lithium survival

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lithium survival

Matt Lacey
Hi Guys (and Girls),
 
heres my notes on a customers scooter (Emax 90s) recently that you may find
interesting:
 
The scooter had been on of action for some time due to a failed motor.
 
for one reason or another, I had forgot to pull the main fuse, so it battery
slowly discharged over a long period of time.
 
the battery is a 16 cell 90Ah pack (TS 90AHA, old case size, 2008 build
date)
when I got to it, the battery pack voltage was a mere 6.3v
each cell was between 0.2v and 0.5v.
 
it was using an EVworks BMS.
each cell module draws ~10mA (IIRC) to run the little green LED, but only
when the voltage is above ~2.6v.
the master module has a internal DC-DC that draws a small current until the
pack voltage is below ~30v.
the scooters internal dc-dc for running the controller micro also draws a
small current until pack voltage is below ~30v.
the cycleanalyst I installed draws a small current until pack voltage gets
down to ~6.5v.
 
the battery charger only works when the battery is above ~35v.
 
after fiddling around with various powesupplies to get the pack voltage high
enough, it went on to recharge at full rate.
it appears all the cells survived.
further testing will determine whether any capacity has been lost.
 
the pack was top balanced when the motor failed.
it is likely the bottom balancing effect of the LEDs prevented any of the
cells from reversing when the cycleanalyst pulled the voltage down so low.
 
These cells sure can survive a fair bit of inattention.
 
Matt
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Re: lithium survival

Lee Hart
On 6/6/2010 8:00 AM, Matt Lacey wrote:

> Hi Guys (and Girls),
>
> heres my notes on a customers scooter (Emax 90s) recently that you may find
> interesting:
>
> The scooter had been on of action for some time due to a failed motor.
>
> for one reason or another, I had forgot to pull the main fuse, so it battery
> slowly discharged over a long period of time.
>
> the battery is a 16 cell 90Ah pack (TS 90AHA, old case size, 2008 build
> date)
> when I got to it, the battery pack voltage was a mere 6.3v
> each cell was between 0.2v and 0.5v.
>
> it was using an EVworks BMS.
> each cell module draws ~10mA (IIRC) to run the little green LED, but only
> when the voltage is above ~2.6v.
> the master module has a internal DC-DC that draws a small current until the
> pack voltage is below ~30v.
> the scooters internal dc-dc for running the controller micro also draws a
> small current until pack voltage is below ~30v.
> the cycleanalyst I installed draws a small current until pack voltage gets
> down to ~6.5v.
>
> the battery charger only works when the battery is above ~35v.
>
> after fiddling around with various powesupplies to get the pack voltage high
> enough, it went on to recharge at full rate.
> it appears all the cells survived.
> further testing will determine whether any capacity has been lost.
>
> the pack was top balanced when the motor failed.
> it is likely the bottom balancing effect of the LEDs prevented any of the
> cells from reversing when the cycleanalyst pulled the voltage down so low.
>
> These cells sure can survive a fair bit of inattention.

Be aware that after a lithium cell is pulled this low in voltge, it can
develop a higher internal resistance. When you charge it, or draw load
current from it, this extra resistance makes it heat up much more than
normal. If you don't have any sort of individual cell temperature
measurement, you can even get a fire during charge or discharge!

--
Lee A. Hart | Ring the bells that still can ring
814 8th Ave N | Forget the perfect offering
Sartell MN 56377 | There is a crack in everything
leeahart earthlink.net | That's how the light gets in -- Leonard Cohen

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Re: lithium survival

Seth Rothenberg
Related question -

With Matt's story, I have hopes of reviving one cell
that was connected to a bms module that was damaged.
(warning: don't put a bms module on backwards! :-)

Lee's comment gives pause to this idea.   My question is...

If a cell is bad (about .5 V), and is NOT being used,
is  it NECESSARY to unbolt it from the "5-pack"?
i.e., if it's NOT being used with its potential higher
internal resistance, does it present a danger?

My inclination is that it doesn't, but it is still
an extra thing with extra exposed metal which is
a magnet to all sorts of metal things.

Thanks
Seth

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Re: lithium survival

tomw
In reply to this post by Lee Hart
This is not an issue.  I have discharged cells down to less than 1V and after recharge they behaved similar to the other cells.  Bill Dube reported on this forum not long ago discharging cells down to low voltage and they were still behaving well two years later.  You can get swelling of cells from severe over-charging which increases internal resistance, but typically not enough to dissipate as much energy in Joule heating as typical lead acid cells, let alone enough to start a fire.  Some people treat all cells which contain lithium as if they are the same.  Laptop batteries are lithium cobalt and behave quite differently than the large prismatic LiFePO4 cells used in most evs.  

"Be aware that after a lithium cell is pulled this low in voltge, it can
develop a higher internal resistance. When you charge it, or draw load
current from it, this extra resistance makes it heat up much more than
normal. If you don't have any sort of individual cell temperature
measurement, you can even get a fire during charge or discharge!"
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Re: lithium survival

Lee Hart
In reply to this post by Seth Rothenberg
Seth Rothenberg wrote:
> If a cell is bad (about .5 V), and is NOT being used,
> is it NECESSARY to unbolt it from the "5-pack"?
> i.e., if it's NOT being used with its potential higher
> internal resistance, does it present a danger?
>
> My inclination is that it doesn't, but it is still
> an extra thing with extra exposed metal which is
> a magnet to all sorts of metal things.

That would be my guess as well. A dead cell is just as combustible as a
full cell, but it doesn't contain the energy needed to start a fire. It
would fuel a fire if something else started it, though.

The risk of leaving the dead cell in the pack is that someone may not
know it's bad, and so try a high current charge or discharge. Forcing
high current through a bad cell with high resistance *could* generate
the heat to start a fire!
--
Lee A. Hart | Ring the bells that still can ring
814 8th Ave N | Forget the perfect offering
Sartell MN 56377 | There is a crack in everything
leeahart earthlink.net | That's how the light gets in -- Leonard Cohen

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