Ok so which is "ok" 0.1, 0.01, 0.001 or? for balancing lifepo batteries?

[Deleted member 9967]

I have seen many videos and read much [a books worth so far] in forums.
What I want to know is this.
Some say balancing a large battery of 280Ah to 0.1 volt is fine. Others say you have to balance to 0.01 volt. And still others say that 0.001 is safest for the batteries.

While I am sure that the tighter the tolerances the better in the end.
What is acceptable and just fine?

Thanks to all the geniuses here for your answers. I have learned so much that I never could have done without all of your help.
You guys rock.

 

[zorlig]

I trigger active balancing are .01 with my test setup, but that's only because a lower number would be overwhelmed by a .3a balance current.

 

[HRTKD]

As I recall, my BMS setting is .015.

 

[toms]

If you can adjust the settings for your active balancer, i’d go for 0.05V

The variation in the flat part of the curve is often transient - if you turn off the active balancer and log your cells while you cycle for a while you will see that there is small voltage variations between cells across the cycles, but they are not always the same.

 

[Deleted member 9967]

Ok so maybe it would be better to ask what is the lowest you would still feel safe about then?
I realize the higher the better as in 0.001 is better than 0.1. But at what point is it "good enough"?

 

[Maast]

IMO trying to achieve anything more than a .01 balance between cells is pretty much worthless. If you ever watch the individual cell voltages even under a moderate charge or discharge you'll see that the .xx# numbers flicker up/down so fast anyway you can't even see them.

Personally I'm happy with a .02 differential between cells - again because they change so fast and a differential of .02 or less has no functional impact on the performance of the LFP battery bank and .02v could just as easily be within the error 'bars' of the measuring equipment.

 

[Deleted member 9967]

IMO trying to achieve anything more than a .01 balance between cells is pretty much worthless. If you ever watch the individual cell voltages even under a moderate charge or discharge you'll see that the .xx# numbers flicker up/down so fast anyway you can't even see them.

Personally I'm happy with a .02 differential between cells - again because they change so fast and a differential of .02 or less has no functional impact on the performance of the LFP battery bank and .02v could just as easily be within the error 'bars' of the measuring equipment.

Ok thanks. So you prefer 0.02 as to 0.1 then. Got it and thanks again.

 

[Zil]

Top balance. My practice is steady 0.01 with +/- 0.003

 

[Deleted member 9967]

Top balance. My practice is steady 0.01 with +/- 0.003

That seems pretty tight. Three decimal places. How long did that take to balance?
Why such tight tolerances?
Thanks

 

[zorlig]

That seems pretty tight. Three decimal places. How long did that take to balance?
Why such tight tolerances?
Thanks

He means that there is no significant current flow between paralleled cells so that they show the same voltage. That should happen quickly.

 

[Deleted member 9967]

He means that there is no significant current flow between paralleled cells so that they show the same voltage. That should happen quickly.

OH, ok. Thanks

 

[Zil]

He means that there is no significant current flow between paralleled cells so that they show the same voltage. That should happen quickly.

Not exactly. As soon as the cells are parallel the voltage is the same. I use a adjustable voltage supply. I charge the parallel cell pack by stepping up voltage to 3.60n. Then series the cells and discharge to 3.20n. I then check each cell over a few days to see they hold 3.20n. If necessary, I repeat the process. They must be balanced to hold 2 decimal places with third decimal within 3 or 4 numerals. Then they pass my QC.

 

[Deleted member 9967]

Not exactly. As soon as the cells are parallel the voltage is the same. I use a adjustable voltage supply. I charge the parallel cell pack by stepping up voltage to 3.60n. Then series the cells and discharge to 3.20n. I then check each cell over a few days to see they hold 3.20n. If necessary, I repeat the process. They must be balanced to hold 2 decimal places with third decimal within 3 or 4 numerals. Then they pass my QC.

That makes sense. But is there any significance to charging to 3.6 and discharging to 3.2?
Or is it just that you had to pick numbers and those two got elected?

 

[Zil]

I charge to 3.45 with regulated power. Then reset and charge to 3.6v. While present and observing this final charge. It doesn't matter if it is 3.60V, 3.65V or even 3.70V. No consequence or interest if you are actively monitoring the process, because these values are often reached seconds apart.
There are differing practices than mine, but Discharging down is needed as LiFePo do not like 100%+ SOC.

 

[Deleted member 9967]

I charge to 3.45 with regulated power. Then reset and charge to 3.6v. While present and observing this final charge. It doesn't matter if it is 3.60V, 3.65V or even 3.70V. No consequence or interest if you are actively monitoring the process, because these values are often reached seconds apart.
There are differing practices than mine, but Discharging down is needed as LiFePo do not like 100%+ SOC.

Have no intention of doing 100% charge or discharge.
I did not know it took seconds to go from 3.45 to 3.7
THAT is scary. Or did you mean 3.6 to 3.7?
I thought it would take half an hour or something.
Ok so when it gets to 3.4ish I should just turn it all off I think?
I do not yet have a BMS and yet I have to charge the batteries.
But when I do get the BMS in the coming months Then I will be able to us this advice.
Was thinking of parallel balancing actually.

 

[MisterSandals]

Ok so when it gets to 3.4ish I should just turn it all off I think?

When one cell gets to 3.4v watch it like a hawk. You will be able to learn which cell(s) are weaker and stronger. Take good notes.
Depending on the weakest cell and your rate of charge, my guess would be 15 minutes to a half hour before the first one gets to 3.6v.
Then maybe 5 minutes to 3.65v.

Please report back.

And do yourself a favor, label your cells and take good notes! You will be kicking yourself if you do not take notes, particularly noting weakest cell(s) and total voltage when one gets over 3.45v (a good candidate for max charge cutoff on SCC).

 

[Deleted member 9967]

When one cell gets to 3.4v watch it like a hawk. You will be able to learn which cell(s) are weaker and stronger. Take good notes.
Depending on the weakest cell and your rate of charge, my guess would be 15 minutes to a half hour before the first one gets to 3.6v.
Then maybe 5 minutes to 3.65v.

Please report back.

And do yourself a favor, label your cells and take good notes! You will be kicking yourself if you do not take notes, particularly noting weakest cell(s) and total voltage when one gets over 3.45v (a good candidate for max charge cutoff on SCC).

Thanks. However with this new info I might decide to stop the charging at 3.4 or 3.5 [if still safe] until I at least get a proper BMS anyhow.
The speed from 3.6 to 3.65 is to fast for me to want to risk destroying my cells.

Thanks for the heads up. This is the kind of info I am looking for.

 

[Deleted member 9967]

And yes, copious notes.

 

[MisterSandals]

And yes, copious notes.

I like labeling cells with letters (A, B,...) because i also note, when wired in series, the cell position which is a number.
And for reasons i do not know, cell #1 is the negative side of the battery.

Odd chit usually happens on cell #1... you’ll see.

 

[Deleted member 9967]

LOL

I like labeling cells with letters (A, B,...) because i also note, when wired in series, the cell position which is a number.
And for reasons i do not know, cell #1 is the negative side of the battery.

Odd chit usually happens on cell #1... you’ll see.

Ok thanks