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Inverter Charging Settings
#1
Hey Guys,

I need a little help, I have a Axpert 3KVA 24V inverter, been running on Lead Acid till now, I am about to switch over to a 4 x 7s80p 18650 power wall and just want some validation on the settings I need to configure on the inverter, This particular inverter does have a Li-Ion option it has AGM and FLd. and a User-Defined option, it's here where I can configure the voltages.

So based on this manual I set the following settings :
http://voltronicpower.com/content/downlo...manual.pdf

05 - USE  "User Defined"
26 - 28.2v "Bluk Charge" This is the max the Charger will accept"
27 - 28.7v "Floating Charge 4.1v per cell"
29 - 21v "Low DC cut-off voltage   (3.0v per cell)

Does this seem right ?
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#2
(07-18-2020, 07:28 PM)rolfbartels Wrote: 26 - 28.2v "Bluk Charge" This is the max the Charger will accept"
27 - 28.7v "Floating Charge 4.1v per cell"
29 - 21v "Low DC cut-off voltage   (3.0v per cell)

Does this seem right ?
A typical lithium-ion cell can go from 4.2v (max charge) to 3.0v (minimum charge).
However, if you do this, the life (number of cycles of charge/discharge) will be *greatly* reduced.   

Max v per cell considerations:
* If you allow charge up to 4.2v/cell (29.4v) then the 7 cells in series will need to be perfectly balanced so that one or more cells (of 7 in series) don't go to 4.24v or 4.27v while another is at 4.1v -  its not good / fire risk to go above 4.2v/cell.  
* There is hardly any power between 4.15v and 4.2v / cell - so forcing things to 4.2v will greatly reduce life-span and not get you much power.
* Read this article - https://batteryuniversity.com/learn/arti...0V%2Fcell.  and you'll see research in Table 4 that says 4.2v = 300-500 cycle BUT 4.1v = 600-1000 cycles and 4.0v = 1,200 to 2,000 cycles.   Clearly, a lower max voltage (along with higher min voltage) can add 1) safety to avoid any cell going over 4.2v and 2) greatly increase your life span.


>26 - 28.2v "Bluk Charge" This is the max the Charger will accept"
>27 - 28.7v "Floating Charge 4.1v per cell"
Based on the above - these settings are perfectly good.   You may even want to lower them to 4.0v/cell if you can afford less daily power but want longer life.  Its up to you.   I would measure the battery with a volt meter to see battery voltage that results from these settings...  the battery itself could be up to 0.5v higher or lower depending on various factors.   Let it get in to 'float' and then go out and measure the battery v...  and then you can adjust accordingly if needed Smile


Min v per cell considerations:
* Yes, cells can typically go down to 2.8v-3.0v *BUT* a little bit similar to the max voltage situation the life span will be reduced if you go to the bottom every day.  
* The power curve - e.g. the power available in the cell based on voltage takes a sharp drop at 3.4-3.5v/cell.  This means that there isn't much power after you get below 3.4v/cell (23.8v) but yet you risk shortening the life of the cells.

>29 - 21v "Low DC cut-off voltage   (3.0v per cell)
Personally - I would suggest the 3.4v/cell (23.8v)  or 3.5v/cell (24.5v) as a cut-off setting.     You can actually make a chart of your battery bank by simply putting a steady load on the battery and measuring voltage at regular intervals.  You will see voltage drop steadily per time until 3.4 or 3.3 ... and then voltage drop very rapidly.    Then you can use this 'discharge knee' in the curve to guide your decision. 


Discussion:
Perfectly OK to experiment with 4.0 or 4.1 etc and 3.4 or 3.5 or 3.3...  min/max settings and just measure / track your voltages vs power output.   There is no 'one right answer' as long as you don't exceed the min/max specs for your cells - its just the lifespan that is affected the most.   This voltage range is called the DOD (depth of discharge).   If you look at Figure 6 on that Battery University link above - you can see a set of data showing the dramatic affect of life-span on hi/low voltage ranges.

For me - I'm 100% off-grid, so my goal is to consume all the PV power I create thru the night that I didn't use during the day - and I use my battery bank to store the excess daily PV power.    The larger the battery, the less DOD it takes.    I have designed my system to use < 50% DOD on a daily basis...  and after experimentation I've found that 4.0v max and 3.54v min works for me.
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#3
Exactly as OffGridInTheCity says.
I only have 1 Pack so I run mine up to 4.05 and down to 3.45 right now.
Once I get my second pack installed I will change that to 4.0 and 3.5

Wolf
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#4
Thanks for the response guys, I will have 19kw when done in about 3 weeks time I only consume maybe 10wk a day, so yeah Ill drop my max and in crease my min.
thanks again for the help
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#5
(07-18-2020, 10:20 PM)OffGridInTheCity Wrote: * Read this article - https://batteryuniversity.com/learn/arti...0V%2Fcell.  and you'll see research in Table 4 that says 4.2v = 300-500 cycle BUT 4.1v = 600-1000 cycles and 4.0v = 1,200 to 2,000 cycles.   Clearly, a lower max voltage (along with higher min voltage) can add 1) safety to avoid any cell going over 4.2v and 2) greatly increase your life span.

It is true that - unlike most battery electochemistries - Li-ion is distinguished by the fact that we can greatly prolong its life by reducing the depth of (dis)charge (and centering it around 50% SOC). But the numbers quoted by BU = BatteryUniversity are misleading (and useless) because these numbers strongly depend on many parameters (e.g. chemistry, rates, temperature). 

As is typical, BU gives no hint of this, nor do they even give a proper citation where one can find this necessary info (often only "courtesy of Cadex" - the author's charger company). If you wish to learn more then follow the above link to a post where I explain these matters a bit further, and give links to peer-reviewed publications.

Due to this and other detriments, generally Battery "University" is not regarded as a reliable source of information on battery technology. It is even blacklisted by Wikipedia. It's raison d'etre seems to be marketing the author's company (whose name is plugged around 450 times) rather than seriously teaching battery technology.

Generally there is a lot of misleading info on Li-ion batteries on the web because manufacturers refused to release it to the general public, so that left a knowledge vacuum - which was filled by self-proclaimed "experts", charlatans, pseudoscientists etc. The result is that we have to be very careful evaluating what we read on the web (much more so than for other scientific topics). But at least nowadays there are many journals freely accessible online. so we can (somewhat) workaround this hoarding of knowledge by the manufacturers  - which was also motivated by patent wars, e.g.  

Quote:Michel Armand is still furious. To Armand, patents "are tearing the community apart." The saga of lithium iron phosphate is a "horror story" of "meanness and greed." "Oh, yeah, of course, he feels very angry," Goodenough said. "Because I must say Michel Armand was the one who recognized that the LiFePO4 was potentially very interesting." ...

"You need to talk to Michel Armand! He's just a broken, broken man. The last time he was here, you know what he said? He said, ‘The number-one property of lithium iron phosphate is that it is an excellent catalyst for human greed.

The Li-ion patent wars have been raging since the dawn of the technology. You can read about them in many popular accounts, e.g. above is an excerpt from Gizmodo's The Knock-Down, Drag-Out Fight Over the Next Generation of Batteries, which is excerpted from Seth Fletcher's book Bottled Lightning: Superbatteries, Electric Cars, and the New Lithium Economy - a must read for anyone interested in the history of Li-ion batteries.

By the way, LiFePO4 (lithium iron phosphate) may end up winning the battle after all, since rumor has it that Telsa has made some recent breakthroughs that reach the holy grail of $100/kWh combined with super-long life - so long that it enables car batteries to be recycled as powerwalls. Evil cobalt may soon be a thing of the past...
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#6
I am looking to understand the bulk and float charge settings I need to set
I want to have the max voltage 4.05 and the Min voltage 3.4v

I have set the following setting

05 - use
26 - 28v
27 - 28.35v
29 - 23.8v

29 I get as that's 3.4v per cell cut off, 26 and 27 I'm not sure do I set the float to 4.05v per cell and the bulk to 4.1v ? At the moment my bulk is at 4v and the float at 4.05, I'm not really sure if bulk needs to be less than my desired end volt or more, and I assume float is my 4.05v ?
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#7
(07-22-2020, 02:07 PM)rolfbartels Wrote: I am looking to understand the bulk and float charge settings I need to set
I want to have the max voltage 4.05 and the Min voltage 3.4v

I have set the following setting

05 - use
26 - 28v
27 - 28.35v
29 - 23.8v

29 I get as that's 3.4v per cell cut off, 26 and 27 I'm not sure do I set the float to 4.05v per cell and the bulk to 4.1v ? At the moment my bulk is at 4v and the float at 4.05, I'm not really sure if bulk needs to be less than my desired end volt or more, and I assume float is my 4.05v ?
I'm not an expert in your exact equipment..  but bulk charge is a CC (constant current) charge and float is a CV (constant voltage) charge. 
*  CC is max current going into the battery...  which requires the charger to raise it's voltage higher than the battery voltage by a certain amount...  to encourage max current to flow.   
* CV is a 'topping off' - e.g. the charger keeps the voltage steady and as the battery catches up...  the current flow will get less and less. 

For Lithium ion I don't recommend that Bulk/CC or Float/CV go above 4.2v.   So for me, I set my Bulk and Float voltage as close as I can to each other.  In my case it requires me to set Bulk 0.01v higher than Float - e.g. it doesn't matter...  I just set them both for the top voltage I want the battery to achieve.  In your case - 4.00v or 4.05v/cell ...  will charge your battery up to 4*7 = 28.0v or 4.05*7 = 28.35v.    28.0v vs 28.35v is trivial - doesn't really matter. 

Again, I would suggest you set things to 4.00v or 4.05v/cell and charge up 100% and then *measure your battery directly with a voltmeter* or read the voltage somehow to confirm what it actually does.     Let's say you set 4.05v and the battery reads a bit higher  - then you could accept that or back the setting off a bit to make the the battery be the actual voltage you want as in 28.0v.     

>29 I get as that's 3.4v per cell cut off,
29 / 7 = 4.14v.   Maybe you're saying you are going to use 3.4v * 7 = 23.8v as your cutt-off?   That sounds fine. 

I hope that helps..
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#8
Thank you, for your help
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