esaj

Battery voltages and capacities etc (Split from NEW Gotway Monster 100V

6 posts in this topic

Thank you!!

Some questions:

1. Won't you have a cell voltage of ~4.2V for 24 serial cells to make 100V? (Same for 20 to get to 84V, or 16 for 67V) You're using 3.7V cells - I guess there's both?

2. I guess this is more a physics question... as it comes down to this:

53 minutes ago, esaj said:

When cells are put in series, the voltage goes up, but the capacity does not increase.

WHY?????????????:pooping:

Makes sense you'd measure battery capacity in Ah (C if you like) so it's just the amount of charge. Which, one would think, is an absolute quantity (???). So why do the electrons get stuck in there if you put the cells in a series?

This is what irks me, why you can't just get the battery "capacity" (better to say energy aka Wh [Joule if you are a better person:P] number) when you know only the number of cells, and what cells exactly they are (like 2.9 Ah,4.2 V). How can it conceptually be explained that the battery configuration matters at all?

[Maybe your really nice post and this discussion should be split into a separate "battery math" or so thread, it's a bit hidden in here and probably getting too offtopic]

 

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Posted (edited)

1 hour ago, meepmeepmayer said:

Thank you!!

Some questions:

1. Won't you have a cell voltage of ~4.2V for 24 serial cells to make 100V? (Same for 20 to get to 84V, or 16 for 67V) You're using 3.7V cells - I guess there's both?

Yes, when using the maximum voltage. That's what the wheel voltages are usually reported in. 4.2V is the (typical) maximum voltage a lithium-based cell should be charged to, although the actual "precise" value might differ somewhat between different chemistries, as not all Lithium-Ions are the same, although they're all sold generally under the "umbrella" term of Li-Ion. For more details, see for example http://batteryuniversity.com/learn/article/types_of_lithium_ion . You'll notice that there are slightly different values for the nominal (3.6...3.8V) and maximum voltages (4.2...4.3V), with the exception of Lithium Iron Phosphate (LiFePo4) and Lithium Titanate (Li4Ti5O12), that both use much lower voltages. I don't know much of the actual chemistry or physics to tell you why that is, though :P

But, as the above calculations show, for battery capacity (watthour) calculations, it seems that (at least with the wheels) the nominal voltage is used instead of the maximum. Even then, they're theoretical, as you cannot squeeze the battery all the way to "empty": if the voltage drops below 2.5V (for the kinds with around 3.7V nominal voltage), the cell may be damaged permanently. My best guess is that repeated experimentation for the actual watthours you can get out of the pack is pretty close to what you get by calculating using the nominal voltage and the cell capacity in amphours?

 

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2. I guess this is more a physics question... as it comes down to this:

WHY?????????????:pooping:

Makes sense you'd measure battery capacity in Ah (C if you like) so it's just the amount of charge. Which, one would think, is an absolute quantity (???). So why do the electrons get stuck in there if you put the cells in a series?

The electrons don't get "stuck" in there (current is the flow of electrons, or more specifically electric charge, that's usually carried by moving electrons). I don't think of electronics circuits in the sense of electrons moving or such, I'm looking at voltages between two points (nodes, usually one or the other being the "common ground"), currents flowing through different parts, power dissipations to make sure the components don't break... I don't go to that "low level" as to think of the actual molecular/atomic/whatever efffects... ;) Um... probably it's better to give links to some relevant pieces than try to explain it myself, as I'd just get the details wrong :P :D  I was just taught something along the lines that because of conservation of energy "the voltage over two parallel components or sub-circuits must be the same" and "the current flowing through components in series or sum of entering and exiting the same point must be the same", and "just believe that" :whistling:

https://electronics.stackexchange.com/questions/67478/why-do-batteries-in-series-add-up-voltage

https://www.quora.com/Why-do-cells-give-more-current-when-connected-in-parallel-and-more-voltage-when-connected-in-series

https://www.lifewire.com/basic-circuit-laws-818993

If you really want to dig deep, you can probably find tons of articles about electrochemical reactions (in batteries and otherwise) and galvanic cells and whatnot ;)  The battery university -articles are a good place to start too: http://batteryuniversity.com/learn/   Although you might get stuck reading those for hours (at least I did :D)

 

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This is what irks me, why you can't just get the battery "capacity" (better to say energy aka Wh [Joule if you are a better person:P] number) when you know only the number of cells, and what cells exactly they are (like 2.9 Ah,4.2 V).

EDIT: The total energy amount in the packs is the same, regardless of the configuration, if that's what you meant:

64 cells * 3.7V per cell * 3.5Ah per cell * 1 pack = 828,8Wh  (64S1P)

16 cells per pack * 3.7V per cell * 3.5Ah per cell * 4 packs = 828,8Wh  (16S4P)

1 cell per pack * 3.7V per cell * 3.5Ah per cell * 64 packs =  828,8Wh  (1S64P)

 

Well, you could say that you get "about" 3.7V (the nominal voltage) * 2.9Ah = 10.73Wh from one single cell when it's discharged from about 4.2V to around 2.5...3V. In reality, if you pull lots of current out of it, you get less (rest of the energy is wasted on heating the cell because it has internal resistance), or more if you discharge it more slowly... 

But sure, there is some absolute (finite) amount of energy in the battery at all times, but how deep do you want to go, how much energy would be released if all the atoms int he battery could be split? (I have no idea) ;)

I think someone somewhere (don't remember where I read it) put it eloquently by saying something along the lines of "Electronics and engineering in general isn't that much about 'absolute' values rather than ballpark-figures, varying tolerances, uncertainties and error margins, and dealing with them."  (Of course in somethings really, really precise values are needed, but you can't take all the real world effects into account, can you? ;))

 

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How can it conceptually be explained that the battery configuration matters at all?

If you think of conservation of energy, the energy amount per cell is about the same, so in that sense, the battery configuration doesn't matter; energy doesn't disappear, it just changes form (like from chemically stored energy in batteries to electricity to magnetic field to movement in motor, or back to the battery through regenerative braking, ie. back-EMF causing a backwards current flow, all the while energy is  also "wasted", but not actually lost, as heat in every part of the way).

But water analogies are pretty generally used in explaining electricity/electronics (like voltage is pressure, current is water flow and resistance restricts current flow by making the pipe smaller etc.), I think the hydraulic analogy in one of the electronics.stackexchange -answers puts it in relatively easy to understand terms:

A cell or a battery is essentially a charge "pump". Now, to help form an intuition for the answer to your question, fall back to the hydraulic analogy.

Two water pumps in parallel can produce twice the water flow of one (ideally).

Two water pumps in series can produce twice the pressure (or head) of one (ideally).

 

 

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[Maybe your really nice post and this discussion should be split into a separate "battery math" or so thread, it's a bit hidden in here and probably getting too offtopic]

Yeah, probably :P 

Edited by esaj
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Thank you again! I know how much work goes into such long posts, with all the "wait I need to add this/change that/I should go to bed" later edits, etc, so I appreciate it very much.

So you use maximal voltage to get the battery configuration/serial cells... makes sense, the manufacturer must use components that work in the worst case (at least theoretically they should...*cough*) and higher numbers sound better anyways.

And you use the the nominal voltage to get the capacity energy... because nominal means you can't constantly get more than that out of the batteries. Or some other reason.

24 minutes ago, esaj said:

I don't think of electronics circuits in the sense of electrons moving or such

...

I don't go to that "low level" as to think of the actual molecular/atomic/whatever efffects... ;) 

Actually, I tried to go as high level as possible. "Why is the overall energy content depending on the cell configuration, it should only depend on number and type of the cells?" was the question. Sounds plausible, doesn't it?  Answer (by physics): F**K YOU DEAL WITH IT *storms out of the room*

The "low level" (just trying to make it as simple as possible and stay with simple units) is just an attempt at understanding why it does not work like this.

--

Well, I'll read up a bit, maybe something comes out of that.

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1 minute ago, meepmeepmayer said:

Thank you again! I know how much work goes into such long posts, with all the "wait I need to add this/change that/I should go to bed" later edits, etc, so I appreciate it very much.

Yeah, I should go to bed, it's 3:30AM here ;)

1 minute ago, meepmeepmayer said:

So you use maximal voltage to get the battery configuration/serial cells... makes sense, the manufacturer must use components that work in the worst case (at least theoretically they should...*cough*) and higher numbers sound better anyways.

You can get the amount of cells in series typically by dividing the voltage the manufacturer reports with 4.2V, but that's just because they seem to use the maximum voltage in advertising their wheels  (higher number sounds better in marketing? ;)). And yes, for measuring the components, the "worst case" scenarios should be used, so probably maximum voltage there too (in most cases, at least).

 

1 minute ago, meepmeepmayer said:

And you use the the nominal voltage to get the capacity energy... because nominal means you can't constantly get more than that out of the batteries. Or some other reason.

Something like that... or maybe the average voltage over a steady discharge is around the nominal voltage -value. I have absolutely no idea how the nominal value for the cells is arrived at, just that it's typically said to be 3.7V for (most) lithium-ions.

 

1 minute ago, meepmeepmayer said:

Actually, I tried to go as high level as possible. "Why is the overall energy content depending on the cell configuration, it should only depend on number and type of the cells?" was the question. Sounds plausible, doesn't it?  Answer (by physics): F**K YOU DEAL WITH IT *storms out of the room*

The "low level" (just trying to make it as simple as possible and stay with simple units) is just an attempt at understanding why it does not work like this.

I added this edit to the above post, probably after you were already reading it so you didn't see it... I probably understood your question wrong from the get-go :D

"

EDIT: The total energy amount in the packs is the same, regardless of the configuration, if that's what you meant:

64 cells * 3.7V per cell * 3.5Ah per cell * 1 pack = 828,8Wh  (64S1P)

16 cells per pack * 3.7V per cell * 3.5Ah per cell * 4 packs = 828,8Wh  (16S4P)

1 cell per pack * 3.7V per cell * 3.5Ah per cell * 64 packs =  828,8Wh  (1S64P)

"

 

1 minute ago, meepmeepmayer said:

Well, I'll read up a bit, maybe something comes out of that.

There's so much information available in the internet these days, that you'll learn a lot more and a lot faster than I could ever try explaining :D  And you should always remember, that I'm just a hobbyist, not a professional, when it comes to batteries and electronics. Most of my work experience and lots of hobby background is in the software engineering-field, which is very different than fields concentrating in the "real world" ;)

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So.. everything is good then?

Hmm...

Now I'm confused why I was confused. Maybe mixing up capacity as in Ah with capacity as in energy (Wh)?

I'll read this again tomorrow and do some concrete examples.

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