Batrium LongMon's accuracy

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Mysik

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Jul 29, 2019
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Hi All,

I have my diy powerwall (18650 cells 14s120p), for about 2 weeks. So as you can see I'm at the beginning of my journey. I'm learning more and more about the way how thesystem is working and I'm tuning the settings to make it work the way I want.

I'm using batrium BMS (WM4 + Longmons) and I've noticed that the voltage accuracy is not great. In my case all my longmons reading to low. I've measured my cells with my Fluke multimeter (which was calibrated a month ago), and the best longmon is -0.01V off (which I'm OK with) and the worst one is -0.03V off (which I'm not happy with).

From what I've noticed of the forum, everyone (including myself)is configuring system voltage with accuracy to second decimal place.
Being off 0.03V onthe entire pack is not an issue, but if you are out 0.03V on individual cells is not great. According to spec longmons can be as much as +-0.1V (typical +-0.05V off), which I didn't know before I've purchased my bms. It looks that I'm actually quite luck that my readings are just 0.03V off (the worst case).

From spec 0.05V accuracy for 3.7V cell is 1.3% measurement error, which is huge comparing to my Fluke179 which is 0.09% accurate.
Batrium is quite expensive (my set cost me over 700) and I would expect better accuracy from it, especially that the system is using values with 2 decimal places (or at least an option to adjust the reading in software to much true values).

Does anyone ever re-calibrated longmons? Does anyone know how to do it? I can't find anydocumentation on batrium website.
I've sent them a question about it and waiting for reply, but I thought I will ask community what is your experience with longmons accuracy.

Regards
 

daromer

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It's not possible to re calibrate them from our point of view.

The accuracy is good enough for general application. The voltage does not state much at all to be honest. It does not show if the capacity is off by 1% or whatever voltage is off.

You cannot compare to a Fluke. A fluke cost alot more than 1 longmon cost. Quite different type of AD inside and components ;)
 

OffGridInTheCity

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Dec 15, 2018
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1,653
I have WM4+Expansion + 84 longmons :) Interesting to hearabout 0.03v 'difference'. I've noticed that one must expect a bit of loosness inDIY solar/battery power production. The panels hardly every produce 100% (sun, clouds, angle - whatever...), the 18650 batteries cannot be run to 100% DOD if you want safety/longevity, power in/out of batteries is not 100%,the inverters are not 100% efficient,and on and on.

Sometimes its important to allow <100% perfection (but its cool to know the details) and remember what's actually being accomplished at DIY level and DIY prices. Recently I was able to go 8 days of 24/7, whole house, solar power without interruption - and I might get all summer (3-4 months) of this. Now that's a result!
 

Mysik

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Jul 29, 2019
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I've use Fluke just as an example, but also my chinese multimeter which cost $15 is more accurate (0.5% error). OF course multimeter is more complex device, but is also capable of measuring more than LongMon.

I will use my example to show you where I see a problem...

I set my bypass to 4.05V:
cell #1 is 4.065V - longmon is reading 4.03V
cell #2 is 4.06V - longmon is reading 4.05V and start bypassing
the outcome of this is that bms trying to balance cell that doesn't need to be balanced, and cell that longmon is reading low (and should be balanced)will never reach balance voltage because max pack voltage is set to 56.8V (4.06V/cell). Shunt is reading spot on and will not raise the voltage.

That is why from my perspective 0.03V off is a problem.
If you have any idea or sugestion how I can overcome this issue feel free to comment.
 

daromer

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that will only happen the first time. The second time they are in balance to the error the longmons are in. And at top and bottom voltage those 1% margin equates to 0.05% capacity at most.

Chasing decimals are nice but you still can not compare it to a voltmeter. Especially not 1 chinese that could have been lucky.

I think the problem is mostly related to how you see it and not real world scenario. If you need more precise setup you need to pay alot more for the BMS to be honest :)
 

Mysik

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Jul 29, 2019
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daromer said:
I think the problem is mostly related to how you see it and not real world scenario.

:) you probably right :p

Well, as I said at the beginning, I'm at the start of my DIY powerwall journey, so I need to adjust not just my system settings but also my expectations :D

Thx guys for sharing your opinions,I appreciate it.
 

daromer

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:) If the error would cause balance issues on each cycle. Lets say you have a charger that charges each pack on its own and then batrium manage balance on its own. Then it would be a fight between them. In this that wont happen since batrium operates alone on cell level basis.
 

w0ss

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Oct 15, 2019
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I just asked Batrium about calibrating the WM5/MM8. They said the same to me that it isn't allowed. I agree it's inconsequential but it bothers my OCD that my second battery will never bypass since it is reading 15mv low on all 14 cells. When you have a longmon and each is slightly off it all balances out but in my case since the MM8 is low it seems to affect all 14 cell monitors equally.
 

daromer

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It Will bypass.. it Will even out perfectly in the end but to the variance so if you ocd the screen its not a problem. Let batrium balance to its variance and its fine.

I think you have missed how it works ;)
 

Wolf

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I think I will chime in with some interesting findings.

So I got my system all set up and everything was running swimmingly except for 2 packs. #4 and #6 which were indicating 0.06 V difference.
I went out with my "FLUKE"DMM and checked the actual V and all cells where within 0.02V. I was reading this thread and came to the conclusion by all the post that this was going to be it and I could do nothing about it.

image_srkmah.jpg


But havingMysik's OCD and not being happywith this as it reallydoes play with the balancing at the top end I had#6 constantly blowing of steam as it read 4.11 Vand the rest of the cells never getting that high as my charge controller was set at 4.05V. Not acceptable. Here I have 1 Longmon turningred hot on #6 while #4 is strugglingto get up to voltageaccording to Batrium. My Fluke meters where saying otherwise 0.02 V difference between packs.
The rest of the pack wasjust idling along perfectly.
What to do? Order another 14 longmons as I will use them anyway on my 2nd pack? and see if they are better?

Then I said well as I need to finish the lexan sides on 3 of my packs #6included I decided to switchthe longmons between #4 and #6.
Resynced everything and well I don'tknow what happenedbut my pack voltage differencewent from 0.04V/0.06V to 0.01V/0.03V which is"Acceptable" Now my balancingworks perfectly bouncingaround ALL packsas directed by Batrium. Now it looks like a symphony with all the instruments participating not just a solo artist.

image_zzvdrp.jpgimage_orrgdq.jpg

image_zmrbci.jpg


So Mysik try moving the longmons between your highestreading longmon and lowest and see if that makes a difference.

Wolf
 

OffGridInTheCity

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Again - very interesting. I now have 84 longmons... and while I haven't go all out balancing things (0.04v diff is OK with me), I too have noticed weirdness.

I have a couple of packs that just seem to go a bit higher and a bit lower each day - no matter what I do. I've done individual pack charging to 'nudge it' (for example) up with others but doesn't seem to make things better. I hadn't considered the longmons may be 'just a touch weird' themselves - but after 2 yrs of this I've generally come to accept that 0.03v - 0.06v fluctuation each day between 4.1v high and 3.5v low

If you look at pack #26 - you can see one of my weird ones - it just sticks out, but if I pull it down it will just be low in the other direction.
NOTE: I just added a new 14s battery yesterday (#1 thru #14 below), so I'm using Auto-Level to bring it all within 0.04v difference on lower end.

image_dodcbz.jpg


I do try to keep my OCD in check as I have no plans to go above 4.0v or lower than 3.5v for long life - so a 0.04v-0.06v dif makes no difference at all and its *stable* over many months of charge/discharge... so the packs must be OK.

But I do find this topic very interesting...
 

daromer

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Your balancing did work before as well. Its just a graphicaly intepretation. What you saw when 1 pack balanced more will happen in time when it drifts out of the callibrated scope.

OCD on the graphs is one thing and how it works in real life is something else. No matter if its off 0.05 all cells will have the differences and especially under 4.2V this is normal.

One or many packs will always be off unless you constantly rebuild them or have new cells ;)

With that said if you let the BMS do its job you dont have to bother. I have my system running for many years and the only thing i have done so far is swapped our a couple of longmons that stopped working. The battery bank itself is running.
 

Wolf

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Never to question the master of the Batrium as I have watched every video you have put out on it.

BUT if I have my charge controllers set at 4.05 and my bypass set at 4.07 on auto with a 0.03 difference and 1 of my longmons shows 4.10 something is wrong especially if my fluke shows 4.04V.
This will cause the Batrium to go into bypass when it is not required and venting 1 longmon. That is what I experienced graphical representation or not.
After moving the longmon things settled down, another experience in real life.
After a dreary day yesterday my packs where down to 3.6V very even across the board. Today is a very nice day and I am charging very well with a difference of 0.02. By past experienceas I approach 4.05V per pack there will be a very pretty dance of bypass flickers as the "low" cells catch up never a prolonged bypass by just 1 longmon.
I understand you have been running this system for years compared to me but again I have built my packs with very stringent criteria and hoped to accomplish a very even balanced 14s pack that requires a minimal of intervention. I consider Batrium a battery monitoring system and if neededa little tweaking here and there to maintain an even charge across all packs. I obviously don't consider the Batrium a fixer for poorly designed packs.
As of 11:38Am today. Once I hit 4.05 I will try to post another screen shot of the bypass dance.

image_allach.jpg


Wolf
 

Wolf

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Well as I reach peak saturation of the pack I see that today will not be a balancing day. Sorry :p

101 SOC% and 3.17A charging. 0.01difference in V between packs.
No balance necessary.

image_brywas.jpg


Wolf
 

daromer

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As i said in the beginning this will only happen the first day. Then they will be balanced based on the "wrong" value. So if you look at it that way its nothing special and all values we have are "wrong" values and will be balanced towards that. no matter what if that value is below 4.2V its not a problem

With that said the total voltage may be different so you may callibrate your charger and inverter towards the BMS but when thats done its fine.


So once again the difference you see is not a real world issue if you live with it and only look at 1 source.

Its like taking 1 red Car. that car will look red right? But if you place that Red car beside another Red car from another brand you will most likely say that one of them arent as red as the other... :D So remove them again and you suddenly have red car.

Its all about how you look at something and a value is just a value presented by a set of parameters ;) Yes, one of my interesting areas are numbers and how you can turn them around.
 

Wolf

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Well being the pain in the backsideas I am all in all I do agree with you. :)
However the problem did not resolve itself after the first day nor the second nor the third. That is why I decided to switchthe longmons and I was successful as the problem did resolve itself by doing just that.

As far as red is concerned as RGB its 255,0,0 or in HEX itsFF0000. Now if you change it to 255,1,1 or FF0101 is it still red? Visually yes but digitally no it now has green and blue mixed in. It's all in the numbers :p

Wolf
 

daromer

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Yeah :)


I have painted alot of cars in My days and what One person r is not the same as another hehe. Same with meters
 
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