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Lifepo4 - ideal charging parameters?

Vi s

Learning
Joined
Dec 6, 2020
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Location
Thailand
Hello to everyone!

I am a total newbie to lifepo4 batteries so I need some clarification. I read many posts on this forum and beyond but I am still a bit confused and uncertain about the ideal charging parameters for my setup.

I have following solar setup:

23.6V 20.7W poly solar panel,
Mppt charge module SD30CRMA-18V (I've tested 92% efficiency with 1A max charge current and 96% below 1A. 1A enough and good since below 0.2cc of my 6Ah battery),
New 4 x 6Ah 32650 batteries in series,
Bms bw-4s-S30A with balancer,
Electric energy tester
IMG_20201207_121147.jpg Total cost about 28usd.

I use it to charge the battery during the day (no loads)in order to power led lights and charge 5V usb applications during the night time.

So my question is following, since i have to manually set the battery voltage (eg the voltage where it stops charging the battery I suppose!? )Screenshot_2020-12-04-14-09-59-985_com.lazada.android.jpg
on this simple charge controller module, which Volt parameter should I use in order to charge the battery to about 90% and then stop? I want to get maximum life cycles out of this battery pack. Would 14V be ideal or do I need to go higher?

Much appreciate if you could kindly also explain also your ideal charging voltage suggestion so that I learn something! :)
 
If yours is a 12v system and you want to get to about 90% SOC and do it with a simple charge controller using voltage termination only and not worrying about an absorption charge, then I'd suggest setting the termination voltage to somewhere right around 14.4V should get you close to that 90% state of charge point desired.

xLiFePO4-charging.gif.pagespeed.ic.ZYJX_3h8SJ.webp


Hopefully the graph explains the why I would pick 14.4V as that is roughly the point where the voltage at a 0.5C current gets a LiFePo4 battery to about a 90% SOC.
Here is a link for some additional reading on the topic.
 
To tell the truth, I really don't understand the charge controller you are using. It seems different than anything I have worked with.

The ideal controller will 1) charge to a fixed voltage with a current limit, then 2) let the current drop off to near zero and then 3) stop. It seems like the charger you have is just a voltage source..... but I could be wrong.

You can review this paper to get an idea of the voltages on more conventional LiFePO4 systems

https://diysolarforum.com/resources/lifepo4-voltage-settings-guide-for-bms-chargers-and-loads.121/
 
To tell the truth, I really don't understand the charge controller you are using. It seems different than anything I have worked with.

The ideal controller will 1) charge to a fixed voltage with a current limit, then 2) let the current drop off to near zero and then 3) stop. It seems like the charger you have is just a voltage source..... but I could be wrong.

You can review this paper to get an idea of the voltages on more conventional LiFePO4 systems

https://diysolarforum.com/resources/lifepo4-voltage-settings-guide-for-bms-chargers-and-loads.121/
Thank you for your reply!

I uploaded the rest of information I have to understand this charge controller module.

It has a current limit of 1.08A, which is a little bit less than 0.2c (which would be 1.2A for my battery pack).
I don't know with which voltage it charges the battery but at the solar panel the voltage is kept at around 17 to 18v (mpp). At the battery I observed the voltage slowly climbs till the parameter I have set manually (14v) then it stops at that voltage and no current flows further. After I unhook the battery and let it sit for almost a day the voltage of the battery settles to 13.37v.
Thanks for the link!
IMG_20201207_212004.jpg
That means I could set my end voltage to 13.8v which would be 3.45v per cell which is about 90% according to several posts I read.
 

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If yours is a 12v system and you want to get to about 90% SOC and do it with a simple charge controller using voltage termination only and not worrying about an absorption charge, then I'd suggest setting the termination voltage to somewhere right around 14.4V should get you close to that 90% state of charge point desired.

xLiFePO4-charging.gif.pagespeed.ic.ZYJX_3h8SJ.webp


Hopefully the graph explains the why I would pick 14.4V as that is roughly the point where the voltage at a 0.5C current gets a LiFePo4 battery to about a 90% SOC.
Here is a link for some additional reading on the topic.
Many thanks for your reply!

Yes there is where my trouble starts, which chart should I trust and follow?
IMG_20201207_204705.jpgIMG_20201207_211225.jpg

There they say for example 13.8v is 90% and 14.4v around 98%!

I suppose my charge controller throws the full voltage of the solar panel (17 to 18v) at the battery, and the battery takes what it needs at the constant 1.08A. I don't know how to check what voltage the charge controller throws at the battery!? I can only read before and after the charge controller. Before it's 17 to 18v and after the voltage slowly increasess, 13v till the voltage I have set manually (14v) and then it stops.
 
I suppose I can't adjust the charge voltage but only the cut off voltage. That means if I go according to the charts of https://www.gtmall.com.au/12v-60ah-deep-cycle-lifepo4-battery-rechargeable-7

90% capacity would be reached at 13.7 - 13.8v shown at the battery (means at the in series installed ammeter after the charge controller and before the battery) during charging!

Is 90% now 14.4v or 13.8v? How to make sure?
 
I doubt if your SCC is MPPT.
What is the sales link?
Search for sd30crma
A few examples

I tested it (I have the 18v version) and it holds the voltage for a 21v panel at its mpp at around 17 to 18v most of the time. I measured and calculated its efficiency, it is 96% at 0.6 to 0.8a and 92% at full capacity of 1.08a current.
 
As I understand or have seen it if it would use pwm the voltage at the solar panel would be close to the voltage at the battery. As well the current would be close as well. With this charge controller I measured at the panel for example 0.6 to 0.7a and it charged the battery with 1.08a.
 
I just found these pictures
IMG_20201207_224056.jpg
So there I adjust the output voltage not set the battery voltage or whatever...

IMG_20201207_225119.jpg
Here they suggest to set the charging (= output voltage right!?) voltage at 14.4v for a 12.8v battery like I have. That's for 100%. If I want 90% then 13.8v should be fine right!? Or does it have to be the minimum 14v as mentioned on the solarcity page? The battery manufacturer of my 32650 6ah cells says 3.65v is the charging voltage.
IMG_20201207_225856.jpg
 
The sd30crma probably does "mppt" as the Bq24650 means it is not true mppt but actually mppSET. :)
IMG_20201207_233908.jpg
 
Apparently LFP varies in terms of where the knee of the charge curve actually is. Here is another opinion about whether 14.4 volts is 90% or 99%. You can decide based on your particular cells.
 
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Many thanks for your reply!

Yes there is where my trouble starts, which chart should I trust and follow?
I'd trust the tests done specifically for your cells at the current and voltage that they support. The graph I posted and referenced would only be a general guideline and would most apply to a similar lifepo4 battery of the same capacity and characteristics.

The graphs you posted appear to be for a specific 60AH Sunovo battery you linked to and not about the specific batteries you have as the info given about your batteries in your OP references four 6AH batteries that you are combining together for your system. Does the manufacturer of your specific cells and battery give you any charge characteristic graphs or info?

Not all LiFePo4 batteries have the exact same formulation so there will be variances in specific charge characteristics and you should trust the info given you by the manufacturer of your cells. That said, most LiFePo4 batteries conform to general charging characteristics that you can make some boundary determinations with such as a max charge voltage of 14.6V in a 4S cell configuration or 3.65V per cell. However, the length of time needed to hold that voltage for in a CC/CV charging scenario to get a 100% full charge will vary depending on the charge current and specific characteristics of the battery. If you don't have any published specs for your specific cells then you'll have to make an educated guess knowing that charging at a lower voltage and/or not doing or shortening any absorption cycle will give you something below a 100% SOC for your battery.

The MPPT charge controller you referenced looks to be more a step down power circuit than a true battery charger meaning that it doesn't let you control any absorption phase of the charge so the voltage cut off is the only control you have to determine the end of your charge cycle. This isn't a problem necessarily especially because you desire to charge your battery at less than a 100% SOC and that's really all this charger can ever hope to do. I would test it and make sure it doesn't leak current after it hits the voltage cut off if it does stop all current flow and can be adjusted to stop somewhere between 14.0-14.4 volts then I think you have a working system and should count it good and not be too concerned that you are right at 90% SOC.
 
Not all LiFePo4 batteries have the exact same formulation so there will be variances in specific charge characteristics and you should trust the info given you by the manufacturer of your cells
I agree completely. The only thing I would add is to track his cells specifically to see how will they stay matched in terms of voltage as they approach the top of the charge curve. My particular cells have significant variations, therefore I am conservative on where I cut off voltage. I have had packs from various manufacturers and each one has its own unique characteristics.
 
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Thank you all for your kind input!!

They came with no charts (seller offered also no info) but having seen manufacturers info from alike 32650 cells the stated charge voltage is 3.65v.

I see. So all I could do to be sure they are around 90% would be getting a coulomb counter (that thing is more expensive than my batteries , charger and panel combined :/) and check after the charge cycle at a specific set cut off voltage the true capacity charge of the battery. Then adjust accordingly to the result. As well check each cell separately as well to see if there are any significant or undesirable differences between the cells. ?

Anyone having an more affordable and smart way to determine the almost true capacity level of one's batteries than having to buy an expensive coulomb counter?
 
I have got a set of these, nothing special, cheap batteries but they seem to work fine so far. No documentation whatsoever. Just that they are 6Ah cells.
IMG_20201129_173742.jpg
 
Apparently LFP varies in terms of where the knee of the charge curve actually is. Here is another opinion about whether 14.4 volts is 90% or 99%. You can decide based on your particular cells.
Thanks!

Do you know a clever easy way to determine whether one's cells are 90 or 99% at 14.4v without a coulomb counter?

That chart contains numbers while discharging. Also useful but I need numbers while charging which are different as I understand it.
 
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Thanks!

Do you know a clever easy way to determine whether one's cells are 90 or 99% at 14.4v without a coulomb counter?

That chart contains numbers while discharging. Also useful but I need numbers while charging which are different as I understand it.
I do have an Ahr counting meter and also a PowerLab 6 which would give me the exact number. I could look up some tests I did a few months ago but it doesn't matter what the exact number is to me. What I have found is that when I try to go above 3.4 volt per cell on my 16S pack there is one or two cells that seem to want to sprint to 3.6. I thought I did a pretty good job of parallel top balancing but as others have said these cells have bigger variations during the knee of the charge curve. The chart I had linked to actually said that 3.375 or 13.5 for a 4 cell pack was 98%. I can't tell from the data whether that was a charge or discharge chart. Whomever plotted it decided to plot the chart from the table which just had values in descending order. The scale of the chart does not appear to be linear so it flattens the typical knees which are steeper if the horizontal scale was linear.
The other thing to understand is that it depends on the current and I don't have testing equipment that can go up to the real time current that my system can put on my pack in normal use.
 
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I do have an Ahr counting meter and also a PowerLab 6 which would give me the exact number. I could look up some tests I did a few months ago but it doesn't matter what the exact number is to me. What I have found is that when I try to go above 3.4 volt per cell on my 16S pack there is one or two cells that seem to want to sprint to 3.6. I thought I did a pretty good job of parallel top balancing but as others have said these cells have bigger variations during the knee of the charge curve. The chart I had linked to actually said that 3.375 or 13.5 for a 4 cell pack was 98%. I can't tell from the data whether that was a charge or discharge chart. Whomever plotted it decided to plot the chart from the table which just had values in descending order. The scale of the chart does not appear to be linear so it flattens the typical knees which are steeper if the horizontal scale was linear.
The other thing to understand is that it depends on the current and I don't have testing equipment that can go up to the real time current that my system can put on my pack in normal use.
I see, thank you!

I have an electric energy tester. It also counts ah and wh.
So I would have to totally deplete the battery and then charge it fully to determine its max capacity. After that I would have to apply an end voltage limit in order to reach around 90% of the max capacity.
Would that be about right!? Or is there a faster way to figure out the threshold I am looking for?
IMG_20201207_121232.jpg
 
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