# [EVDL] LiFePO4 sensitivity to overcharge



## EVDL List (Jul 27, 2007)

it's been said that lithium can't stand overcharge like lead can. does 
anyone have any hard info on that or is that an urban legend?

how sensitive are they

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## EVDL List (Jul 27, 2007)

They will quickly lose capacity, or they can start a lithium battery fire.

Don't do it. Like at all. People have talked about just making a pack without bothering with the overcharge protection, and it's just not an option. The cost in pack damage is higher than the management system cost. The possibility of fire cannot be measured in cost.

Also be aware that lithium batts have a finite calendar life. Its self-degradation is greatest when they're left at full charge and at warmer temperatures. This may limit the batt's lifespan to a few years even if not used often, but it varies with technology as well as the circumstances they're maintained under.

Danny



> ---- Dan Frederiksen <[email protected]> wrote:
> > it's been said that lithium can't stand overcharge like lead can. does
> > anyone have any hard info on that or is that an urban legend?
> >
> ...


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## EVDL List (Jul 27, 2007)

Here's how it works for "standard" Lithium (LiPo and LiCoO2):

The higher the final charge voltage, the shorter the life span of the
batteries (significantly so). Also, if you charge them too high, it
can ignite the batteries.

I don't know how well that translates over to LiFePO4. My guess is
that the lifespan will still be reduced, but the fire hazard won't be
there because of the higher thermal stability of the iron phosphate.

-Morgan LaMoore



> Dan Frederiksen <[email protected]> wrote:
> > it's been said that lithium can't stand overcharge like lead can. does
> > anyone have any hard info on that or is that an urban legend?
> >
> ...


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## EVDL List (Jul 27, 2007)

> Morgan LaMoore wrote:
> > I don't know how well that translates over to LiFePO4.
> that's just it. LiFePO4 is so different you can pierce it with a metal
> rod and not catch fire. this might translate to other strengths as well.
> ...


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## EVDL List (Jul 27, 2007)

No, I don't. Neither do I feel any motivation to test it.

I will be using a BMS if/when I get a Lithium pack, and I believe that
it's well worth the cost for others, too.

Even if overcharge has little effect on LiFePO4, if you already have a
BMS to protect against other things, you may as well only charge to
manufacturer specifications, too.

-Morgan LaMoore



> Dan Frederiksen <[email protected]> wrote:
> > Morgan LaMoore wrote:
> > > I don't know how well that translates over to LiFePO4.
> > that's just it. LiFePO4 is so different you can pierce it with a metal
> ...


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## EVDL List (Jul 27, 2007)

> Morgan LaMoore wrote:
> > No, I don't. Neither do I feel any motivation to test it.
> >
> > I will be using a BMS if/when I get a Lithium pack, and I believe that
> ...


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## EVDL List (Jul 27, 2007)

> Dan Frederiksen wrote:
> > that's just it. LiFePO4 is so different you can pierce it with a metal
> > rod and not catch fire. this might translate to other strengths as well.
> > if it is genuinely robust to soft overcharge we need to know
> ...


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## EVDL List (Jul 27, 2007)

Lee, we were talking about reduction of lifespan as a result of 
overcharging, specifically for LiFePO4 cells. if they are not sensitive 
then cascade problems will not follow. we need the core issue first

Dan

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## EVDL List (Jul 27, 2007)

Lee Hart kirjoitti:

> 
> It's like the difference between accidentally lighting one loose match, 
> versus accidentally lighting one in a whole box of matches.

Nice analogy Lee  Now imagine that same match box with 8000 matches in 
it.... ouch!

-Jukka

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## EVDL List (Jul 27, 2007)

It is a known trick to over charge LiFePO4 cells. you can do that and 
even "balance" cells by that way. They are sensitive but will not be unsafe.

Usually the problem is with overcharging that there is just no more 
Lions to transfer or no places for them to go. So they might form a 
metallic Lithium on electrode surface. Not good. Also it will desolve 
electrolyte thus making the cell unusable due too high internal resitance.

Slight "over charging" is even preferred in certain cases. To modify the 
cell parameters. But is't tricky stuff. Nothing that you can do with 
small logic chips.

Also... determine the "over" part. Over what ? Rated capacity ? Real 
capacity ? After how many cycles ? on how high average temperature over 
already lived lifetime ? To what voltage ? With what curent ?

These are jsut general questions. not directed to anyone. Just to crack 
open a bit my point of view. Charging cells safely has not been an issue 
for years. Challenge is far greater than that.

I decided not to charge cells evenly to same voltages years ago. I took 
other criterias. And suddenly cells were behaving much better. LiFePO4 
cells are still odd stuff. Not at all similar than LiNiCo...

-Jukka



Dan Frederiksen kirjoitti:


> > Morgan LaMoore wrote:
> >> No, I don't. Neither do I feel any motivation to test it.
> >>
> >> I will be using a BMS if/when I get a Lithium pack, and I believe that
> ...


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## EVDL List (Jul 27, 2007)

Aren't we talking about this because you'd like to run the lithiums
without BMS?

Brian 

-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of Dan Frederiksen
Sent: Wednesday, January 02, 2008 4:17 PM
To: Electric Vehicle Discussion List
Subject: Re: [EVDL] LiFePO4 sensitivity to overcharge

Lee, we were talking about reduction of lifespan as a result of
overcharging, specifically for LiFePO4 cells. if they are not sensitive
then cascade problems will not follow. we need the core issue first

Dan

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## EVDL List (Jul 27, 2007)

if possible yes. obviously.
to know the implications of the choice at hand. to be informed. is that 
such a foreign concept

Gilbert, Brian D (GE Infra, Energy) wrote:
> Aren't we talking about this because you'd like to run the lithiums
> without BMS?
> 

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## EVDL List (Jul 27, 2007)

Well, don't. It'll be an expensive and dangerous waste of time!

The difficulty is not just that you need to sense when to stop charging and shut off. Nor can you put a zener on there to just limit the voltage. Neither of these meet the required charging algorithm. 

The required algorithms I saw are constant current at first (this one is easy, the whole string can be one fixed current) then cell-by-cell they must switch over to constant-voltage (such as a voltage shunt, but zener shunts are not really available in this voltage range with the required accuracy), and sense when the current to that cell drops below a threshold and shut off entirely. That could be done by measuring the current going through a voltage shunt- batt string current minus the cell's shunt current tells you the batt's charging current. That's essential because we can't put a shunt on each batt to measure each batt's current; those shunts would be in series with the pack's discharge path during use. Then you'd "shutoff" that cell by putting the shunt in a constant-current mode equal to the string's current so there's no net battery current. A significant problem with that idea is that any difference in calibration between the shunt path's current shunt and the !
whole-string's current shunt would result in a net charging or discharging current in that shutoff mode. If the string has 5 amps through it and the cell's shunt in "shutoff" mode tries to absorb 5A but only absorbs 4.9A due to a shunt or voltage reference issue in the manager, then the cell's still receiving 100mA of overcharge when it's supposed to be shut off completely..

Danny



> ---- Dan Frederiksen <[email protected]> wrote:
> > if possible yes. obviously.
> > to know the implications of the choice at hand. to be informed. is that
> > such a foreign concept
> ...


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## EVDL List (Jul 27, 2007)

Jukka J=E4rvinen skrev:
> It is a known trick to over charge LiFePO4 cells. you can do that and =

> even "balance" cells by that way. They are sensitive but will not be unsa=
fe.
>
> Usually the problem is with overcharging that there is just no more =

> Lions to transfer or no places for them to go. So they might form a =

> metallic Lithium on electrode surface. Not good. Also it will desolve =

> electrolyte thus making the cell unusable due too high internal resitance.
> =

do you have data that shows diminished lifespan as a result of slight =

overcharging?

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## EVDL List (Jul 27, 2007)

[email protected] skrev:
> The required algorithms I saw
saw where?


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## EVDL List (Jul 27, 2007)

I want to make it clear that in spite of the following positive experience
with LiFePO4, I don't recommend blindly using them without BMS at this point
in development. I basically am getting away with murder (or rather, not
murdering cells), but at my own risk I think it shows that through quality
control and further developmental maturity of the chemistry, it is not
unreasonable to expect that the requirement of an elaborate BMS may be
relaxed in the future (OK, I'll be more committal after a few hundred more
cycles).

Experimenting with a string of 15 40Ah cells, for the first charge I put
them in parallel on rails, simply monitoring total current draw and voltage
as they were brought up to a 4.2V float shelf and held there for an hour.

Next they were installed in series as a 48V nominal pack on the vehicle and
conservatively discharged down to about 80% SOC monitoring only pack
voltage. The only protection on discharge is setting the under voltage
cutoff on the controller, which would not likely give warning for any
individual deviant cells.

On the first series charge cycle, I manually metered individual cells as
they climbed just to make sure they were staying in their voltage window of
safety. They wandered on the steep slope at the very end of the charge
cycle by as much as 0.1V deviation. I was just about to stop the charge
when the charger, calibrated for a 48V Pb pack, shut itself off.

After manually 'spot' monitoring for the first few cycles, I reasoned that
due to the nature of the charge/discharge voltage curve, there isn't much
difference in SOC between a relaxed 53V pack and a 60V pack. If the cells
were all hypothetically equal the danger at the top would be over 63V and
under 38V at the bottom. So, I let the charger trickle at 3A overnight and
shut itself off when it brings the pack up to 58V, which to date has proven
to be an adequate safety margin while virtually filling the pack.

I've been operating at my own risk without BMS for about 5 months and at
least a few dozen cycles with no consequences (yet. . .wait, Maynard, don't
hex it!). This week I checked and even when "full", all cells still
individually relax to 3.75V +/- <0.01V each.

Obvious to note that I'm only watching the voltage window here-not thorough
science. In typical use, I know I'm discharging typically between 1C and
3C, with daily bursts up to 10C (to keep up with highway cars while crossing
a bridge with no bike lane on my way home).

"Dum Luck randomly happens to those who are reckless"
-Anonymous

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf
Of Gilbert, Brian D (GE Infra, Energy)
Sent: Wednesday, January 02, 2008 4:34 PM
To: Electric Vehicle Discussion List
Subject: Re: [EVDL] LiFePO4 sensitivity to overcharge

Aren't we talking about this because you'd like to run the lithiums
without BMS?

Brian 

-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of Dan Frederiksen
Sent: Wednesday, January 02, 2008 4:17 PM
To: Electric Vehicle Discussion List
Subject: Re: [EVDL] LiFePO4 sensitivity to overcharge

Lee, we were talking about reduction of lifespan as a result of
overcharging, specifically for LiFePO4 cells. if they are not sensitive
then cascade problems will not follow. we need the core issue first

Dan

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## EVDL List (Jul 27, 2007)

very interesting to hear Sam.
it would be expected that trickle overcharge would equalize. as I see it 
the real test is if they drop in capacity faster than they would 
otherwise. if you have the means you can periodically take out a few 
specific cells and measure their capacity somehow. and/or internal 
resistance (measure current when discharging into a resistor). only if 
they live as long will the assumption of robustness be validated.

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## EVDL List (Jul 27, 2007)

Dan is a skeptic. 

What he really wants to see is some sort of documentation, a graph or perhaps a live video of a test, not anecdotal evidence or what I call "tribal knowledge" (My friend's, cousin's, brother said...). I understand that need and I don't think there's anything wrong with that. 

What I don't understand is why he doesn't simply request the public, written battery specifications or test data from the manufacturer of the battery he's curious about instead of asking us. Does he think that they're lying? Does he think that the manufacturers are incompetent? If the manufacturer's written specification states a particular maximum, or finishing voltage or charging curve for optimal performance, I don't understand why that's not good enough. It's not like the manufacturer pulled those values out of their collective arses. Such a specification implies that there will be a consequence for non-adherence by it's very existance.

I mean, he's basically asking us (who have no written proof because we didn't develop or make the damned things), and then tells us that we don't know what we're talking about just so he can tout how smart and scientific he is.

On the other hand Dan, if you're asking specifically WHAT the deletrious effects of overcharging are (instead of asking IF there are any), you basically have 3 choices:

1. Write the manufacturer and ask them.
2. Buy your own cell and perform your own overcharge and discharge test.
3. Ask the list if anyone has performed this specific test and what proof they have.

The problem for you Dan, is that choices #1 and #3 require a certain amount of trust or faith in other people which you seem to lack so that leaves option #2 as your best option. Please let us know how it all works out for you.
"Is a man not entitled to the sweat of his own brow? No, says the man in Washington. It belongs to the poor.
No, says the man in the Vatican. It belongs to God.
No, says the man in Moscow. It belongs to everyone."

Rich A.
Maryland
http://www.austinev.org/evalbum/371.html


------------------------------------------------------------------

Message: 32
Date: Wed, 2 Jan 2008 19:31:28 -0600
From: [email protected]
Subject: Re: [EVDL] LiFePO4 sensitivity to overcharge
To: Electric Vehicle Discussion List [email protected]
Message-ID:
[email protected]
Content-Type: text/plain; charset=utf-8
Well, don't. It'll be an expensive and dangerous waste of time!
The difficulty is not just that you need to sense when to stop charging and shut off. Nor can you put a zener on there to just limit the 
voltage. Neither of these meet the required charging algorithm. 
The required algorithms I saw are constant current at first (this one is easy, the whole string can be one fixed current) then cell-by-cell 

they must switch over to constant-voltage (such as a voltage shunt, but zener shunts are not really available in this voltage range with 

the required accuracy), and sense when the current to that cell drops below a threshold and shut off entirely. That could be done by 

measuring the current going through a voltage shunt- batt string current minus the cell's shunt current tells you the batt's charging 

current. That's essential because we can't put a shunt on each batt to measure each batt's current; those shunts would be in series with 
the pack's discharge path during use. Then you'd "shutoff" that cell by putting the shunt in a constant-current mode equal to the string's 
current so there's no net battery current. A significant problem with that idea is that any difference in calibration between the shunt 
path's current shunt and the !
whole-string's current shunt would result in a net charging or discharging current in that shutoff mode. If the string has 5 amps through it 
and the cell's shunt in "shutoff" mode tries to absorb 5A but only absorbs 4.9A due to a shunt or voltage reference issue in the manager, 
then the cell's still receiving 100mA of overcharge when it's supposed to be shut off completely..
Danny ---- Dan Frederiksen <[email protected]> wrote: > if possible yes. obviously.> to know the implications of the choice at hand. to be informed. is that > such a foreign concept> > Gilbert, Brian D (GE Infra, Energy) wrote:> > Aren't we talking about this because you'd like to run the lithiums> > without BMS?> > > > _______________________________________________> For subscription options, see> http://lists.sjsu.edu/mailman/listinfo/ev
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## EVDL List (Jul 27, 2007)

----- Original Message ----- 
From: "Richard Acuti" <[email protected]>
To: <[email protected]>
Sent: Thursday, January 03, 2008 8:25 AM
Subject: Re: [EVDL] LiFePO4 sensitivity to overcharge


> 
> Dan is a skeptic. 

Understatement of the YEAR and we are only 3 daze into it<g>!

Bob
> 

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## EVDL List (Jul 27, 2007)

fascinating how creative so many are in trying to make something my fault 

while the premise of your criticism is wrong, it is perhaps worth a shot 
to ask the manufacturers. the data you pretend is so readily available 
from them is however not. even the simplest data can be hard to wrestle 
from the incompetent people in the sales departments of the various 
makers. it is not uncommon that I have to patiently go through 3-4 email 
iterations before they realize they have errors in their datasheets no 
matter how obvious. Headway who make these nice big cells sent me cells 
that didn't exist on the list of their products because someone else had 
decided to change the parameters of their products. instead of a 6.5Ah 
cell it was now an 8Ah cell. I pointed out that this would change the 
internal resistance and other specs but she so blindly trusted her data 
that she brushed it off as nonsense on my part. the silly little 
customer, go away. Some of these companies are so stupid that they even 
refuse to sell their products, like A123. "you can't have se duck"
good luck getting data out of those guys. even if they have it. Maybe 
Dube could but take a suggestion from the troll....that's dangerously 
close to admitting mistakes.

but your assumed position raised an interesting point. looking at all 
the datasheets I have collected, not one covers the consequences of 
trickle overcharge. not one warning, not one graph of cycle life. That 
might mean it's not as big a problem as has been rumored. could also 
just be an oversight.

Dan



> Richard Acuti wrote:
> > I mean, he's basically asking us (who have no written proof because we didn't develop or make the damned things), and then tells us that we don't know what we're talking about just so he can tout how smart and scientific he is.
> >
> 
> ...


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