Re: [EVDL] Self discharge never sleeps (was: Curtis 1238-6501...)

[Hoegberg via EV]

Från: EV  för Bill Dube via EV 
Skickat: den 9 augusti 2017 21:01
Till: Electric Vehicle Discussion List
Kopia: Hoegberg via EV; Eva Hakansson
Ämne: [EVDL] Self discharge never sleeps (was: Curtis 1238-6501...)

"Self discharge has an _extremely_ strong function of temperature.
http://batteryuniversity.com/learn/article/elevating_self_discharge "

hmm,
Based on my limited experience that seems a bit high for LFP, 
have you seen this for a123-cells thet have been in long storage ?


/John
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Re: [EVDL] Self discharge never sleeps (was: Curtis 1238-6501...)

[Jukka Järvinen via EV]

Each cell is operated in specific SOC window (say 70%DOD between 20 and 90
%SOC) which is matched to other aging mechanisms. So each cell has its own
operational SOC region. Another cell might be used between 25 and 95 %SOC
and another between 15 and 85 %SOC.

Effectively reducing certain amount of capacity from the pack but cells are
'driven' to more balance. So balancing does not mean one has absolutely
same SOC on each cell. Voltages might be all over the place as the
balancing criteria is matched lifetime and service predictability.

-Jukka


ke 9. elokuuta 2017 klo 23.29 EVDL Administrator via EV 
kirjoitti:

> On 9 Aug 2017 at 20:18, Jukka Järvinen via EV wrote:
>
> > This is why SOC windowing was developed.
>
> Could someone please explain what SOC windowing is and how it works?
> Thanks.
>
> David Roden - Akron, Ohio, USA
> EVDL Administrator
>
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Re: [EVDL] Self discharge never sleeps (was: Curtis 1238-6501...)

[EVDL Administrator via EV]

On 9 Aug 2017 at 20:18, Jukka Järvinen via EV wrote:

> This is why SOC windowing was developed.

Could someone please explain what SOC windowing is and how it works?  
Thanks.

David Roden - Akron, Ohio, USA
EVDL Administrator

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EVDL Information: http://www.evdl.org/help/
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Re: [EVDL] Self discharge never sleeps (was: Curtis 1238-6501...)

[Jukka Järvinen via EV]

This is why SOC windowing was developed. To compensate the 'death rate'
among the cells in the pack. Works with high voltage chemistries but with
LFP and LTO one must do 'other stuff'.

-Jukka

ke 9. elokuuta 2017 klo 22.01 Bill Dube via EV 
kirjoitti:

> Self discharge has an _extremely_ strong function of temperature.
> http://batteryuniversity.com/learn/article/elevating_self_discharge
>
> It is also a function of cell health, age, past abuse, etc. The list of
> factors that alter the rate of self-discharge is seeming endless.
>
> Because it is such a strong function of temperature, small variations in
> the temperature of each the many cells in a high voltage pack can cause
> large imbalance because "self-discharge never sleeps".  It chews away on
> the cells 24-7, regardless of whether they charging or discharging or
> simply open circuit. This is a problem because the end cells (that have
> a thick conductor to the outside world,) and cells on the outside edge
> of the pack, have a different thermal environment than do the inner-most
> cells. The temperature of the outside cells (and the end cells) is often
> starkly different than the inner cells. The self discharge is thus
> greatly different, and is typically dependent on the placement of the
> cell within the pack, and the difference between the outside temperature
> and the average pack temperature.
>
> If your BMS happens to have some sort of cell voltage monitor or some
> sort of LED indicators on the cells themselves, this spacial imbalance
> becomes readily apparent. You can literally see the temperature
> variation that manifests itself as a SOC imbalance across the pack. You
> can watch the LEDs on perimeter of the pack light up before the LEDs on
> the inside of the pack when the outside temperature is cooler than the
> pack temperature. When the outside is warmer than the pack, the opposite
> is observed. It is like a topographic map of the cell temperature since
> the last charge. Even if the cell temperature is uniform at the time you
> actually charge and observe, the BMS LEDs will tell the tale of the cell
> temperature history since the previous full charge.
>
> What is particularly insidious, is that contact resistance of the
> terminals, and internal resistance, also greatly effect the temperature
> of individual cells and thus elevate the self-discharge of those
> specific cells. This is why bad connections cause chronic cell imbalance
> and "weak" cells get out of balance. These cells run hotter than the
> rest, and the self-discharge skyrockets.
>
> Bill D.
>
> On 8/8/2017 6:23 PM, Hoegberg via EV wrote:
> > Hi
> >
> > LFP:
> > You might with some(all?) LFP even find a slight hysteresis in pack
> voltage, at exactly the same SOC..
> >
> >   (most visible if you are in the 30-70% SOC-zone)
> > depending on ..if you have had a regen or a discharge pulse as your last
> event,
> > then the no load voltage seems not to be exactly the same, at the same
> SOC.
> > A higher rest voltage if you did a charge/regen pulse compared to if you
> just did a very short discharge.
> >
> > I agree with the others, count Ah is the way ot go to know the SOC % in
> the flat part of the discharge curve,
> >
> > Also my experience was, that decent cells dont have any / a lot of self
> discharge to balance out when in normal use, only milliamps might be needed
> over time, so if they are well (top)balanced once they seems to stay well
> balanced. But if the cells are damaged / have mfg problems from the
> begining then it might be a different situation,
> >
> > Regarding balancers maximum current:
> >   we had a 5 Amp as the charger minimum current, so we did a pulse
> charge instead of use large balance currents,
> >
> > So if one cell reach the "balancing" voltage then we can just stop the
> charger, and wait for that cell to reach its lower voltage, with only 100mA
> or so as balancer discharge current, then we re-enable the charger(5Amp)
> until any cell(s) again reach the balance-start voltage.
> >
> > If you dont have any cell voltage monitoring , or any kind of signal /
> feedback from the balancers, then it might be tricky to do this, I dont
> have any good solution to shut of the charger in time if we dont know when
> we have a problem. (other than to use a lower charge current than your
> balancers can handle, but if one balancer do fail, then you will probably
> overcharge that cell later)
> >
> >   I would prefer to use some kind of good cell voltage monitoring so you
> can get a warning in time if some cell go to low or to high, and also use
> it to shut of/cut down the charger, or cut back on the trottle if some of
> the cells get to low when driving.
> >
> > in my opinion that should be a minimum when charging a large expensive
> pack..of more than 4 cells in series. :-)
> >
> > If we only use the full pack voltage for the charger to decide whan to
> go in to constant voltage mode, then we can get in troubles, for