# [EVDL] Calb at 3.6V and T-Sky at 3.8V final charge - why the difference?



## EVDL List (Jul 27, 2007)

Hi Folk's,

If CALB and T-sky (Winston) batteries are the same LiFePO4 composition, why is there a 3.6V limit on CALB and a 3.8V limit on T-Sky? (it's easier for the charger to reduce the current to 1A at 3.8V per cell in my 36 cell Ghia)

best regards,
mark 

Date: Thu, 19 Jan 2012 06:48:38 -0800 (PST)
From: tomw <[email protected]>
Subject: Re: [EVDL] top and bottom "loading" of LiFePO4 cells?
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii

David, most of that agrees with notes I took from the CMU professor video: 
Neutral Li is intercalated into the cathode after discharge. It is ionized
during charging and diffuses to the anode. After charging neutral Li is
intercalated into the anode. The phase change, or redox reaction, of
LiFePO4 to FePO4 occurs at 3.4V, so when fully charged there is 3.4V ("rest
voltage" of fully charged cell) driving Li back to the cathode. 

According to my notes, you only get LiC4 formed at the electrodes at
temperatures exceeding 50 - 60 C, which breaks down the electrolyte and
forms LiC4, LiCFx, and others on the electrodes resulting in loss of
capacity due to less volume for Li+ in the films and more difficulty for it
to get in/out. This results in larger Rs and more cell heating during
discharge.

He also said that using thinner films reduces the Warberg effect, or
concentration polarization of charge, due to diffusion-limited transport of
Li+ out of the anode during discharge, which results in lower capacity at
higher discharge rates. I assume this is the trade-off you see in high
specific power/high specific energy LiFePO4 cells - higher specific power
using thinner films, higher specific energy using thicker films. I think
cylindrical cells give more surface area per unit volume which also helps.


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## EVDL List (Jul 27, 2007)

My guess is that it is a way to claim more energy storage than they
should have put into them for long term life. In reality, however, it
probably isn't much energy difference if my measurements are anywhere
near close. If charging is properly cut off when the current drops to
0.05C it won't be much. Not worth pushing the cells that high and
definitely not worth balancing at that voltage on every charge.

TS used to say 4.00V. I charged a TS-LFP40AHA from that era to 4.00V
and terminated the charge when the current dropped to 0.015C. After
two weeks the OCV was still 3.533V so the cell was overcharged. If it
wasn't overcharged the OCV would have been at or just under 3.4V.

On Fri, Jan 27, 2012 at 11:18 AM, Mark Hanson <[email protected]> wr=
ote:
>
> Hi Folk's,
>
> If CALB and T-sky (Winston) batteries are the same LiFePO4 composition, w=
hy is there a 3.6V limit on CALB and a 3.8V limit on T-Sky? (it's easier fo=
r the charger to reduce the current to 1A at 3.8V per cell in my 36 cell Gh=
ia)
>
> best regards,
> mark
>
> Date: Thu, 19 Jan 2012 06:48:38 -0800 (PST)
> From: tomw <[email protected]>
> Subject: Re: [EVDL] top and bottom "loading" of LiFePO4 cells?
> To: [email protected]
> Message-ID: <[email protected]>
> Content-Type: text/plain; charset=3Dus-ascii
>
> David, most of that agrees with notes I took from the CMU professor video:
> Neutral Li is intercalated into the cathode after discharge. It is ionized
> during charging and diffuses to the anode. After charging neutral Li is
> intercalated into the anode. The phase change, or redox reaction, of
> LiFePO4 to FePO4 occurs at 3.4V, so when fully charged there is 3.4V ("re=
st
> voltage" of fully charged cell) driving Li back to the cathode.
>
> According to my notes, you only get LiC4 formed at the electrodes at
> temperatures exceeding 50 - 60 C, which breaks down the electrolyte and
> forms LiC4, LiCFx, and others on the electrodes resulting in loss of
> capacity due to less volume for Li+ in the films and more difficulty for =
it
> to get in/out. This results in larger Rs and more cell heating during
> discharge.
>
> He also said that using thinner films reduces the Warberg effect, or
> concentration polarization of charge, due to diffusion-limited transport =
of
> Li+ out of the anode during discharge, which results in lower capacity at
> higher discharge rates. I assume this is the trade-off you see in high
> specific power/high specific energy LiFePO4 cells - higher specific power
> using thinner films, higher specific energy using thicker films. I think
> cylindrical cells give more surface area per unit volume which also helps.
>
>
> -------------- next part --------------
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> | Please take those discussions elsewhere. Thanks.
> |
> | REPLYING: address your message to [email protected] only.
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-- =

David D. Nelson
http://evalbum.com/1328
http://2003gizmo.blogspot.com

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## EVDL List (Jul 27, 2007)

While the idea to charge to 4V seems to be ridiculous in the respect
of capacity this OCV example shows exactly why it is a good idea to go
there.

The high SOC should be only visited. Forcing the cell there for
extended period of time will shorten the life. Speed of death depends
on temperature too. So no cooking !



What do we mean by saying 'overcharging' ?

I'd use that when the 100% SOC on nominal has been reached and more
negative effects are taking place than positive ones. With original TS
cells that was not the case. Capacity can be gained by these means
without any noticeable (affecting usage) negative effects. It'll take
still some years to prove same effects on later production.

This is the logic why some manufacturers have claimed 'no BMS'. Which
would be partially OK if there was no other failure mechanisms on the
cells and systems.

-akkuJukka

http://www.google.com/profiles/jarviju#about



2012/1/28 David Nelson <[email protected]>:
> My guess is that it is a way to claim more energy storage than they
> should have put into them for long term life. In reality, however, it
> probably isn't much energy difference if my measurements are anywhere
> near close. If charging is properly cut off when the current drops to
> 0.05C it won't be much. Not worth pushing the cells that high and
> definitely not worth balancing at that voltage on every charge.
>
> TS used to say 4.00V. I charged a TS-LFP40AHA from that era to 4.00V
> and terminated the charge when the current dropped to 0.015C. After
> two weeks the OCV was still 3.533V so the cell was overcharged. If it
> wasn't overcharged the OCV would have been at or just under 3.4V.
>
> On Fri, Jan 27, 2012 at 11:18 AM, Mark Hanson <[email protected]> =


> wrote:
> >>
> >> Hi Folk's,
> >>
> ...


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## EVDL List (Jul 27, 2007)

Hi Jukka,

I'm trying to grasp what is going on because I'm not getting it yet.
Can you help me out a little? questions below

2012/1/27 Jukka J=E4rvinen <[email protected]>:
> While the idea to charge to 4V seems to be ridiculous in the respect
> of capacity this OCV example shows exactly why it is a good idea to go
> there.

I'm not seeing how this shows why it is a good idea to go to 4V. To me
I see the exact opposite. What am I missing?

> The high SOC should be only visited. Forcing the cell there for
> extended period of time will shorten the life. Speed of death depends
> on temperature too. So no cooking !

When you say the high SOC should only be visited, how often and what
should the ending charge current be when visiting 4V? Obviously we
don't want to cook anything so I'm sure the current is lower than
0.05C.

> What do we mean by saying 'overcharging' ?
>
> I'd use that when the 100% SOC on nominal has been reached and more
> negative effects are taking place than positive ones. With original TS
> cells that was not the case. Capacity can be gained by these means
> without any noticeable (affecting usage) negative effects. It'll take
> still some years to prove same effects on later production.

Ok, maybe I'm getting what you are saying. Are you saying that there
are negative effects from not going to 4V (with the older TS-LFP
cells) and that those effects are loss in capacity and/or greater
voltage sag? A full cell is when 100% of the Li is in the anode
graphite structure and there are no more Li ions left in the cathode.
I do not know if there would be any Li+ left in the electrolyte.
Trying to push more charge into the cell after this point would be
overcharging the cell. Initially this would go into the capacitance of
the cell but after that I don't know, Li plating? Any idea how much
capacitance a TS-LFP100AHA cell might have at 4V?

Thanks,

> This is the logic why some manufacturers have claimed 'no BMS'. Which
> would be partially OK if there was no other failure mechanisms on the
> cells and systems.
>
> -akkuJukka
>
> http://www.google.com/profiles/jarviju#about
>
>
>
> 2012/1/28 David Nelson <[email protected]>:
>> My guess is that it is a way to claim more energy storage than they
>> should have put into them for long term life. In reality, however, it
>> probably isn't much energy difference if my measurements are anywhere
>> near close. If charging is properly cut off when the current drops to
>> 0.05C it won't be much. Not worth pushing the cells that high and
>> definitely not worth balancing at that voltage on every charge.
>>
>> TS used to say 4.00V. I charged a TS-LFP40AHA from that era to 4.00V
>> and terminated the charge when the current dropped to 0.015C. After
>> two weeks the OCV was still 3.533V so the cell was overcharged. If it
>> wasn't overcharged the OCV would have been at or just under 3.4V.
>>
>> On Fri, Jan 27, 2012 at 11:18 AM, Mark Hanson <[email protected]>=


> wrote:
> >>>
> >>> Hi Folk's,
> >>>
> ...


----------



## EVDL List (Jul 27, 2007)

Higher OCV after full charge shows you've got nearly all Li-ions out.
It requires more work to get the last ones moving. They're not jumping
back without small discharge pulse. Usually BMS can do that just by
staying powered for some time



> David wrote: "When you say the high SOC should only be visited, how
> often and what should the ending charge current be when visiting 4V?"
> 
> Just to tip the voltage there with low enough current. BMS should
> ...


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