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Cell voltage never over 3.8V? Bad idea or ok?
#1
Do you think it would harm cells (Lithium ion 4.2V) if they were never charged over 3.8 Volts? I think I was reading an article from Battery University where they told that 'issues' occure when cells are not charged over 3.9 Volts. Whats your opinion?

Asking because I have a micro inverter that should not get more than 54 Volts and I'm building a 14S system. And merry Christmas btw Big Grin
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#2
Of course it won't harm them. You'll get ridiculously long life span from doing it. However, that's slightly lower than peak performance on capacity. You'll loose more capacity from 3.8-4.0 than you'd loose from 4.0 - 4.2.
The draw back is when a load is applied, it could voltage drop too low. could

At 54V, you can ran 13s if you'd like. That would charge all the cells to 4.15V. Voltage drop should be around 4.08 - 4.1V on initial load so you'd have good run time.
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#3
Just to echo @Korishan - For 2019, I set my 14s Lithium Ion battery with a 49.5v low/inverter-cuttoff and 56.4v hi/float.   That's 3.54v/cell - 4.03v/cell.  In practice, I averaged a high of 53v (3.79v/cell) for the year and I'm well pleased with this as I'm hoping it will lead to a long life for my battery bank.

However, Solar power + consumption can vary greatly day to day.   In spite of my low overall average, I did on occasion go up to 4.03v/cell because of high PV power and low consumption on those days.   My goal is to consume 100% of all PV power produced by the panels.  You need to design your system accommodate a range of scenarios depending on your goals.     

Design:
If the inverter is 54v max you could do as @Korishan suggested and enforce this via 13s (4.15v/cell max).   However you didn't say the low for the inverter - for 13s it will need to work down to at least 45.5v (13 x 3.5v) or even lower (13 x 3.1 = 40.3v) to access the lower range of your battery bank.     You could instead enforce the 54v max (as I do) by the charge controller settings. 

The problem with 13s vs 14s is that 'most inverters' work very well in the 14s voltage range - and you might find you need to buy newer/larger inverters in the future and will likely have more options if you go 14s now.
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#4
Yes, I just tested a 100p cell from 3.8V to 4.0V. In this voltage area I have around 50Ah what is equal to 1/4 of the capacity from 3.3V to 4.0. But if it would still make my cells live longer it'll be fine. 3.3V to 4.2 Volts is also much more capacity. About 280Ah but it will kill the cells earlier.

I was just afraid that the cells wouldn't last as long if going even deeper in voltage.

When pulling 30A out of my 100p pack its a voltage drop around 0.250V. But on 14S 30A is alot. Around 1.5 kWs if my calculation is right.
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#5
(12-22-2019, 07:19 PM)Maniac_Powerwall Wrote: Yes, I just tested a 100p cell from 3.8V to 4.0V. In this voltage area I have around 50Ah what is equal to 1/4 of the capacity from 3.3V to 4.0. But if it would still make my cells live longer it'll be fine. 3.3V to 4.2 Volts is also much more capacity. About 280Ah but it will kill the cells earlier.

I was just afraid that the cells wouldn't last as long if going even deeper in voltage.

When pulling 30A out of my 100p pack its a voltage drop around 0.250V. But on 14S 30A is alot. Around 1.5 kWs if my calculation is right.


>I was just afraid that the cells wouldn't last as long if going even deeper in voltage.
The general consensus is to stay above the 'sharp bend' in the discharge curve - which is 3.5-3.6v/cell. There's not much power after you hit the knee - e.g. 3.5 -> 3.0 - anyway. The other thought is that the lower you charge them up (above 3.5/3.6) the longer they will last. Most agree that 4.0 max vs 4.1 max will lengthen life.

3.9v vs 4.0v - not as clear... we all see the Battery University article that indicates the lower you go as your max - e.g. 3.8v vs 3.9v - will increase your number of cycles... but really there isn't much hard data on 2,000(5.5yrs) or 3,000 or 5,000 cycles - and especially on DIY powerwall scenarios. In my case - I'm just designing for lower top but trying not to obsess too much. My top goal is to consume 100% of PV energy I produce.


>30A is alot. Around 1.5 kWs 
if my calculation is right.
Agree with your calculation.
 
>30A is alot
My system is designed to pull a max of 250a from my 14s (12,000watt inverter).   I don't see it as a question of 30a being 'a lot' but rather....

1) Number of cells vs amps.   You suggest 100cells per each pack?  then that's 30a/100cells = 300ma/cell - which is a nice light load.    I don't know your cell specs but I'm guessing that 300ma is well within specs.     I have enough cells that circuit breaker enforces 250a = 600ma/cell and typically the system peaks at 150a range = 415ma/cell.    Used OPUS at 500ma discharge to test/verify all the cells used - so I'm confident that 500ma/cell is perfectly OK. 

2) Overall battery busbars/wiring and fusing.   I have a 250a circuit breaker to enforce the max amps and wiring to match. 
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#6
Its not really only about voltage span. Its about the dod ( depth of discharge) or even better the capacity used per cycle.

So the shorter span the more cycles you get. And that works best in middle of the Soc due to how the chemistry is built Up. Most of the lithium cells work like that
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#7
My first setup will only be used to power a computer a NAS and some electronic goodies. Not taking more than 2-300 Watts :-) So in this case 30A is alot

Yes I know it is about SOC but what I can messure the easiest way is the voltage. And on Lithium Ion this is quite equal to SOC.
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#8
In top and bottom yes but in middle its flatter. Also you can only do it after 5 min testing. So what you Will see is above 70 and below 40. The other is Hard to say for sure without colloumb counter
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#9
Yes that's clear for me.
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#10
at what point do we extend the cycle life so much we hit calendar life?
there has to be a sweet spot where we use up cycle life before old age takes them out.
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