# 3.7v vs 3.2v Lithium Batteries?



## frodus (Apr 12, 2008)

3.7V is Lithium Polymer
3.2V is Lithium Iron Phosphate

Two different chemistries with different requirements for BMS and charging. I don't know of any LiPo prismatics.... most of the one's I've seen are pouch cells.

Where did you find 30Ah lipo that is available to the consumer?


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## ndplume (May 31, 2010)

Travis,
I saw them at this link
http://www.electriccarinternational.com/Lithium-Pouch-Batteries.php

I haven't purchased anything from them, but just saw them there on a search. Anyone else purchased these and tested them?

Mike


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## dougingraham (Jul 26, 2011)

ndplume said:


> Travis,
> I saw them at this link
> http://www.electriccarinternational.com/Lithium-Pouch-Batteries.php
> 
> ...


Too expensive. Should be less than $1.50 per AH to be close to competitive.

At that price the only people who should be interested are racers where cost is not nearly so important.


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## frodus (Apr 12, 2008)

For lipo, it should be well under $2/Ah. The 30Ah 3C continuous cells (10C pulse) are $3/Ah and the 30Ah 20C cells are $4.40/Ah.

I wouldn't use the 3C/10C batteries to race though, IMHO.

Would be even better if they showed discharge graphs to see what the voltage drop is under load.


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## LithiumaniacsEVRacing (Oct 9, 2010)

The Chinese producer also produces a 30C Lipo cell with very large tabs. Not sure about these cells, but atleast they are featured products directly from the producer, not rejects. I might buy some just to test.


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## Semper Vivus (Apr 13, 2011)

frodus said:


> 3.7V is Lithium Polymer
> 3.2V is Lithium Iron Phosphate
> 
> Two different chemistries with different requirements for BMS and charging. I don't know of any LiPo prismatics.... most of the one's I've seen are pouch cells.
> ...


Hello,
there is some mistake with your explanation.
It's right that 3.2V is the nominal voltage of the Lithium-Ion Iron Phosphate chemistry. But "Lithium Polymer" is no cell chemistry. It is the construction method inside a cell. 

Lithium-Ion Polymer:
With Lithium-Ion Polymer you have a solid Polymer between the cathode and the anode as a the separator. This material acts also as the electrolyte. Because of this construction you are able to create a packaging for these three layers (anode, polymer, cathode) that just consists of a foil. Because of that a Lithium-Ion Polymer cell has a higher energy density as a traditional cell.

Lithium-Ion (traditional)
Traditional cells have a slim porous separator in combination with an fluid electrolyte between anode and cathode. This requires the heavy metal case to hold all the things together.

Lithium-Ion cell chemistries:
The difference between the chemistries is the material with that the cathode (aluminium foil, positive terminal of the cell) is coated.
There are four commonly used cell chemistries:
- iron phosphate as you mentioned
- manganese
- cobalt
- nickel-cobalt-manganese
For the first three ones from up till down, the safety decreases and the energy density increases. Because of this for example cobalt is used in cell phones, manganese in power tools.
The nickel-cobalt-manganese is a mix of three components to join the advantages of different components.

The chemistry is independent from the cell construction. So you can get polymer cells, cylindrical cells and prismatic cells with all of these chemistries.

Regards,
Tom


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## frodus (Apr 12, 2008)

Tomas,

I'm well aware of what LiPo means as well as chemistries and the differences (I have to, it's part of my business). I test batteries (and have built packs) for people and I find it's easier to keep explanations simple rather than get into a huge explanation of construction type, chemistry etc. I try to keep things related to what is actually out there commercially available and relate to that. *Since he said 3.7V and Pouch, I felt it was safe to assume it's a lithium polymer.*

You are correct, LiPo is the construction type, but within that type, there are only a couple of commercial options of chemistry, LiCoO2 or LiMn2O4, both at ~3.7V. It is true however that there are other Lithium Ion types that are not Lithium Polymer (cylindrical non-polymer LiMn and LiCo) that have a voltage of ~3.7V, but my statements are still true with respect to Lithium Polymer. *I just didn't include non-polymer because he mentioned 30Ah 3.7V pouch cells.*

http://en.wikipedia.org/wiki/Lithium_ion_polymer_battery

Notice that my "hunch" about the 3.7V batteries was indeed correct.... the link he gave was Lithium Polymer, pouch cells at 3.7V nom.

Go read read the first post from ndplume, and you can see where I got my assumptions.


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## Semper Vivus (Apr 13, 2011)

frodus said:


> Tomas,
> 
> I'm well aware of what LiPo means as well as chemistries and the differences (I have to, it's part of my business). I test batteries (and have built packs) for people and I find it's easier to keep explanations simple rather than get into a huge explanation of construction type, chemistry etc. I try to keep things related to what is actually out there commercially available and relate to that. *Since he said 3.7V and Pouch, I felt it was safe to assume it's a lithium polymer.*
> 
> ...


Hello Frodus,
with the context of the first post of ndplume it is right.
When I'm writing an answer I always assume the other readers are blinkered as I am...probably the fewest are .

Regards,
Tom


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## frodus (Apr 12, 2008)

It's good knowledge to share. Mainly that there are other chemistries in non-polymer construction that offer ~3.7Vnom.


I'm a little confused about the LiFePo4 cell from ABF on that link that has 3.7V nominal.... that to me is a little "off".... seems like a misprint.


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## ndplume (May 31, 2010)

Hey Tom and Travis, 
Thanks for sharing the additional insight, I'm new to the battery landscape and wasn't aware that the construction was actually different. Its likely that many people are in my shoes so you've helped enlighten them.

Does the construction explain why LiPo are NOT made into "Prismatics"? Is it that the solid polymer can't be wound up like the LiFeOn separator? 

All said and done, I'm back to the principal that fewer connections are fewer points of failure/maintenance. So prismatics have fewer connections than pouch cells. And a higher voltage cells would require less cells = less connections too. So the ideal solution would be a high cell voltage, prismatic cell. 

Since a 3.7V cell would reduce cell count by ~15%, then they could be 15% higher in price and have the same effective pack price. You would have fewer connections as langinappe!

Looks like at this point in time, there aren't cost effective 3.7V cells for DIYers?

Thanks again
Mike


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## Semper Vivus (Apr 13, 2011)

ndplume said:


> Does the construction explain why LiPo are NOT made into "Prismatics"? Is it that the solid polymer can't be wound up like the LiFeOn separator?


I assume that is one reason. In fact I've never seen a LiPo cell or pouch cell (just another word for LiPo) that has a different construction as the commonly known ones.

But I'm not sure if we are both talking about the same "prismatics".
A prismatic cell has the same basic construction as a cylindrical cell, just twisted to a prismatic package instead of a round package (metal case, anode, cathode, separator, electrolyte).
Maybe you mean batteries like the big yellow ones from Thunder Sky?
In fact that are also LiPos, just packaged in a plastic case (several in parallel).



ndplume said:


> All said and done, I'm back to the principal that fewer connections are fewer points of failure/maintenance. So prismatics have fewer connections than pouch cells. And a higher voltage cells would require less cells = less connections too. So the ideal solution would be a high cell voltage, prismatic cell.


That's right.
In general you mentioned the higher energy densitiy because of the higher voltage (compared to the LiFePO chemistry with 3,2V nominal).
The other advantage of the higher voltage chemistries is the greater gradient of the charge/discharge curve. This makes SOC determination with the help of the voltage much easier. This for example is important in a hybrid car. There you can't charge up to 90 or 100 percent (because of the regenerative braking). So you won't reach the end of the charging curve where every chemistry has a very great gradient.

That is the reason why the most car manufacturers use higher voltage chemistries (for example nickel-manganese-cobalt) with a prismatic or a polymer construction.



ndplume said:


> Looks like at this point in time, there aren't cost effective 3.7V cells for DIYers?


I don't know big prismatics for DIY car applications, only the LiPos.
Think it's the better safety and the cheaper price of the LiFePO chemistry why manufacturers like Thunder Sky use this technology.

Regards,
Tom


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