Effects of UNDERcharging LiFe4Po batteries?

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Effects of UNDERcharging LiFe4Po batteries?

Richard Acuti

I am curious if anyone has any data on the effect or consequence of slightly UNDERcharging Lithium batteries. I am accepting wild theories, anecdotal evidence and tribal knowledge at this time. At the moment, I have no printed literature from any vendors to refer to. When I get closer to an actual purchase, I will dig deeper and make contact with the vendors.
 
The reason I ask this, is because I'm wondering if it's possible to use a cheaper system of battery regs instead of a BMS OR run without any sort of BMS is I run the batteries within a narrower window between the minimum/maximum voltages specified by the vendor.
 
Based on one vendor I looked at a while back (and lost the documentation for), I'd need about 40 units to build a pack. I was thinking that it might be possible to charge/discharge between a narrower, safe window during the week and equalize individually on the weekend. At least until I could afford the real BMS. However, if not fully charging these batteries is harmful then the plan is moot.
 
What say 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.Marylandhttp://www.austinev.org/evalbum/371.html
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Re: Effects of UNDERcharging LiFe4Po batteries?

Dan Frederiksen-2
Richard Acuti wrote:
> I am curious if anyone has any data on the effect or consequence of slightly UNDERcharging Lithium batteries. I am accepting wild theories, anecdotal evidence and tribal knowledge at this time.
it's not hard data but it is indicative that GM has made the preliminary
decision to do just that for the Chevrolet Volt. I think it was
something like from 30%-80% SOC using only half the capacity.

Dan

PS. you should probably specify at least that it's LiFePO4 chemistry.
some lithium chemistries are so different that they can't even be
recharged at all...

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Re: Effects of UNDERcharging LiFe4Po batteries?

Morgan LaMoore
In reply to this post by Richard Acuti
As far as I know, undercharging them just reduces capacity for that
cycle; it won't harm the cells.

I'm basing this on "regular" LiCoO2 cells; for them, storage at high
SOC ages cells faster; the lower the SOC, the slower they age, down to
an optimal storage SOC of 40%.

If this holds over to LiFePO4, your cells may actually be better off
being undercharged (at a penalty to range).

-Morgan LaMoore

On Jan 3, 2008 9:02 AM, Richard Acuti <[hidden email]> wrote:

>
> I am curious if anyone has any data on the effect or consequence of slightly UNDERcharging Lithium batteries. I am accepting wild theories, anecdotal evidence and tribal knowledge at this time. At the moment, I have no printed literature from any vendors to refer to. When I get closer to an actual purchase, I will dig deeper and make contact with the vendors.
>
> The reason I ask this, is because I'm wondering if it's possible to use a cheaper system of battery regs instead of a BMS OR run without any sort of BMS is I run the batteries within a narrower window between the minimum/maximum voltages specified by the vendor.
>
> Based on one vendor I looked at a while back (and lost the documentation for), I'd need about 40 units to build a pack. I was thinking that it might be possible to charge/discharge between a narrower, safe window during the week and equalize individually on the weekend. At least until I could afford the real BMS. However, if not fully charging these batteries is harmful then the plan is moot.
>
> What say 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.Marylandhttp://www.austinev.org/evalbum/371.html
> _________________________________________________________________
> Don't get caught with egg on your face. Play Chicktionary!
> http://club.live.com/chicktionary.aspx?icid=chick_wlhmtextlink1_dec
> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev
>

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Re: Effects of UNDERcharging LiFe4Po batteries?

Richard Acuti
In reply to this post by Richard Acuti

Thanks Dan,
 
That was actually quite helpful. You're right, I should specify which chemistry. I know very little about all the new chemistry batteries other than the fact that you're supposed to use a BMS to keep them equal and healthy.
 
My understanding (and I could be wrong) is that when (any chemistry) batteries drift apart, over time the differences between cells are amplified so even if you attempt to stay within the 30-80% window, some are going to fall out of that. I'm guessing that I would still need to equalize eventually.
 
Currently, I am still trying to comprehend why ordinary battery regulators that some people are using won't work.
"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.Marylandhttp://www.austinev.org/evalbum/371.html------------------------------ Message: 6Date: Thu, 03 Jan 2008 16:45:07 +0100From: Dan Frederiksen <[hidden email]>Subject: Re: [EVDL] Effects of UNDERcharging LiFe4Po batteries?To: Electric Vehicle Discussion List <[hidden email]>Message-ID: <[hidden email]>Content-Type: text/plain; charset=ISO-8859-1; format=flowed Richard Acuti wrote:> I am curious if anyone has any data on the effect or consequence of slightly UNDERcharging Lithium batteries. I am accepting wild theories, anecdotal evidence and tribal knowledge at this time. it's not hard data but it is indicative that GM has made the preliminary decision to do just that for the Chevrolet Volt. I think it was something like from 30%!
 -80% SOC using only half the capacity. Dan PS. you should probably specify at least that it's LiFePO4 chemistry. some lithium chemistries are so different that they can't even be recharged at all...   ------------------------------
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Re: Effects of UNDERcharging LiFe4Po batteries?

Jukka Järvinen
Generally speaking:

Less voltage. Less electrolyte dissolving. Less impedance growth. Less
capacity available. Longer calendar life.

Valid on most (if not all) organic electrolytes.

Do not even try to play with anything else than LiFePO4. Only hten
you'll be safe. We need all enthustiasts alive here :)

-Jukka


Richard Acuti kirjoitti:
> Thanks Dan,
>  
> That was actually quite helpful. You're right, I should specify which chemistry. I know very little about all the new chemistry batteries other than the fact that you're supposed to use a BMS to keep them equal and healthy.
>  
> My understanding (and I could be wrong) is that when (any chemistry) batteries drift apart, over time the differences between cells are amplified so even if you attempt to stay within the 30-80% window, some are going to fall out of that. I'm guessing that I would still need to equalize eventually.
>  
> Currently, I am still trying to comprehend why ordinary battery regulators that some people are using won't work.
> "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.Marylandhttp://www.austinev.org/evalbum/371.html------------------------------ Message: 6Date: Thu, 03 Jan 2008 16:45:07 +0100From: Dan Frederiksen <[hidden email]>Subject: Re: [EVDL] Effects of UNDERcharging LiFe4Po batteries?To: Electric Vehicle Discussion List <[hidden email]>Message-ID: <[hidden email]>Content-Type: text/plain; charset=ISO-8859-1; format=flowed Richard Acuti wrote:> I am curious if anyone has any data on the effect or consequence of slightly UNDERcharging Lithium batteries. I am accepting wild theories, anecdotal evidence and tribal knowledge at this time. it's not hard data but it is indicative that GM has made the preliminary decision to do just that for the Chevrolet Volt. I think it was something like from 30
%!
>  -80% SOC using only half the capacity. Dan PS. you should probably specify at least that it's LiFePO4 chemistry. some lithium chemistries are so different that they can't even be recharged at all...   ------------------------------
> _________________________________________________________________
> Share life as it happens with the new Windows Live.
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Re: Effects of UNDERcharging LiFe4Po batteries?

Jeff Shanab
In reply to this post by Richard Acuti
I hope I am not "stating the obvious"

I think "State of charge" and "balance" are separate issues; even though
balance is relative state of charge.

The failure mode for most lithium ion chemistries is that high voltage
disassociates the electrolyte and allows the chemicals to fowl up the
pores on the electrode that accept the intercalated ions during charge.
LIfepo4 is more tolerant than other lithium chemistry as it is actually
lithium starved at end of charge and its resistance shoots up to limit
current.(as opposed to thermal runaway)

For any single cell, charging to max allowed voltage, gets you the most
charge. It has a bit of the destructive part associated with it.
For any single cell, charging to a lower voltage, lessens this mechanism
at the expense of capacity.

The real problem is that if you start off with cells in series that are
all at 80% SOC and run them all down to 30% SOC for the pack, some will
be at 29% and others will be at 31%. This is caused by different
temperatures or different internal resistances.  Those same conditions
probably exist on the next charge so when you get back to "pack is at
80% SOC" the ones at 29% have come up to 78% and the ones at 31% are now
at 82%.

The balance issue is true of any string of series batteries charged as a
pack, regardless of chemistry.

I think keeping an entire pack between 30% and 80% state of charge will
actually be more complicated than 30% to 100%. You will have to keep
track of the columbs in and out of each cell and measure during
operation the internal resistance for each cell. Then actively balance
in advance of hitting the 80% SOC mark.  If you are allowed to go to
100% then it is easier to bypass some and let others continue to get
them all at the same point.

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Re: Effects of UNDERcharging LiFe4Po batteries?

Dan Frederiksen-2
Jeff Shanab wrote:
> I think keeping an entire pack between 30% and 80% state of charge will
> actually be more complicated than 30% to 100%. You will have to keep
> track of the columbs in and out of each cell and measure during
> operation the internal resistance for each cell. Then actively balance
> in advance of hitting the 80% SOC mark.  If you are allowed to go to
> 100% then it is easier to bypass some and let others continue to get
> them all at the same point.
>  
you could just charge to 100% every tenth time or so and balance them
with a trickle overcharge, all without any BMS

Dan

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Re: Effects of UNDERcharging LiFe4Po batteries?

David Beard
In reply to this post by Richard Acuti
my understanding is that there should be no problem with undercharging, as a matter of fact leaving a little space might be a good thing.....especially if you live at the top of a big hill and use regen.....hybrids are designed for it....
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