Re: [EVDL] Phd in Materials Science on LiFePO4 batteries video WAS: Lithium 60ah 144v
AMPhibian <[email protected]> wrote:
> Not sure why you feel you need a BMS to do what your controller and charger
> should do. The controller should start limiting current when pack voltage
> drops low enough, and if your cells are bottom balanced or simply well
> matched there will be no damage.
The problem is, that's a lot of "if" and "should." In a perfect world a BMS
would not be required as all cells would be roughly the same and would age
that way too. However, history has not shown reality to work that way.
Hobbyists also usually don't have perfectly matched cells. Remember, a
matched set of cells is matched in both capacity and series resistance.
That's something GM can do but your average hobbyist is going to find that
harder to reliably get.
Getting a pack into an initial reasonable state is only the beginning. You
can then either religiously check every cell in the pack ever so often to be
sure that you don't have a problem or you can have a BMS do this for you.
And, remember that the big automotive companies do still use BMS systems on
their vehicles! They almost certainly aren't shunt enabled and maybe don't
monitor every single cell individually but they do monitor the pack.
> My Curtis 1238 AC controller did just that
> once when I pushed my range, limited my current such that I could creep
> at 20 mph, cells at 1.77V under load, 2.45 resting, but bottom balanced so
> no single cell dropped near zero. Likewise the charger is set to
> undercharge the cells to about 3.4V per cell and shut down. Some cells
> go a bit higher, some lower, but none in the danger zone.
You really should set your charge point higher than that. 3.4v is not a
sufficient average to actually charge lithium cells. At that point you could
easily have a most every cell never get to the knee and thus you have no
real idea what is going on with them. They end up in a
virtually unmeasurable limbo. Charging a lithium cell (LiFePO4) with 3.4V is
like charging a 12v lead acid battery with 12v. There is a reason that
people tend to charge to at least an average of 3.6v per cell. If you have a
smartphone or laptop then get a voltage program and watch the cell voltage
while it charges. You'll see that they most certainly go into the high side
knee. At your voltage you would have to time the charger for how long it
should take based on charger efficiency and pack capacity and hope it works
out. This is all beside the current point though I guess...
Also, if you aren't using a BMS then your 1.77V under load is an average.
How do you know if one of the cells has a larger series resistance than the
rest and is sagging to 0.5V while the rest are at 1.8V? Though, most BMS
systems I've seen would long since have dropped cell monitoring at that
voltage so they'd be worthless at that point too (and to top it off it might
even damage the BMS monitor)
> The main point that I got from the video is to get closely matched cells
> not bother with a BMS and stay away from the ends of the curve, which is
> same thing I've felt all along. If you use a BMS to push your cells
> you will probably shorten their life, not prolong it. The way I see it a
> BMS is a band aid for mismatched cells, and the proper solution is matched
Note that he NEVER says to forget the BMS. In fact he says that the
attendees should be using one. He says he uses them. He just *also* says
that he tries to properly match cells first. This is good advise but does
not mean that you should immediately forget the rest of what he said. It
seems to me that the best course of action is to get a matched pack and then
monitor it with a BMS to be sure that no trouble pops up.
I'll agree that shunting of current should be unnecessary. Per cell voltage
monitoring seems like a good idea however. Dr Whitacre said that being able
to monitor each cell voltage was engineering wise the right choice if one
could afford it.
I don't know that I can say that I've found a BMS that I think is perfect or
super wonderful yet. I do have one and it seems to work reasonably well.
Hasn't burned the car down but it has helped me to find a couple of weak
cells so that I can replace them. As Martin said, BMS systems allow us to
get data from the pack now which helps to determine probable failure modes,
cell quality, driving data, etc.
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