> From: michael.e.r...@gmail.com
> Date: Tue, 22 Apr 2014 17:48:33 -0400
> To: ev@lists.evdl.org
> Subject: Re: [EVDL] self-discharge of CALB 180Ah LiFePO4 cells
>
> What is a BMS? Very good question. The small packs I have are a complete
> unknown. Each cell is wired with a tiny wire so I presume they are making
> an attempt at top balancing (I am told that is a very large assumption).
> They both have low cut offs, and they appear to communicate with the
> chargers they are shipped with so a green light comes on when "charged."
> In both cases if the chargers were not cutting off they would be reaching
> a voltage that is less than would overcharge any cell in the pack. One
> cuts off at 56V and the other 60V on 16 LFP cells.
>
I think we agree,
Im not 100% sure what you meant, but in general I would not charge LiFePO4 (or
any lithiums..!) without at least a "cell by cell" individual monitoring that
CAN take the "oooh shiiit"-emergency control over the charger or regen and cut
back when any cells conditions is not so normal any more.
(But I also like a throttle down feature at discharges, when the "oh shit"
happens as I drive around here in disasterland ).
For example, it will be a potential problem if we charge more than about 4s of
LiFepo4 cells with a normal lead acid smartcharger and have a 14.0 V cutoff,
say if 3cells was 50% discharged for any reason, and 1 cell till was close to
full.
More than 4 LiFePo4 cells in series for chargers with a "total voltage cut off
only",, will in my experience be bad(and/or dangerous)
Example:
8 LiFePo4 cells in series.
Charging with a constant.current & 3.60V per cell constant.voltage = 28.8V
(28.8 Volt for leadhead-compability here at the list :-)
Assume we have a situation like this:
6 cells, that still have a some % to go before they reach "full", the voltages
are for example 3.50 or 3.55V.
2 cells, that are *now* at 3.60V, almost full ..and they now starts to act
almost as it was capacitors, the voltage will go up much faster and faster at
the already full cells.
-Aha?? So what will the charger do now, in this situation?
It will now continue to push up the poor 3.60-cells trough the roof @ constant
full current..
like this example:
3Cells @ 3.50V = 10.50V
3cells @ 3.55V = 10.65V
Gives a total of 21.15V
+ the 2 already full cells.
-Aha? So what voltage will that 2 cells have now, in this example?
That 2 cells , at the upper limit before, they will be forced up to:
28.8V -21.15V = 7.65V at 2 cells = 3.82 V each.
-Ooops..
So what about my 16Cells pack?
16cells * 3.60V = 57.60 V in total and it was set as the chargers max voltage
(great, if all cells was good and in a perfect and even top-balance.)
But in this Unbalanced charging example we have:
15cells * 3.50 Volt = 52.5
1 cell @ ??? Volt.
Hmm, lets see, we had 57.60 total - 52.5( the 15cells)
= That last cell in this example will therefore see 5.1 Volt..
-ugh.. , =:-o
Ok. So what about a 36 Cells pack then?
Yes, we assume that this charger was set to the recommended 3.650 Volt / cell
= 131.40 V cutoff.
And the situation was full current charging:
32 cells out of 36 measures at the moment about 3.55V /cell = 113.60 V
But 4 cells did already reach almost full, and now they must go on and share
the reminding 17.80 V
= 4.45 V per cell, or maybe even more for some cell.
-Ouch pouch..Oh my dear popcorning antivacuuum bags..! :-/
But how about my 96 Cell pack?
/John
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