[EVDL List]

Note: I am not a Li-ion cell chemist; (all I know if from the research I did
for the "Battery Management Systems for Large Lithium-Ion Battery Packs"
book).

In general, Li-ion cells suffer irreversible chemical damage if their OCV
drops below a give threshold (1.9 V for LiFePO4). (Note that a cell can be
below that limit under heavy load, as long as it then snaps back to an OCV
above 1.9 V.)

However, A123 M1 (26650) cells are different and have been shown to not only
survive being discharged to 0, but actually have a lower internal resistance
afterward. I have no knowledge on how that affects their lifetime and
capacity.

Regardless, all cells will be very badly damaged by reverse voltage.

There is a significant difference depending on how cells are discharged:
1) If cells are discharged by a load across them (a balancer with a high
stand-by current), they will be discharged to 0 V and stop.
2) But is the complete battery is discharged by an external load, the first
cells to reach zero volts will then start charging in the reverse direction
(powered by the cells that still have some voltage), and will be very badly
damaged.

I believe that in your case (ALM 12V7 batteries) it is situation #1.

Again, just because a cell appears to recover, doesn't mean that its life or
its capacity won't have been reduced.

Please ask Bill Dube' for more insights on this.

-----
Davide Andrea
Elithion
--
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[EVDL List]

[quote]Lee Hart wrote:

> I can imagine a single cell not being harmed by 0 volts; a few other
> types of cells can survive this. But it is much harder to imagine a pack
> of series cells surviving it. For example, individual nicad cells will
> being run down to 0 volts. But a series string of nicads can't be run
> dead without a significant chance that one or more cells will get
> reversed, and short out.
>
> The pack Mark Hanson has *is* a series string.

Bill can clarify if I have this correct or not, but from what he has said, my understanding is that the *cells* in Mark's pack were *NOT* at 0V.

The *terminal* voltage read 0V because the internal BMS had disconnected the cells from the terminals due to their voltage being below some non-zero threshold.

Either the BMS provides a path for charge current to flow into the cells, or reconnects the cells to the external terminals when it detects an external source of power (charger); either way, connecting a charger to the apparently 0V module allows the cells to be charged from their non-zero but low state back into a normal operating range.

Mark did not open the module to check individual cell voltages, and the module is not a simple series string of cells connected directly between the external module temrinals.

Cheers,

Roger.


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[quote]Bill Dube wrote:
> On the ALM 12V7, the BMS is set to turn off the output terminals if
> any cell goes below something around 2 volts...After the output shut-off, cells
> will continue to self-discharge. They may go to zero volts
> eventually. You don't care if they do or not.

The internal BMS may turn off the output, but won't it continue to load
the cells itself? These cells have a very slow self-discharge rate; I
have A123 cells that have sat for over 3 years and are still above 3v.

If these *new* batteries have shut down because of a low cell, their BMS
must draw a lot of power. And it may continue to draw power even after
it has turned off the external terminals. That worries me. All the BMS
chips I've seen draw power from all cells, and aren't smart enough to
stop if one cell is totally dead. In other words, the BMS itself can
reverse a cell.

The description of Phoenix Contact's design so far does not inspire
confidence. The pack was delivered dead. Their charger won't recharge a
dead pack. The float voltage appears to be too low.

> I also should emphasize that the cells themselves really are not
> damaged by discharging to zero volts.

Do you have any test data on this? I would like to know what effect on
capacity, cycle life, etc. going to 0 volts will have.

--
An engineer can do for a nickel what any damn fool can do for a dollar.
-- Henry Ford
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart at earthlink.net

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[EVDL List]

Lee, you are applying you many years of battery experience from other
chemistries mistakenly to LiFePO4. The LiFePO4 chemistry behaves in a
_completely_ different manner than any other battery chemistry. Completely.

For LiFePO4, the OCV only changes very near the end points of the SOC
curve. Voltage is pretty much flat for most of the SOC curve.
Completely unlike any other battery chemistry. Thus, the cells will
read over 3 volts for years. The voltage only goes low at the very
end of the SOC.

There is darn close to zero remaining capacity below 2 volts OCV.
Even a tiny BMS load or simply self discharge will bring a 2 volt
cell down to zero volts much more quickly than this might occur in
other battery chemistries.

I have no clue about the details of the ALM 12V7 BMS. Perhaps it
loads the cells after output shut-off. Perhaps it doesn't. It really
doesn't matter because the cells are unharmed by going to zero volts.

>>>> personal anecdote <<<

I have personally discharged a 12 volt LiFePO4 battery to zero volts.
(Glove box light left on.) The battery was at zero volts for about
two weeks. Since the goal of building that 12v battery was to test it
under typical consumer use, I jump started the car with a
fully-charged 12 volt lead-acid battery, just like a consumer might
do. The jumper cables became noticeably warm from the large current
flow into the zero-volt LiFePO4 battery. Started the engine and
quickly disconnected the jumper cables, allowing the full 100 amps
from the alternator into the LiFePO4 battery. This would have been
certain battery death, likely an explosion, and surely a fire with
LiPo. Five years and 60,000 miles later, that 12v LiFePO4 battery
still working perfectly.

Going to zero volts (but not below!) with LiFePO4 does not harm the
cells in any way. It is a fact.

Bill D.


> > On the ALM 12V7, the BMS is set to turn off the output terminals if
> > any cell goes below something around 2 volts...After the output
> shut-off, cells
> > will continue to self-discharge. They may go to zero volts
> > eventually. You don't care if they do or not.
>
>The internal BMS may turn off the output, but won't it continue to load
>the cells itself? These cells have a very slow self-discharge rate; I
>have A123 cells that have sat for over 3 years and are still above 3v.

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[EVDL List]

Lee Hart wrote
>
> All the BMS chips I've seen draw power from all cells
>

In general, you're right. However, the Linear Technology LTC6803 may be
powered separately, or you can place a switch between the battery voltage
and the IC's power input pin, which can be opened when the battery is not in
use or empty.

And don't forget that not all BMSs use an off-the-shelf BMS IC.


Lee Hart wrote
>
> the BMS itself can reverse a cell.
>

Not with a distributed BMS (MiniBMS, Elektromotus, EV power, Elite,
Elithion, Pacific EV, Tritium...), because each cell board gets power from
its own cell, not from the series string.

Now, yes, a DC-DC converter that powers the BMS could reverse a cell, but I
can't see a way that the distributed BMS itself can do so.




-----
Davide Andrea
Elithion
--
View this message in context: http://electric-vehicle-discussion-list.413529.n4.nabble.com/A123-ALM-12V7-Batteries-now-charging-tp4656170p4656219.html
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