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Batrium LongMon's accuracy
#1
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 the system 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 on the 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 any documentation 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
OffGridInTheCity likes this post
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#2
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 Wink
OffGridInTheCity likes this post
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#3
I have WM4+Expansion + 84 longmons Smile    Interesting to hear about 0.03v 'difference'.    I've noticed that one must expect a bit of loosness in DIY 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!
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#4
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.
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#5
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 Smile
The Ultimate DIY Solar and build place
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Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 100kWh LiFePo4 | 20kWh 14s 18650 |  66*260W Poly | ABB S3 and S5 Trip breakers
Upcoming: 14S 18650~30kWh
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#6
(05-13-2020, 05:28 PM)daromer Wrote: I think the problem is mostly related to how you see it and not real world scenario.

Smile  you probably right  Tongue

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  Big Grin

Thx guys for sharing your opinions, I appreciate it.
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#7
Smile 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.
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Upcoming: 14S 18650~30kWh
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#8
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.
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#9
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 Wink
The Ultimate DIY Solar and build place
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Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 100kWh LiFePo4 | 20kWh 14s 18650 |  66*260W Poly | ABB S3 and S5 Trip breakers
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#10
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. 


But having  Mysik's OCD and not being happy with this as it really does play with the balancing at the top end I had #6 constantly blowing of steam as it read 4.11 V and 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 turning red hot on #6 while #4 is struggling to get up to voltage according to Batrium. My Fluke meters where saying otherwise 0.02 V difference between packs.
The rest of the pack was just 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  #6 included I decided to switch the longmons between #4 and #6. 
Resynced everything and well I don't know what happened but my pack voltage difference went from 0.04V/0.06V to 0.01V/0.03V  which is "Acceptable" Now my balancing works perfectly bouncing around ALL packs as directed by Batrium. Now it looks like a symphony with all the instruments participating not just a solo artist.



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

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