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Making your battery cycle properly despite useage pattern
#1
I'd like to see what comments I get on this.?

My LiFePo4 system is that I build in protection like an algebraic equation. Brackets ,inside brackets ,inside brackets.

In the inner brackets I have a voltage controlled switch on the Photovoltaic negative line.
In the next set of brackets I have the charge controllers settings.
In the outside brackets is the BMS settings as last resort.

Like this on 24 volts LiFePo4
(BMS low 20v (Charger Low 25v ( PVSwitch Low 26.5v... High 27.6v ) Charger High 27.7v) BMS High 28.2v)

This ensures that when not used heavily the battery cycles down to at least 26.5v before any charge can come from the panels.
Its disconnects the panels when my battery reaches about 3.5volts per cell

Should something go wrong then the chargers settings will cut off charge to the battery but leave the battery connected.

Should all that fail high or low , the BMS would then disconnect the entire battery.
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#2
>In the inner brackets I have a voltage controlled switch on the Photovoltaic negative line
Do you mean you have a CB on PV array coming into the charge controller(s)?.    If so, then that makes sense so you can isolate the Charge Controller from the PV array to work on it.  However, this is not needed for the battery because....

>In the next set of brackets I have the charge controllers settings.
A good Charge Controller will 'protect' the battery from the incoming PV power thru it's settings.  The Charge Controller will turn on or off the charging current based on these settings.

In my case, I use the Charge Controller to adjust the operating voltage range of the daily cycle - and agree that this range is smaller than the BMS range.    As you say, the BMS is the outer edge / protection for hi/low volts and temp.

Note: I do have CBs between the charge side of the Charge Controller and the Battery - but again this is to isolate the Charge Controller to work on it rather than protection for the battery.
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#3
Yep the point is the voltage controlled switch in there is
1/ Extra protection wrapper
2/ Allows you to make the battery discharge down to say 40% SOC before it can start to take power from the PV cells.

This is handy on a boat where when on board you use lots of power and when not there the battery is being continually topped up , which despite contrary opinions is not good for LiFePo4 or LiIon.

Same with a holiday house or the like.

You set the voltage it switches on and off . Its not a CB exactly
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#4
This is a solution searching for a problem that does not exist.
ajw22 likes this post
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#5
(09-10-2020, 12:43 AM)tytower Wrote: 2/ Allows you to make the battery discharge down to say 40% SOC before it can start to take power from the PV cells.
I don't get what's gained by forcing a battery to go down to 40% SOC.    Seems like it would be better to set long-life top such as 75% DOD (or something like that) and then use PV power (instead of battery power) if its available - and perhaps the battery won't go down to 40% SOC as often....  extending it's life.   

But then I often don't get things Smile   Maybe you could explain more?
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#6
(09-10-2020, 02:15 AM)OffGridInTheCity Wrote: But then I often don't get things Smile   Maybe you could explain more?

Length of life is most often associated with the number of cycles . Ions moving into charge accepting molecules . The molecules expand and contract and gradually lose their ability to take in and release ions due to cracking in the molecule .
A full cycle for LFP and Li Ion is considered better than holding the SOC of the cells high . 
This method does that but don't take my word for it ,read up on it.
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#7
Depends on the "high" you take the cells to.
What voltage do you charge your LiFePo4's to?
Running off solar, DIY & electronics fan :-)
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#8
(09-10-2020, 02:54 AM)tytower Wrote: Length of life is most often associated with the number of cycles . Ions moving into charge accepting molecules . The molecules expand and contract and gradually lose their ability to take in and release ions due to cracking in the molecule .
A full cycle for LFP and Li Ion is considered better than holding the SOC of the cells high . 
This method does that but don't take my word for it ,read up on it.
Please cite sources
later floyd
Crimp Daddy likes this post
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#9
(09-10-2020, 02:54 AM)tytower Wrote: Length of life is most often associated with the number of cycles . Ions moving into charge accepting molecules . The molecules expand and contract and gradually lose their ability to take in and release ions due to cracking in the molecule .
A full cycle for LFP and Li Ion is considered better than holding the SOC of the cells high . 
This method does that but don't take my word for it ,read up on it.

Likely counterproductive.
Those artificial charging condition you put in just increases the number of shallow-cycles, which likely degrades the cell more than simply letting the charger keep it "steady" at 3.5V.
"Steady" in quotes, because in a way, the charger is already doing what you're trying to do, but in super-shallow-cycles with mV and kHz precision - let it do what it was designed for.

Yes, leaving the cells at 100% charge (3.65V?) degrades the cells, but that issue is significantly reduced at 3.5V.
If you need the cells to last even longer, just set the charger to keep it at max 3.45V or whatever.
Modular PowerShelf using 3D printed packs.  60kWh and growing.
https://secondlifestorage.com/showthread.php?tid=6458
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#10
Cmon guys give it a break . How you going to learn with bricks for heads.
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