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Re: [EVDL] Phd in Materials Science on LiFePO4 batteries video WAS: Lithium 60ah 144v

Collin Kidder wrote:
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
> 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.An occasional check and possible re-balance if needed is simple and
> free, and so far unnecessary.

> You really should set your charge point higher than that. 3.4v is not a
> sufficient average to actually charge lithium cells. Actually it is. 3.4
> average means that a few of the smaller capacity cells will get near 3.45
> which is into the knee. Since lithium prefers to be undercharged and
> there isn't much capacity above 3.45 anyway there is no good reason to go
> higher. My cells are all within less than 4% same capacity, if they were
> closer to 2% I probably would go higher than 3.4 average as they would
> come up closer together, probably 3.45. As it is if I did that the
> smaller cells would start going beyond 3.55 while the larger cells were
> still just above 3.40 I should point out the actual voltage point depends
> on the charger. The Manznanita PFC's don't hold the voltage precisely and
> allow it to drift up so some leeway must be allowed for. I believe some
> other chargers hold the voltage more tightly. An external pack level
> voltage monitor that shuts down the PFC would probably be useful to make
> it more precise, especially when charging at different currents.

> 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? Because with
> bottom balanced cells the differences in resistance will not cause that
> great a voltage difference. After hitting 1.77 under load they all sprang
> back to around 2.45 and climbing.

>> The main point that I got from the video is to get closely matched cells
>> and
>> not bother with a BMS and stay away from the ends of the curve, which is
>> the
>> same thing I've felt all along. If you use a BMS to push your cells
>> further
>> 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
>> cells.
> 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.
I just think we'll get more bang for the buck by pushing for more closely
matched cells, even if we have to pay extra, than paying for a monitoring
system which doesn't actually fix the problem of mismatched cells. I'm ok
with occasional manual checking, and most of the time that only means
checking the 3 or 4 smallest capacity cells in my pack.
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