diy solar

diy solar

Almost everyone is charging their LFP wrong!

shvm said:
Charge termination current varies with Voltage.

Charging Marine Lithium Battery Banks | Nordkyn Design

nordkyndesign.com nordkyndesign.com
Manufacturer-specified termination condition3.65V @ 0.033C3.65V @ 0.05C
Cell voltage (V)Termination current (C)Termination current (C)
3.370​
0.000​
0.000​
3.400​
0.004​
0.005​
3.425​
0.006​
0.010​
3.450​
0.009​
0.014​
3.475​
0.012​
0.019​
3.500​
0.015​
0.023​
3.525​
0.018​
0.028​
3.550​
0.021​
0.032​
3.575​
0.024​
0.037​
3.600​
0.027​
0.041​
3.625​
0.030​
0.046​
3.650​
0.033​
0.050​
Click to expand...

@sunshine_eggo you know much more about charging than I do. Does the article/link here make sense to you? It seems to be where OP is basing some of their theory from.

What I’m wondering is if the article is valid perhaps OP has misread it. Or if the article is invalid or bad advice, then OP has read it correctly but is misinformed.

Thoughts?
 
Tomthumb62 said:
What I’m wondering is if the article is valid perhaps OP has misread it. Or if the article is invalid or bad advice, then OP has read it correctly but is misinformed.
Click to expand...
You should never put your trust in random internet strangers, me included. Read the cell manufacturer's datasheet instead. There has to be a reason behind every single (decent) server-rack battery sold today coming with a RS485/CAN communication port.

EVE:
http://www.dcmax.com.tw/LF105(3.2V105Ah).pdf (Section 5.3)
(Section 4.2)

CALB:
https://cdn.shopify.com/s/files/1/1820/0269/files/L160F100B_data_sheet.pdf?v=1622844198 (Section 4.2.1)

All three say the exact same thing about charging. It is only a matter of reading them.
 
shvm said:
You should never put your trust in random internet strangers, me included. Read the cell manufacturer's datasheet instead. There has to be a reason behind every single (decent) server-rack battery sold today coming with a RS485/CAN communication port.

EVE:
http://www.dcmax.com.tw/LF105(3.2V105Ah).pdf (Section 5.3)
(Section 4.2)

CALB:
https://cdn.shopify.com/s/files/1/1820/0269/files/L160F100B_data_sheet.pdf?v=1622844198 (Section 4.2.1)

All three say the exact same thing about charging. It is only a matter of reading them.
Click to expand...

Oh I agree completely. However, @sunshine_eggo is not a random internet stranger. I have "known" him for over 2 years know, but only on this forum. He has shown me time and time again, that he knows his stuff. I trust him much more than myself when it comes to all this technical battery stuff. I am not going to read the data sheet any more than I already have, because it might as well be written in Greek. It looks like plain enough language but I know it's not. It assumes some basic understanding of electricity and batteries. Understanding that I may or may not actually possess, I don't know. Because I don't know with confidence, I defer to people like sunshine_eggo.

As for trusting you yet, I don't know yet. What you're saying so far doesn't make sense to me. What I am trying to understand here is why it makes sense to you. You seem like a smart person but I wonder if you're misunderstanding something here but are just blind to that (we all do it at times!) There are Electrical Engineers on this forum and others who are nearly so without EE after their name, some of whom have replied to this thread calling out holes in your theory. You haven't convinced anyone here so far, but I am giving you the benefit of the doubt.

Your theory might be golden. But it will be considered unproven until many more people test your theory and get the same results you do. If they don't, then something is off somewhere. Could be all of them who have done something wrong, could be you, it would be hard to say without further inquiry. If you can't convince anyone to actually test your theory (or run your own rigorous scientific study), then this thread will just end up collecting dust.
 
@shvm
Maybe you should rename your post. It might have not come across very well with some. All information is good for discussion
BTW I don't think we've seen your credentials yet?
I have 40 years electrical HVAC experience in installation, service and engineering but I don't know or understand solar like the specialties I have experience with but can identify bogus information that doesn't jive with the electrical facts.
 
shvm said:
There has to be a reason behind every single (decent) server-rack battery sold today coming with a RS485/CAN communication port.
Click to expand...

- Because people ask for it
- Because of regulation pushing for closed loop
- Because companies like Victron want to integrate with their inverter so it becomes part of their ecosystem (see also Pylontech)
- Because other applications of these batteries (telecom/industry/etc.) want remote monitoring and CAN/RS-485 is the standard there
- Because insurance companies will want every possible measure to be in place (these are not just aimed at DIY)
- Because the 'big boys' have it
- Because 'Lithium' is the same to some people and NCM/... and others are more sensitive, but these BMS work with those cells as well.
- ...
 
shvm said:
...
The problem that I faced was the same as everybody else faces here: Cells no longer in balance (showing 0.040 V deviation) even after a 'full charge' to 3.5V the very next day. (you will soon realize the reason behind these quotes later)
Clearly, If I was doing everything 'right' according to what everyone on the internet is doing my cells should retain their balance at least.
Overtime, people have tried to attack this problem by saying passive balancing simply isn't enough and how there is basically a need for active balancing to be the default offering in all BMS's in order to keep the cells in 'daily' balance.
Overtime, this has resulted in the demand for higher Active Balance Current in BMS's.
...
Click to expand...

Excellent post, thanks for sharing this @shvm. The Nordkyn page you refer to also contains a lot of useful information.

Here are my notes and understanding for what it is worth. I have experience with lead acid, but no practical experience with LFP, so it should be taken with a grain of salt. Using the pack I'm building as an example below.

Any charge (absorption) voltage in the acceptable range (3.37 - 3.65 V) will result in a fully charged battery, it just takes longer with a lower voltage, but on the other hand the risk of hitting OVP for a cell is also lower. Charge voltage is just a matter of preference. Most people seem to be in the 3.45 V - 3.50 V range.

Over charging is an issue that can damage the cells. How damaging it is and how sensitive the cells are is a question mark, but why not try to avoid it if we can. It will occur at any charge voltage above the 100% SOC resting voltage (3.37 V) if you keep the cells there long enough. Overcharge is not something you can "see", it just results in cell degradation over time.

Charge should be terminated by measuring the charge current into the cell and stopped when it drops below 0.03C (or what ever the datasheet for your cell says). The 0.03C value is stated for a standardized 3.65 V absorption voltage, so it must be adjusted down if you use a different voltage. For example if you use 3.50 V the value is 0.023 C. (Look at the linked document in the OP). For a 600 Ah pack that is 13.8 A.

This procedure is not possible for most of us, because we don't have a fancy charge controller that can talk to the BMS and get the charge current. The charge controller doesn't know what current flows into the cells, it just knows the sum of what current flows into the cells AND into the inverter.

Since we can't use the current termination method, we use absorption time instead. Set the charge controller to 3.50 V absorption, 1 hr absorption time, and then disconnect the inverter, and start the charging. Go through the bulk, and then as soon as absorption starts (voltage reaches 3.50 V), measure how many minutes it takes to drop to 13.8 A. Stop the charge and enter this value as the new absorption time. This will ensure that even in the worst condition (full charge current and inverter turned off), the absorption stage will never over charge the cells. Most of the time the cells will instead be slightly under charged, which is not damaging.

Set the float to 3.37 V (the resting voltage of a 100% SOC cell) to prevent over charging. This will also prevent the cells from being discharged if the inverter takes a load (for as long as the sun shines). In theory it will also very very slowly get the cells to 100% SOC if that was not done in the absorption stage, but practically this will never happen in a reasonable time frame.
 
BjornM said:
Excellent post, thanks for sharing this @shvm. The Nordkyn page you refer to also contains a lot of useful information.

Here are my notes and understanding for what it is worth. I have experience with lead acid, but no practical experience with LFP, so it should be taken with a grain of salt. Using the pack I'm building as an example below.

Any charge (absorption) voltage in the acceptable range (3.37 - 3.65 V) will result in a fully charged battery, it just takes longer with a lower voltage, but on the other hand the risk of hitting OVP for a cell is also lower. Charge voltage is just a matter of preference. Most people seem to be in the 3.45 V - 3.50 V range.

Over charging is an issue that can damage the cells. How damaging it is and how sensitive the cells are is a question mark, but why not try to avoid it if we can. It will occur at any charge voltage above the 100% SOC resting voltage (3.37 V) if you keep the cells there long enough. Overcharge is not something you can "see", it just results in cell degradation over time.

Charge should be terminated by measuring the charge current into the cell and stopped when it drops below 0.03C (or what ever the datasheet for your cell says). The 0.03C value is stated for a standardized 3.65 V absorption voltage, so it must be adjusted down if you use a different voltage. For example if you use 3.50 V the value is 0.023 C. (Look at the linked document in the OP). For a 600 Ah pack that is 13.8 A.



Since we can't use the current termination method, we use absorption time instead. Set the charge controller to 3.50 V absorption, 1 hr absorption time, and then disconnect the inverter, and start the charging. Go through the bulk, and then as soon as absorption starts (voltage reaches 3.50 V), measure how many minutes it takes to drop to 13.8 A. Stop the charge and enter this value as the new absorption time. This will ensure that even in the worst condition (full charge current and inverter turned off), the absorption stage will never over charge the cells. Most of the time the cells will instead be slightly under charged, which is not damaging.

Set the float to 3.37 V (the resting voltage of a 100% SOC cell) to prevent over charging. This will also prevent the cells from being discharged if the inverter takes a load (for as long as the sun shines). In theory it will also very very slowly get the cells to 100% SOC if that was not done in the absorption stage, but practically this will never happen in a reasonable time frame.
Click to expand...
Finally someone who went to the length of reading things and even more importantly actually understood what is meant.

I can't overstate this enough! Thank you for making a good TL;DR :)
BjornM said:
This procedure is not possible for most of us, because we don't have a fancy charge controller that can talk to the BMS and get the charge current. The charge controller doesn't know what current flows into the cells, it just knows the sum of what current flows into the cells AND into the inverter.
Click to expand...
Precisely! Even with people not having access to BMS-Inverter comms, they can modify existing settings when it comes to when to turn balancing on or which voltage to float to emulate the correct model of charge termination.
 
You can tweak the settings to get a correct cell charge, balance and float but the controllers are generally not that precise.

Not all BMS are equal and batteries will have different parameters so check the battery specs before you go tweaking things.
 
shvm said:
Finally someone who went to the length of reading things and even more importantly actually understood what is meant.
Click to expand...

Or those who better spent their time doing things the way they always have been for years (wrong according to you) with no problems (and have still balanced cells). <shrug>
 
Texas-Mark said:
Or those who better spent their time doing things the way they always have been for years (wrong according to you) with no problems (and have still balanced cells). <shrug>
Click to expand...
Except, there's a difference.

My cells need balancing on the order of weeks to months.
Their cells need balancing every charge cycle. That's how I know what is better and what is wrong.
 
Quattrohead said:
There are some super smart people around here, you need to do a lot more reading and understanding before you start throwing your weight around buddy ?
Click to expand...
Oh, they can be easily recognized by what they say and how they word it.
 
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