Charging speed


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rearden

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Feb 11, 2019
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I have LiFePo4 26650 cells battery 16s around 270ah with a solark 12k charger/inverter. The solark is fairly sophisticated, monitors power flows and can send power to load, battery or grid.
I get about 4 hours of strong sun with pretty sharp tails due to shading. When I observe the charging curve of the battery, it seems like it is unable to charge completely in the 4 hours and that I have excess solar power. I currently have ~4kw of panels, but my 270ah battery may only take 2-3 kw max of power when charging (in a decreasing curve). So I have both excess production and the batteries seem to not accept enough charge to bring them to 100%.

What are some suggestions to improve the system efficiency? I can't really do much about the shading to broaden the solar curve.
Should I oscillate the battery between some other DoD than 40-100%, like 30% to 80% to maximize accepted charge?
What would this to do battery life?
If I add more batteries to max my solar accepted charge input, then I am limited on DoD because it will only charge so fast.

What is a normal charge rate for LiFePo4 and therefore expected time to restore DoD to 100% (or 90%) from say 40%.

rearden
 

Korishan

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Should I oscillate the battery between some other DoD than 40-100%, like 30% to 80% to maximize accepted charge?
This doesn't change "capacity". It changes the "lifespan". By dropping from 100% down to 80%, you can literally add 1000's of life cycles to the cell.
A life cycle is a full charge/discharge/charge.
I would not go lower than 65%. That's around 2.5V. Going down to 30% that is about 1.15V, which is waaaay to low. 40% is 1.52V, which is to low, too.
What is a normal charge rate for LiFePo4 and therefore expected time to restore DoD to 100% (or 90%) from say 40%.
This is dependent on the cells and what they were designed for. If they were for high discharge current, then you can go really high on the Amps. If they were designed for long run time, then you want a lower Amp charge. You need to look up the specs of what those cells are.

There isn't really much you can do to increase efficiency, other than replace your solar charge controller with a more expensive unit that can do better efficiency transfer. Use larger gauge wire to minimize transmission losses. Make sure all your connections are tight to minimize contact resistance. Possibly applying some dielectric grease to help with that transfer as well.
 

TheBatteries

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I would not go lower than 65%. That's around 2.5V. Going down to 30% that is about 1.15V, which is waaaay to low. 40% is 1.52V, which is to low, too.

Most LiFePO4 batteries are considered empty or 0% state of charge at 2.50V per cell. If you take them down to 1.15V or 1.52V, you're pretty much destroying your batteries...
 

Roland W

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Oct 9, 2017
Messages
146
For a 270ah battery, 3kW at charging is fairly normal. That's a C rate of around 0.2. I would stay within 3.0V at bottom and 3.4V at the top. That gives you a usable capacity of around 85-90% and will increase you battery life dramatically.
In my opinion, if you really need or want to make sure your pack is getting full to the mentioned limits, you would probably have to add a few panels and max out the charger in the inverter, or have another small array feeding in with an other individual charger. Would that be an option?
If the solar day is too short and the battery capacity too big, you cannot do much by tweeking efficiancy ;)
 
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J_Mack58

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Apr 7, 2021
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145
I charge my 16S30P, 26650 cell banks up to 55.5 volts, if balanced that is 3.47 volts/cell and I discharge down to 45.5 volts, that's 2.84 volts/cell. This seem to work for me, I'm just about to get a year out of some used 26650's that was given to me. I will say Rearden, I have a hodge podge of batteries all parallel up they are Nissan Leaf, 18650's and 26650's, when charging to 55.5 volts the 26650's are the last to stop receiving current. The Leaf cells charge up quick the capacity is not high only 64 Ah. The 18650's are 14S80P I'm getting 190 Ah out of them. The 26650's are 16S30P or 90 Ah that seem to need to draw a charge current forever. I have the sun in California so I just let it do what it do but like you I scratch my head why. I would think my 18650 bank at 190 Ah would be the last to stop drawing current. I say it is the battery chemistry and roll with it.
 

cak

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Mar 14, 2021
Messages
133
Without knowing al the details the thing I would add from my experience with very short days in the long dark winters of Northern MN is that getting the battery fully charged is asking a bit much because of how the charge controller starts reducing the charge current as the battery nears full and so I am not getting the full capacity of the panels as it tops off. With short days, 2-4 hours of productive sun in winter, this results in the battery never really topping off. My work around has been a combination of accepting that I still have most of the charge and also during the winter I set the bulk cutoff voltage higher which worsens the wear on the batteries but maximizes how much current can flow in during a small time and then the day ends very quickly and the voltage sags back down to better levels.
 

Korishan

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because of how the charge controller starts reducing the charge current as the battery nears full and so I am not getting the full capacity of the panels as it tops off
Then I would add more battery capacity. Better than increasing the SoC percentages.
My work around has been a combination of accepting that I still have most of the charge
That's part of it :)
during the winter I set the bulk cutoff voltage higher which worsens the wear on the batteries
yeah, that's the biggest issue. At least you're aware of it.

Now, the shortening of life cycles is if this would be done a regular basis. Just having these charge voltages for a couple months won't kill the cells in short order. You could still get many years of service from them. For example, perhaps 6 years instead of 7 years ;)

I know that not everyone has the means to just expand their battery capacity. Cells aren't (always) cheap/free (for good quality ones, usually). Or easy to get a hold of. So do what can be done with minimizing degradation as much as possible by doing what you can when you can.
 

J_Mack58

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Apr 7, 2021
Messages
145
Without knowing al the details the thing I would add from my experience with very short days in the long dark winters of Northern MN is that getting the battery fully charged is asking a bit much because of how the charge controller starts reducing the charge current as the battery nears full and so I am not getting the full capacity of the panels as it tops off. With short days, 2-4 hours of productive sun in winter, this results in the battery never really topping off. My work around has been a combination of accepting that I still have most of the charge and also during the winter I set the bulk cutoff voltage higher which worsens the wear on the batteries but maximizes how much current can flow in during a small time and then the day ends very quickly and the voltage sags back down to better levels.
This has been my fix too, I raised almost every parameter by 1 volt trying to get full charge in the few sun hours. I use EPEVER solar charge controller and I have my summer and winter parameter files I trade off in the controller...forever tweaking.
 
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