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

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Discussion Starter · Dec 14, 2010

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

I have 46 TS cells running in parallel with 12 50Ahrs spiral AGMs . I do not
run BMS & never run my bat setup below 120V. I do a manual equalise of the
LIPs every few months but have found it largely not needed. I use a NG3
Zivan with appropriate user adjusted programme. David

--------------------------------------------------
From: "AMPhibian" <[email protected]>
Sent: Tuesday, December 14, 2010 11:46 AM
To: <[email protected]>
Subject: Re: [EVDL] Phd in Materials Science on LiFePO4 batteries video WAS:
Lithium 60ah 144volts vs 100ah 120 volts?

>
> 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
> cells.
>

> 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
>> http://www.evalbum.com/2092
>> www.winlow.co.uk
>>
>>
>>
>>
>>
>
> --
> View this message in context:
> http://electric-vehicle-discussion-list.413529.n4.nabble.com/Phd-in-Materials-Science-on-LiFePO4-batteries-video-WAS-Lithium-60ah-144volts-vs-100ah-120-volts-tp3084043p3086307.html
> Sent from the Electric Vehicle Discussion List mailing list archive at
> Nabble.com.
>
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