Well... Hysteresis is there and you can maintain the CV phase forever.
It will keep the cell at full.
We did single cell charger programs for large prismatic cells with
CC/CV curve in a way that when the LFP cell reaches 4.3V and current
less then 2A the charger goes to 'wait' -mode. Charger waits for few
minutes or longer (depends on the selected program) and sets the
voltage at the terminals as CV point. I_max was set to 1A. Then it
kept cell fully charged until it was removed from the charging system.
This is the way how we initiated cells after long transport.
Systematically all cells. (There were failsafes too to detect bad
cells)
Doing this (not exactly as described above but close) after few years
of operation it refreshens the cell. Regains capacity.
You can use the LiFePO4 cells nearly all the ways you can and they'll
provide the lifetime of the conversion.
-akkuJukka
http://www.google.com/profiles/jarviju#about
2012/7/11 David Dymaxion <xxx@xxx.xxx>:
> Jack Rickard's chart: http://23.21.184.60/evtv-word-press/wp-content/uploads/2012/07/CAchargecurve.jpgshows going to 3.6V put 187 Ahr into a 180 Ahr cell, and that continuing to hold 3.6V put in an addition 1.6% capacity (up to 190 Ahr). So yes, stopping at 3.6V does slightly undercharge, but not by much and that could be safer than trying to run right to the limit (Lithium is less tolerant of overcharge than lead).
>
>
>
> ________________________________
> From: Roger Stockton <xxx@xxx.xxx>
> To: Electric Vehicle Discussion List <xxx@xxx.xxx.edu>
> Sent: Wednesday, July 11, 2012 9:06 AM
> Subject: Re: [EVDL] Lithium concerns
>
> Cor van de Water wrote:
>
>> It may sound strange, but you may be overcharging
>> the cells by keeping them at 3.6V, because the
>> charging protocol is to charge them at a certain
>> current, for example 0.1C (which would be 10A in your case)
>> but I forget how much it exactly is - see the manufacturer
>> spec or search the archives, and as soon as the cell reaches
>> the 3.6V level you should stop charging it.
>> By keeping it at 3.6 a BMS will over-charge the cell.
>
> This is incorrect. The typical charge protocol for lithium chemistries is a constant current/constant voltage regimen. This involves charging at a high rate until the voltage reaches a target level, then *holding* this level until the current drops below some minimum threshold. This threshold is generally on the order of 0.01CA.
>
> If the cell is not held at the voltage target until the current falls to a low level, then they will not be fully charged. As you note, some people prefer not to fully charge their cells.
>
> Holding the cells at the voltage target for a long time may indeed overcharge the cells, but terminating charge immediately upon reaching the voltage target will, generally, undercharge them.
>
> Cheers,
>
> Roger.
>
>
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