Lithium drop in replacements.

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Lithium drop in replacements.

Electric Vehicle Discussion List mailing list
This carbon lead issue is dead for me.  I asked Jim at American Battery in Hayward about carbon/lead batteries and he said wait a couple weeks as totally protected drop in replacements changing a 35 ah form factor into a 50 ah battery with many more cycles made from lithium was coming.  These are only a little more ah but much lighter.  We will see.  I have seen others.  If they are close to 300 dollars they might be a thing.  Lawrence Rhodes
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Re: Lithium drop in replacements.

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On 9 May 2018 at 21:21, Lawrence Rhodes via EV wrote:

> wait a couple weeks as totally protected drop in replacements changing
> a 35 ah form factor into a 50 ah battery with many more cycles made
> from lithium was coming.

I've seen a lot of these around already.  I almost bought a small one about
a year ago, but then went in another direction.

https://www.amazon.com/12-Volt-Rechargeable-Lithium-Battery/dp/B00JK06CK8/

They're 12v (sometimes 6v) worth of lithum cells, often LiFePO4, with some
kind of BMS internally so you can (supposedly) charge them with a lead
battery charger.  Actually, in many cases all you need is a bare power
supply.

You have to watch out for them though.  There are a lot of cheap Chinese
batteries like this that have just "protection" circuits, not true BMSes.  
The cells soon get out of balance, and the usable capacity falls.

David Roden - Akron, Ohio, USA
EVDL Administrator

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Re: Lithium drop in replacements.

Electric Vehicle Discussion List mailing list
Do you have any evidence of these batteries failing because of out of balance conditions or is that speculation

Sent from my iPhone

> On May 9, 2018, at 9:22 PM, EVDL Administrator via EV <[hidden email]> wrote:
>
>> On 9 May 2018 at 21:21, Lawrence Rhodes via EV wrote:
>>
>> wait a couple weeks as totally protected drop in replacements changing
>> a 35 ah form factor into a 50 ah battery with many more cycles made
>> from lithium was coming.
>
> I've seen a lot of these around already.  I almost bought a small one about
> a year ago, but then went in another direction.
>
> https://www.amazon.com/12-Volt-Rechargeable-Lithium-Battery/dp/B00JK06CK8/
>
> They're 12v (sometimes 6v) worth of lithum cells, often LiFePO4, with some
> kind of BMS internally so you can (supposedly) charge them with a lead
> battery charger.  Actually, in many cases all you need is a bare power
> supply.
>
> You have to watch out for them though.  There are a lot of cheap Chinese
> batteries like this that have just "protection" circuits, not true BMSes.  
> The cells soon get out of balance, and the usable capacity falls.
>
> David Roden - Akron, Ohio, USA
> EVDL Administrator
>
> = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
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Re: Lithium drop in replacements.

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On 10 May 2018 at 6:35, paul dove via EV wrote:

> Do you have any evidence of these batteries failing because of out of balance
> conditions or is that speculation

The battery I mentioned ("Dakota" brand) was advertised as having a true
balancing BMS.

Discussion of PCBs vs BMSes here:

http://ka7oei.blogspot.com/2013/05/lithium-iron-phosphate-lifepo4.html

David Roden - Akron, Ohio, USA
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Re: Lithium drop in replacements.

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Thanks that was interesting. It’s more likely that the Dakota BMS was causing the imbalance with parasitic unbalanced loads, however.

Sent from my iPhone

> On May 11, 2018, at 11:56 AM, EVDL Administrator via EV <[hidden email]> wrote:
>
>> On 10 May 2018 at 6:35, paul dove via EV wrote:
>>
>> Do you have any evidence of these batteries failing because of out of balance
>> conditions or is that speculation
>
> The battery I mentioned ("Dakota" brand) was advertised as having a true
> balancing BMS.
>
> Discussion of PCBs vs BMSes here:
>
> http://ka7oei.blogspot.com/2013/05/lithium-iron-phosphate-lifepo4.html
>
> David Roden - Akron, Ohio, USA
> EVDL Administrator
>
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Re: Lithium drop in replacements.

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Paul,

Do you have any evidence of this or is it speculation?

(Sorry, I just couldn't help myself.)

Bill D


On 5/11/2018 12:47 PM, paul dove via EV wrote:

> Thanks that was interesting. It’s more likely that the Dakota BMS was causing the imbalance with parasitic unbalanced loads, however.
>
> Sent from my iPhone
>
>> On May 11, 2018, at 11:56 AM, EVDL Administrator via EV <[hidden email]> wrote:
>>
>>> On 10 May 2018 at 6:35, paul dove via EV wrote:
>>>
>>> Do you have any evidence of these batteries failing because of out of balance
>>> conditions or is that speculation
>> The battery I mentioned ("Dakota" brand) was advertised as having a true
>> balancing BMS.
>>
>> Discussion of PCBs vs BMSes here:
>>
>> http://ka7oei.blogspot.com/2013/05/lithium-iron-phosphate-lifepo4.html
>>
>> David Roden - Akron, Ohio, USA
>> EVDL Administrator
>>
>> = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
>> EVDL Information: http://www.evdl.org/help/
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>> reach me.  To send a private message, please obtain my
>> email address from the webpage http://www.evdl.org/help/ .
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>>
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Article on BMS (was: Lithium drop in replacements.)

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In reply to this post by Electric Vehicle Discussion List mailing list
David,

Very nice plain language article on BMS's.
Thank you for posting it.

Bill D.

> The battery I mentioned ("Dakota" brand) was advertised as having a true
> balancing BMS.
>
> Discussion of PCBs vs BMSes here:
>
> http://ka7oei.blogspot.com/2013/05/lithium-iron-phosphate-lifepo4.html
>
> David Roden - Akron, Ohio, USA
> EVDL Administrator
>
> = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
> EVDL Information: http://www.evdl.org/help/
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> email address from the webpage http://www.evdl.org/help/ .
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Re: Lithium drop in replacements.

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In reply to this post by Electric Vehicle Discussion List mailing list
Actually I do have evidence. I did extensive testing with LiFePO4 cells. I found no evidence of self discharge or of balance issues with cells connected in series.

Sent from my iPhone

> On May 11, 2018, at 4:43 PM, Bill Dube via EV <[hidden email]> wrote:
>
> Paul,
>
> Do you have any evidence of this or is it speculation?
>
> (Sorry, I just couldn't help myself.)
>
> Bill D
>
>
>> On 5/11/2018 12:47 PM, paul dove via EV wrote:
>> Thanks that was interesting. It’s more likely that the Dakota BMS was causing the imbalance with parasitic unbalanced loads, however.
>>
>> Sent from my iPhone
>>
>>>> On May 11, 2018, at 11:56 AM, EVDL Administrator via EV <[hidden email]> wrote:
>>>>
>>>> On 10 May 2018 at 6:35, paul dove via EV wrote:
>>>>
>>>> Do you have any evidence of these batteries failing because of out of balance
>>>> conditions or is that speculation
>>> The battery I mentioned ("Dakota" brand) was advertised as having a true
>>> balancing BMS.
>>>
>>> Discussion of PCBs vs BMSes here:
>>>
>>> http://ka7oei.blogspot.com/2013/05/lithium-iron-phosphate-lifepo4.html
>>>
>>> David Roden - Akron, Ohio, USA
>>> EVDL Administrator
>>>
>>> = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
>>> EVDL Information: http://www.evdl.org/help/
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>>> reach me.  To send a private message, please obtain my
>>> email address from the webpage http://www.evdl.org/help/ .
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Re: Lithium drop in replacements.

Electric Vehicle Discussion List mailing list
I also did tests on LiFePO4 cells and while self-discharge was low, I was able to prove from my measurements over many weeks, that there is was about a factor 2 difference in self-discharge current between the best and worst cell. Sample size was over 40 cells.
This is exactly what the author of the quoted article found, that the capacity of the cells had not degraded, but the cells had gotten out of balance, so the protection circuitry allowed an ever shrinking overlap between best and worst cell. Simply balancing the cells brought back the original available pack capacity, which is logical.
Hope this clarifies,
Cor.

BTW, this email address will soon cease to work, please find me at [hidden email]


On May 11, 2018 10:58 PM, paul dove via EV <[hidden email]> wrote:
Actually I do have evidence. I did extensive testing with LiFePO4 cells. I found no evidence of self discharge or of balance issues with cells connected in series.

Sent from my iPhone

> On May 11, 2018, at 4:43 PM, Bill Dube via EV <[hidden email]> wrote:
>
> Paul,
>
> Do you have any evidence of this or is it speculation?
>
> (Sorry, I just couldn't help myself.)
>
> Bill D
>
>
>> On 5/11/2018 12:47 PM, paul dove via EV wrote:
>> Thanks that was interesting. It’s more likely that the Dakota BMS was causing the imbalance with parasitic unbalanced loads, however.
>>
>> Sent from my iPhone
>>
>>>> On May 11, 2018, at 11:56 AM, EVDL Administrator via EV <[hidden email]> wrote:
>>>>
>>>> On 10 May 2018 at 6:35, paul dove via EV wrote:
>>>>
>>>> Do you have any evidence of these batteries failing because of out of balance
>>>> conditions or is that speculation
>>> The battery I mentioned ("Dakota" brand) was advertised as having a true
>>> balancing BMS.
>>>
>>> Discussion of PCBs vs BMSes here:
>>>
>>> http://ka7oei.blogspot.com/2013/05/lithium-iron-phosphate-lifepo4.html
>>>
>>> David Roden - Akron, Ohio, USA
>>> EVDL Administrator
>>>
>>> = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
>>> EVDL Information: http://www.evdl.org/help/
>>> = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
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>>> reach me.  To send a private message, please obtain my
>>> email address from the webpage http://www.evdl.org/help/ .
>>> = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
>>>
>>>
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Re: Lithium drop in replacements.

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I have used headway cells, headway specs them as 5% per month self
discharge! so have close to zero, worse is 5 in practice. found it to be
a big problem, if you charge every day it's ok, but the ebike i got them
for may go a while without charging, also, If it is not left on charge
long enough they still will not equallize. at the moment, there is no
way to tell when eq is complete, could be 400+ hours if they are out by
their capacity! I won't use them again.



On 12-May-18 6:09 PM, Cor van de Water via EV wrote:

> I also did tests on LiFePO4 cells and while self-discharge was low, I was able to prove from my measurements over many weeks, that there is was about a factor 2 difference in self-discharge current between the best and worst cell. Sample size was over 40 cells.
> This is exactly what the author of the quoted article found, that the capacity of the cells had not degraded, but the cells had gotten out of balance, so the protection circuitry allowed an ever shrinking overlap between best and worst cell. Simply balancing the cells brought back the original available pack capacity, which is logical.
> Hope this clarifies,
> Cor.
>
> BTW, this email address will soon cease to work, please find me at [hidden email]
>
>
> On May 11, 2018 10:58 PM, paul dove via EV <[hidden email]> wrote:
> Actually I do have evidence. I did extensive testing with LiFePO4 cells. I found no evidence of self discharge or of balance issues with cells connected in series.
>
> Sent from my iPhone
>
>> On May 11, 2018, at 4:43 PM, Bill Dube via EV <[hidden email]> wrote:
>>
>> Paul,
>>
>> Do you have any evidence of this or is it speculation?
>>
>> (Sorry, I just couldn't help myself.)
>>
>> Bill D
>>
>>
>>> On 5/11/2018 12:47 PM, paul dove via EV wrote:
>>> Thanks that was interesting. It’s more likely that the Dakota BMS was causing the imbalance with parasitic unbalanced loads, however.
>>>
>>> Sent from my iPhone
>>>
>>>>> On May 11, 2018, at 11:56 AM, EVDL Administrator via EV <[hidden email]> wrote:
>>>>>
>>>>> On 10 May 2018 at 6:35, paul dove via EV wrote:
>>>>>
>>>>> Do you have any evidence of these batteries failing because of out of balance
>>>>> conditions or is that speculation
>>>> The battery I mentioned ("Dakota" brand) was advertised as having a true
>>>> balancing BMS.
>>>>
>>>> Discussion of PCBs vs BMSes here:
>>>>
>>>> http://ka7oei.blogspot.com/2013/05/lithium-iron-phosphate-lifepo4.html
>>>>
>>>> David Roden - Akron, Ohio, USA
>>>> EVDL Administrator
>>>>
>>>> = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
>>>> EVDL Information: http://www.evdl.org/help/
>>>> = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
>>>> Note: mail sent to "evpost" and "etpost" addresses will not
>>>> reach me.  To send a private message, please obtain my
>>>> email address from the webpage http://www.evdl.org/help/ .
>>>> = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
>>>>
>>>>
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Re: Lithium drop in replacements.

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In reply to this post by Electric Vehicle Discussion List mailing list
Another thing, I see the ebike chargers have 2 voltage pots, I think one
is for the ballance charge, at reduced current. Have anyone seem this?


On 12-May-18 6:47 AM, paul dove via EV wrote:

> Thanks that was interesting. It’s more likely that the Dakota BMS was causing the imbalance with parasitic unbalanced loads, however.
>
> Sent from my iPhone
>
>> On May 11, 2018, at 11:56 AM, EVDL Administrator via EV <[hidden email]> wrote:
>>
>>> On 10 May 2018 at 6:35, paul dove via EV wrote:
>>>
>>> Do you have any evidence of these batteries failing because of out of balance
>>> conditions or is that speculation
>> The battery I mentioned ("Dakota" brand) was advertised as having a true
>> balancing BMS.
>>
>> Discussion of PCBs vs BMSes here:
>>
>> http://ka7oei.blogspot.com/2013/05/lithium-iron-phosphate-lifepo4.html
>>
>> David Roden - Akron, Ohio, USA
>> EVDL Administrator
>>
>> = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
>> EVDL Information: http://www.evdl.org/help/
>> = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
>> Note: mail sent to "evpost" and "etpost" addresses will not
>> reach me.  To send a private message, please obtain my
>> email address from the webpage http://www.evdl.org/help/ .
>> = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
>>
>>
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>> UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub
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Re: Lithium drop in replacements.

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In reply to this post by Electric Vehicle Discussion List mailing list
>From: Cor van de Water via EV <[hidden email]>
>I also did tests on LiFePO4 cells and while self-discharge was low, I was able to prove from my measurements over many weeks, that there is was about a factor 2 difference in self-discharge current between the best and worst cell. Sample size was over 40 cells.

This is what I have found as well. Brand new cells, all bought at the same time from a quality source are very similar. Their amphour capacity, internal resistance, and self-discharge rates are very close.

But cells from cheap or low-quality sources have a much broader spread in characteristics. Cells also get worse as they get old, or get cycled, or as the temperature changes. Differences between cells accumulate over time, getting worse and worse. You may get by without a BMS initially, but it gets needed more as the differences between cells grows Not having a BMS means shorter life.

>This is exactly what the author of the quoted article found, that the capacity of the cells had not degraded, but the cells had gotten out of balance.

Yes. The BMS was too primitive to do its job of balancing the cells. Only having upper and lower voltage limits prevents against catastrophic failures; but does not compensate for differences between cells.


--
Excellence does not require perfection. -- Henry James
--
Lee A. Hart http://www.sunrise-ev.com
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Re: Lithium drop in replacements.

Electric Vehicle Discussion List mailing list
Hey thanks for your input. I’m sure a lot of people have varied experiences. I must’ve met there probably differences between manufacturers. However that was not my experience. I bought Bestgo sales hundred amp hour. I did extensive testing before installing them in a vehicle. I put 40 for 100 amp our cells in a 1986 Toyota Celica. I charge them 3.65 V per cell or 160.6 V.  After sitting for a while the voltage dropped 148.7 v or 3.38 volts per cell. I drove the vehicle every day for two years. There was never a variance at the end of charge. All the cells measured 3.38 V several hours after charging. I took the cells out of the vehicle And they said on the shelf for year and a half or so. I measure the voltage and they were all 3.38 V. However they do experience reversible capacity fade. I discharged all the cells and Got 45amp hours the first time. The second cycle I got around 65 amp hours and it continued to increase for five cycles. On the last cycle all the cells measured Close to 100 amp hours.

Sent from my iPhone

On May 12, 2018, at 12:23 PM, Lee Hart via EV <[hidden email]> wrote:

>> From: Cor van de Water via EV <[hidden email]>
>> I also did tests on LiFePO4 cells and while self-discharge was low, I was able to prove from my measurements over many weeks, that there is was about a factor 2 difference in self-discharge current between the best and worst cell. Sample size was over 40 cells.
>
> This is what I have found as well. Brand new cells, all bought at the same time from a quality source are very similar. Their amphour capacity, internal resistance, and self-discharge rates are very close.
>
> But cells from cheap or low-quality sources have a much broader spread in characteristics. Cells also get worse as they get old, or get cycled, or as the temperature changes. Differences between cells accumulate over time, getting worse and worse. You may get by without a BMS initially, but it gets needed more as the differences between cells grows Not having a BMS means shorter life.
>
>> This is exactly what the author of the quoted article found, that the capacity of the cells had not degraded, but the cells had gotten out of balance.
>
> Yes. The BMS was too primitive to do its job of balancing the cells. Only having upper and lower voltage limits prevents against catastrophic failures; but does not compensate for differences between cells.
>
>
> --
> Excellence does not require perfection. -- Henry James
> --
> Lee A. Hart http://www.sunrise-ev.com
> _______________________________________________
> UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub
> http://lists.evdl.org/listinfo.cgi/ev-evdl.org
> Please discuss EV drag racing at NEDRA (http://groups.yahoo.com/group/NEDRA)
>

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Self-discharge (was: Lithium drop in replacements.)

Electric Vehicle Discussion List mailing list
Measuring voltage on a LiPOFe4 cell has *_no bearing_* on the
state-of-charge. Only at near 100% SOC or near 0% SOC does the voltage
change significantly.

You have to actually fully cycle a LiFePOFe4 cell to determine its SOC.
If you don't do a full, 100% - 0% - 100% cycle, you can't determine the
SOC. Time consuming and tedious, especially for many cells in series.

The patience and care which is required to measure self-discharge in
LiPOFe4 cells is probably the root of many misconceptions about
self-discharge, balance, and the need for a BMS.

The self discharge for LiPOFe4 cells is indeed small, but it is real and
it is a strong function of temperature. It also varies from cell to
cell. Just because it is small, does not mean it can be neglected. We
have all hear the familiar saying "rust never sleeps". Well, "self
discharge never sleeps" either. Given time, it will ruin a series pack,
or in worse case cause a fire.

If self-discharge were equal, and remained equal, there would be no need
for a BMS, but it is unequal. As the number of cells in series
increases, the problem becomes more pronounced, and more deadly.

Here is a link showing the temperature dependence of self-discharge:
http://www.batteryspace.com/prod-specs/9444.pdf
You can see that the self discharge gets _much_ greater as temperature
increases. (There is a cornucopia of self-discharge information
available. All you have to do is look.)

What most people don't realize is that the cells in a battery pack do
not stay at uniform temperature. The end-most cells, for example, are
connected to the outside environment by large cables. These cables make
the end cells swing in temperature with the outside temperature, while
the middle cells tend to stay at the average pack temperature. The
middle most cells, are somewhat insulated from the outside, and tend to
heat up when the pack is cycled. When the temperature rises, the self
discharge goes wild. When it gets cold, the self-discharge returns to a
tiny rate. Each temperature rise gets the end cells more out of balance.
Kind of like a ratchet. (The cold swings don't "correct" for the warm
swings, unfortunately. The exponential nature of the Arrhenius curve is
to blame.)

If you log the individual cell temperatures, this disparity becomes
obvious. Without any BMS or any sort of battery monitoring, you don't
realize there is any problem at all, until your garage burns down. :-)

Bill D.

On 5/14/2018 7:18 AM, paul dove via EV wrote:

> Hey thanks for your input. I’m sure a lot of people have varied experiences. I must’ve met there probably differences between manufacturers. However that was not my experience. I bought Bestgo sales hundred amp hour. I did extensive testing before installing them in a vehicle. I put 40 for 100 amp our cells in a 1986 Toyota Celica. I charge them 3.65 V per cell or 160.6 V.  After sitting for a while the voltage dropped 148.7 v or 3.38 volts per cell. I drove the vehicle every day for two years. There was never a variance at the end of charge. All the cells measured 3.38 V several hours after charging. I took the cells out of the vehicle And they said on the shelf for year and a half or so. I measure the voltage and they were all 3.38 V. However they do experience reversible capacity fade. I discharged all the cells and Got 45amp hours the first time. The second cycle I got around 65 amp hours and it continued to increase for five cycles. On the last cycle all the cells measured Close to 100 amp hours.
>
> Sent from my iPhone
>
> On May 12, 2018, at 12:23 PM, Lee Hart via EV <[hidden email]> wrote:
>
>>> From: Cor van de Water via EV <[hidden email]>
>>> I also did tests on LiFePO4 cells and while self-discharge was low, I was able to prove from my measurements over many weeks, that there is was about a factor 2 difference in self-discharge current between the best and worst cell. Sample size was over 40 cells.
>> This is what I have found as well. Brand new cells, all bought at the same time from a quality source are very similar. Their amphour capacity, internal resistance, and self-discharge rates are very close.
>>
>> But cells from cheap or low-quality sources have a much broader spread in characteristics. Cells also get worse as they get old, or get cycled, or as the temperature changes. Differences between cells accumulate over time, getting worse and worse. You may get by without a BMS initially, but it gets needed more as the differences between cells grows Not having a BMS means shorter life.
>>
>>> This is exactly what the author of the quoted article found, that the capacity of the cells had not degraded, but the cells had gotten out of balance.
>> Yes. The BMS was too primitive to do its job of balancing the cells. Only having upper and lower voltage limits prevents against catastrophic failures; but does not compensate for differences between cells.
>>
>>
>> --
>> Excellence does not require perfection. -- Henry James
>> --
>> Lee A. Hart http://www.sunrise-ev.com
>> _______________________________________________
>> UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub
>> http://lists.evdl.org/listinfo.cgi/ev-evdl.org
>> Please discuss EV drag racing at NEDRA (http://groups.yahoo.com/group/NEDRA)
>>
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Re: Lithium drop in replacements.

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In reply to this post by Electric Vehicle Discussion List mailing list
I have some headway cells in the original boxes, charge them every few
months, if I wait too long some will be 2V and others 3.65, from being
equal the time before. I have jigs to measure self discharge directly, a
4 wire very accurate charger with a current measurement, I calculate the
self discharge from that. some have zero, i can't remember the worse
number, but it corresponded to 5% per month, as per manufacturer's spec.
I wrote to them, asking if I can get them in tighter groups, but no answer.
   The reason it this started was that a pack Verne got for his bike was
very low capacity, found no cells were actually faulty, just at the
limits of self discharge.


On 15-May-18 1:18 AM, paul dove via EV wrote:

> Hey thanks for your input. I’m sure a lot of people have varied experiences. I must’ve met there probably differences between manufacturers. However that was not my experience. I bought Bestgo sales hundred amp hour. I did extensive testing before installing them in a vehicle. I put 40 for 100 amp our cells in a 1986 Toyota Celica. I charge them 3.65 V per cell or 160.6 V.  After sitting for a while the voltage dropped 148.7 v or 3.38 volts per cell. I drove the vehicle every day for two years. There was never a variance at the end of charge. All the cells measured 3.38 V several hours after charging. I took the cells out of the vehicle And they said on the shelf for year and a half or so. I measure the voltage and they were all 3.38 V. However they do experience reversible capacity fade. I discharged all the cells and Got 45amp hours the first time. The second cycle I got around 65 amp hours and it continued to increase for five cycles. On the last cycle all the cells measured Close to 100 amp hours.
>
> Sent from my iPhone
>
> On May 12, 2018, at 12:23 PM, Lee Hart via EV <[hidden email]> wrote:
>
>>> From: Cor van de Water via EV <[hidden email]>
>>> I also did tests on LiFePO4 cells and while self-discharge was low, I was able to prove from my measurements over many weeks, that there is was about a factor 2 difference in self-discharge current between the best and worst cell. Sample size was over 40 cells.
>> This is what I have found as well. Brand new cells, all bought at the same time from a quality source are very similar. Their amphour capacity, internal resistance, and self-discharge rates are very close.
>>
>> But cells from cheap or low-quality sources have a much broader spread in characteristics. Cells also get worse as they get old, or get cycled, or as the temperature changes. Differences between cells accumulate over time, getting worse and worse. You may get by without a BMS initially, but it gets needed more as the differences between cells grows Not having a BMS means shorter life.
>>
>>> This is exactly what the author of the quoted article found, that the capacity of the cells had not degraded, but the cells had gotten out of balance.
>> Yes. The BMS was too primitive to do its job of balancing the cells. Only having upper and lower voltage limits prevents against catastrophic failures; but does not compensate for differences between cells.
>>
>>
>> --
>> Excellence does not require perfection. -- Henry James
>> --
>> Lee A. Hart http://www.sunrise-ev.com
>> _______________________________________________
>> UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub
>> http://lists.evdl.org/listinfo.cgi/ev-evdl.org
>> Please discuss EV drag racing at NEDRA (http://groups.yahoo.com/group/NEDRA)
>>
> _______________________________________________
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>


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