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

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. My Curtis 1238 AC controller did just that
once when I pushed my range, limited my current such that I could creep home
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 will
go a bit higher, some lower, but none in the danger zone. Even if capacity
drops over time the voltage should still have the same relationship to SOC.
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

martinwinlow wrote:
> My 2c worth - Yes, with good initially well balanced LiFePo4 cells,
> kept together or equally well insulated and thermally managed and
> never imbalanced by unequal parasitic loads on one or more cells
> within the pack... you can get away with just switching off the
> charger at pack size (in cells) times 3.6V or whatever. But if you
> are serious about your EV being a 'real' car rather than an expensive
> experiment or 'grand project' - especially if you expect anyone else
> to drive it - you MUST have protection against over discharge in the
> form of a BMonS to provide a warning of impending doom or a BMMS to
> take it out of the drivers hands.
> If you go down Jack's route of basing your 'empty' pack state on Ah
> counting, eventually, as the pack deteriorates, you will kill one or
> more cells. It seems as obvious to me as it is inevitable. Perhaps
> the cells won't ever deteriorate to the point where Ah counting
> without a BMonS causes a failure. It is entirely possible if you never
> discharge the cells below 70% or so (in the lifetime of the EV at
> least). But one day...
> Perhaps I have missed Jack's point - I'm sure he will set me straight
> if I have!
> Regards, Martin Winlow
> Herts, UK

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