times Amps times time.

That's all.
 
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