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Hello everybody,
I have an offgrid system, running for 1 year, made of a 8kWh BYD LVS battery, a MPPT RS450-100, and a Multiplus II 5000 VA.
Everyday, after the battery reaches 100%, I observe a strange increase of the max cell voltage, during hours :
Is it normal ? Does anyone have an idea of what happens ? The BMS should be balancing the cell's voltage instead, no ?
Thank you very much for your help
Honestly, as much as I like how new BYD LVS Premium Gen4 how it is done from housing to inside parts, all nice and tidy, all sealed, pressure blower hidden in the handle, inside cables tight, screws with locks, stainless legs, battery can slide out of housing, pass through made by busbars, and comparing to older LVS model, extra board with ON/OFF switch.
Unfortunately, the biggest disappointment is the balancer itself. First I tried like you to find information about it, nothing available, BYD support after much pressing and explaining I am a expert doing 20 years industrial automation, they told me, how the balancer works is a trade secret :-) honestly, this was a joke... and it also made me think I need to do it myself because if it is a secret, then it likely would not work properly...
So I opened the box and investigate myself. BYD LVS 4 is a has passive balancer for each individual of 16 cell, implemented by using Analog Devices LTC6811 with bus communication to module BMS. The major issue is, it is very weak passive 33R per cell balancer, so it can do around 100mA per cell to decrease the voltage over time. In order to work it needs to reach certain voltage above 3.45V and stay there. Then the balancer will try to discharge highest cells over the time in order to close the gap to lower cells. You can calculate yourself how long it would take to balance 78Ah battery cell, which is inside...
It also explains why during discharging the lowest sell hits min voltage and the whole 16 modules stop discharging and in opposite one of the cells hits high voltage above 3.65V then it stops charging, no matter than lowest cell is still around 3.3V and most of others close to 3.6V...
Well even with this balancer it would work great, if all cells would be matched and top balanced before assembly, which is definitely not always case as I can confirm with my 16 modules, 2 modules before I did manual balancing disbalance greater than 0.3V between cells.
What you can do? Their repair and replacement warranty service is rather good, so if it is still under warranty, try to replace it. If not, you either keep you battery close to 100% SOC for long time and hope the weakest cell is not so weak like I had. I had to charge it with 5A for several hours the cell reached voltage close to others, so imagine how long it would take to discharge all other cells in module by balancer to close the gap.... What I did was removed the battery from housing, balanced it myself and put it back. I warn you you can do this only if you absolutely know what you do as you can damage the battery, get burned, or put battery with wrong SOC to the rest of modules etc. etc., anyhow it can be done...
All in all, still good battery, believe me, comparing to many DIY boxes another level of quality, more expensive, but I would say almost fairly priced, but the balancer sucks...
I would wish BYD in new LVS generation put there some good active balancer with 2A or more, and maybe offer retrofit to older customers as balancer board can be easily replaced with few minutes of work by removing 4 screws and 3 connectors.
Are you getting any battery warnings?
If not then don't stress about it.
The system is rebulking the batteries during sunny hours which is also normal. And it is staying within the request 56.5v of the bms cvl. So also normal.
I have seen the 3.6v on quite a few different BYD systems. They seem to allow it in software.
Sometimes the graphs render a bit strange when zoomed out so much. Look at a 24 hour period at a time.
Thank you for your answer
Yes I get high voltage warnings, once per day, several day per month, when the battery reaches 100%, like this one :
I made a zoom in the curves :
So it's normal if the difference increases after the 100% ? I don't understand why, some cells are left on the side by the BMS while others continue to gain voltage, even after complete charge ?
I have the same issue with my setup. I have a Quattro with a Fronius on AC OUT and 2 MPPT, and a BYD LVS (8 modules). I get this warning almost every day. The only workaround is to go to DVCC and set "limit managed battery charged voltage" to 55. The battery will reach 99% and you won't have the alarm, but the behavior is the same:
Mine reach 3.7x sometimes if I don't set a limit, I am not sure why.
Conversed with BYD support - they said this is not an issue as the voltage is within permitted operating parameters
You can also avoid the alarm by setting the SOC to below 100%, say 99% - one way to do this is via nodered as otherwise the GUI only allows increments of 5% for SOC target
Snímek obrazovky 2024-04-08 v 17.54.14.pngSnímek obrazovky 2024-04-08 v 17.54.37.pngSnímek obrazovky 2024-04-08 v 17.51.31.pngSnímek obrazovky 2024-04-08 v 17.51.41.pngSnímek obrazovky 2024-04-08 v 17.32.57.png
At what voltage does cell balancing start?I couldn't find this information anywhere.I have the same problem on systems:
2x LVS16
2x LVS12
1x LVS20
All systems have firmware version v1.26
On SOC 99% have cells about 3,36V - all cells
On SOC 99% have cells about 3,39-3,40 V - all cells
BMS see all time from SOC 99% - installed / available capacity 624Ah / 628 Ah ( 2x LVS16)Snímek obrazovky 2024-04-08 v 18.01.27.png
