# Re: [EVDL] A123 ALM 12V7 Batteries now charging



## EVDL List (Jul 27, 2007)

*Re: [EVDL] A123 ALM 12V7 Batteries now charging*



> 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
> ...


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## EVDL List (Jul 27, 2007)

*Re: [EVDL] A123 ALM 12V7 Batteries now charging*



> 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
> ...


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## EVDL List (Jul 27, 2007)

*Re: [EVDL] A123 ALM 12V7 Batteries now charging*

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 (Jul 27, 2007)

*Re: [EVDL] A123 ALM 12V7 Batteries now charging*

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 
--
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Sent from the Electric Vehicle Discussion List mailing list archive at Nabble.com.

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