Re: Effects of UNDERcharging LiFe4Po batteries? (and other questions)

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Re: Effects of UNDERcharging LiFe4Po batteries? (and other questions)

Richard Acuti

Thanks for that info Jeff.
 
I'm sorry if I seem like a cheapskate who's trying to cheat the system by avoiding a BMS. I was just musing if it were possible to keep the pack between say 90 & 30 % and then just disconnect the batteries and equalize them individually every 3-5 days.
 
Ok, so let's skip all that for now and let me start a new line of questions-
 
1. Please explain to me why a system of battery regs like those already sold (Like Rudman Regs) don't work with LiFe4Po? Or do they?2. If I bought batteries from Jukka or Valence or someone, and bought their BMS, what basic parts does that BMS consist of? How does it connect to the pack? Does anyone have any photos?
 
I think the thing that irks me the most, is not being able to buy this stuff piecemeal. I've always been able to buy what I want, -without- putting myself in debt with credit cards or loans by slowly gathering components. I wish I could buy a BMS, then buy a battery or two every month until I had what I need.
 
"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: 32
Date: Fri, 04 Jan 2008 05:56:00 -0800
From: Jeff Shanab [hidden email]
Subject: Re: [EVDL] Effects of UNDERcharging LiFe4Po batteries?
To: [hidden email]
Message-ID: [hidden email]
Content-Type: text/plain; charset=ISO-8859-1
 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 mechanismat 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? (and otherquestions)

Tim Humphrey



On Fri, 4 Jan 2008 10:20:28 -0500, Richard Acuti <[hidden email]> wrote:
>
> Thanks for that info Jeff.
>
> I'm sorry if I seem like a cheapskate who's trying to cheat the system by
> avoiding a BMS. I was just musing if it were possible to keep the pack
> between say 90 & 30 % and then just disconnect the batteries and equalize
> them individually every 3-5 days.
>

Just to beat a dead horse.....

You are contemplating using only half of your pack capacity to avoid purchasing a BMS. That means you need twice the battery capacity for the same range. Which is cheaper, a BMS or twice as much Lithium? Consider, the BMS is essentially reusable.


--
Stay Charged!
Hump
"If you don't "believe" you'll make a difference, than you probably never will!" -- Jim Husted


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Re: Effects of UNDERcharging LiFe4Po batteries? (and otherquestions)

Dan Frederiksen-2
Tim Humphrey wrote:
> Just to beat a dead horse.....
>
> You are contemplating using only half of your pack capacity to avoid purchasing a BMS. That means you need twice the battery capacity for the same range. Which is cheaper, a BMS or twice as much Lithium? Consider, the BMS is essentially reusable.
>  

it appears you don't need a bms either way with LiFePO4

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Re: Effects of UNDERcharging LiFe4Po batteries? (and other questions)

Chuck Homic
In reply to this post by Richard Acuti
Richard Acuti wrote:
>  
> 1. Please explain to me why a system of battery regs like those already sold (Like Rudman Regs) don't work with LiFe4Po? Or do they?2. If I bought batteries from Jukka or Valence or someone, and bought their BMS, what basic parts does that BMS consist of? How does it connect to the pack? Does anyone have any photos?
>  
I'm sure you could.  Let's say you could get the cost of a regulator
down to $30.  Or $10.  Would you put one on each $5 cell?

 

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Re: Effects of UNDERcharging LiFe4Po batteries? (and other questions)

Morgan LaMoore
On Jan 4, 2008 10:55 AM, Chuck Homic <[hidden email]> wrote:
> Richard Acuti wrote:
> >
> > 1. Please explain to me why a system of battery regs like those already sold (Like Rudman Regs) don't work with LiFe4Po? Or do they?2. If I bought batteries from Jukka or Valence or someone, and bought their BMS, what basic parts does that BMS consist of? How does it connect to the pack? Does anyone have any photos?
> >
> I'm sure you could.  Let's say you could get the cost of a regulator
> down to $30.  Or $10.  Would you put one on each $5 cell?

One possibility is putting one regulator on 15-30 parallel'ed cells;
then cost isn't much of an issue.

The risk is that if a cell fails short, it takes out all the cells in
parallel with it. I don't know how likely a LiFePO4 cell is to fail
short, though. If the cells "fail" via reduced capacity or increased
internal resistance, then it isn't a problem; the only risk is a fail
short.

-Morgan LaMoore

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Re: Effects of UNDERcharging LiFe4Po batteries? (and other questions)

Richard Acuti
In reply to this post by Richard Acuti

Ah, yeah I see what you're saying. Except the thing is, the batteries I'm talking about are not these itty bitty cells that everyone is talking about. I was sent some documentation on a larger battery that I would only need 40 of. So would I put a $15 dollar reg on only 40 cells? If a BMS costs $1k, I sure would because that would only be $600.00.
 
It's my fault for not being clearer and posting the battery specs. I'm going to look around and see if I still have the spec sheet.
 
------------------------------Message: 15
Date: Fri, 04 Jan 2008 11:55:57 -0500
From: Chuck Homic [hidden email]
Subject: Re: [EVDL] Effects of UNDERcharging LiFe4Po batteries? (and
other questions)
To: Electric Vehicle Discussion List [hidden email]
Message-ID: <[hidden email]>Content-Type: text/plain; charset=windows-1252; format=flowed
 I'm sure you could. Let's say you could get the cost of a regulator
down to $30. Or $10. Would you put one on each $5 cell?------------------------------------------ Rich A.
Maryland
http://www.austinev.org/evalbum/371.html
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Re: Effects of UNDERcharging LiFe4Po batteries? (and other questions)

Morgan LaMoore
On Jan 4, 2008 2:51 PM, Richard Acuti <[hidden email]> wrote:
>
> Ah, yeah I see what you're saying. Except the thing is, the batteries I'm talking about are not these itty bitty cells that everyone is talking about. I was sent some documentation on a larger battery that I would only need 40 of. So would I put a $15 dollar reg on only 40 cells? If a BMS costs $1k, I sure would because that would only be $600.00.

Any chance you would share your cell source or specs/cost?

-Morgan LaMoore

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