In another thread, @hennephuis posed the question
and suggested a new thread for a separate discussion.
Yes, absorption is very long.
I’m not home now but I’ll post later a graph where you can see that.
After going under the threshold, I have seen 56V from maybe 10:00 until 22:00, when the MPPT can’t hold the house anymore and the battery starts to get lower. The next day it won’t go to 56V but to 54.8V if it didn’t reach the threshold.
I have 15 sec resolution in home assistant.
I only saw a 5 to 6 hour absorption the first charge, now i get just the 2 hours. 12V on a boat and the cells stay within 0.01V so my balance is not drifting, my bstteries normally go down to to 85% SOC and occasionally if I moor for a few days without engine running to 50% so once a month at 2 hours is OK. The battery manual says that if you are heavily cycling to use 8 hours per month, so your suggestion of a 7 day cycle at 2 hours absorption would be good.
For the record to the OP: is the original topic about not syncing to 100% understood now?
If so, we could probably better start en new topic about why the absorption takes so very long. I also experience that it last until the MPPT can no longer deliver enough power (at least an hypothesis), just like @zedamoca reports. What @pwfarnell says about this seems right how it should work, but apparently it does not always do that.
Agree to start a new topic?
I have had an absorption charge today after posting the reply above. I have the default 2 hours still set. My boat has been moored over the past few days unused and gently cycling between 95% and 100% SOC on solar keeping up with a few minor users that were running. The absorption time lasted less than 30 minutes presumably because the batteries were well balanced. A side note, this was carried out by solar.
There is nothing in the BMS documentation that the absorption time is not a hard duration but that it can vary based on the state of balance as it appears to work that way.
I expect that this section copied below that I noted above in the battery manual needs a bit more visibility because both of you may well be needing to have more absorption. I have a feeling that to get the best that the user really needs to do a bit of fine tuning on the repeat absorption interval and SOC threshold and absorption time.
6.1.2. Causes for cell imbalance or a variation in cell voltages
- … … … … … … … … … We recommend a minimum absorption time of 2 hours per month for lightly cycled systems, such as backup or UPS applications and 4 to 8 hours per month for more heavily cycled (off-grid or ESS) systems. This allows the balancer enough time to properly balance the cells.
Thank you very much for sharing your experience. I believe what you describe is as it is designed to work. And partially how it should work; but it doesn’t in reality.
The section of the manual you copied is known to me and it is this section that worries me.
The batteries I got were next to perfectly balanced when they arrived and they have not for a single moment be unbalanced. There is no need for 8h or more balancing, on multiple days within a week.
Although I agree that the absorption time should be a function of the balancing, the general feeling I get when I read the manuals is that this is not the case. In reality, absorption time is a fixed time and the burden is on the installer to make sure that the combination of parameters ensures ‘enough’ time for balancing. Read it back: that is exactly what you said and what is in de section you copied.
This is the problem: absorption should stop after syncing and balancing. But that is not how it works. It stops after a given time. We all just hope that the time is enough to balance, but we can never be sure. And here is the trouble.
What I suspect is that the very same reason why syncing did not take place also causes the absorption never to finish. Because it is time based, it must start a timer when absorption starts and then stop the timer when it reaches say 2h. However, chances are that this works the same as the timer for the synchronization: 56V, less than 4% current and that for two hours.
But the constant rippling lets the voltage drop under 56V often and repeatedly, which causes the timer never to register 2h.
For synchronization we can work around this by setting the monitoring voltage a bit lower to say 55,9V. But the BMS has no setting for the voltage used for measuring the absorption time and it does not, apparently, use the monitoring voltage setting.
A work around would be to create such a setting or let the absorption time use the monitoring setting. Victron would have to do that.
But the real underlying problem I believe is the constant switching between charging/discharging that the system is doing while in ESS mode. That causes a strong ripple ruining some measurements. There are a couple of threads in this forum about that problem. It could be very well the case that this rippling also messes up synchronization and absorption.
W.r.t absorption for balancing: I believe that the BMS should stop absorbing as soon as the cells are balanced and not leave the burden to installers who can never have these setting right, because they shift with different usage patterns.
Added: the above hypothesis is consistent with the observation that absorption stops when finally the MPPT can no longer sustain the loads.
Sure, we can debate separately sync and absorption/float matters
well, as long all the cells are top balanced, I’d believe the absorption is not more needed.
Maybe this is what trigger CVL to 56 or to 54,8V
Same here.
Indeed. This is the same conclusion I have.
I think you got this exact. Well done. This goes in line with my observations. I didn’t found yet the time, but the funny thing is that in 3 phase config, I never noticed this. I started noticing when I changed to parallel and the sync stopped happening.
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If you want to avoid having the BMS (over-) charge your batteries by staying at 56V until sunset, you could try the “Maximum charge voltage” in de DVCC-settings, if it is there. Attached is a screenshot from the Ekrano. Unfortunately, I cannot find any documentation on this setting.
Do not forget to adjust the monitoring voltage accordingly.
I set mine to 55V until this issue is somehow solved or a better workaround is found.
This is how all this story started in the beginning of the other post before the split. Adjusting there.
The thing is that now I understand much better the mechanics, I have adijusted it now to 54,9V 
Thanks to remember be of that setting.
So, now if it goes under the threshold it absorbs at 54,9V, if it doesn’t it floats at 54,8V
this is 3,43V and 3,42V per cell. Much more in line with what I wanted.
Good to hear! But now we have workarounds for both the failing synchronisation and the eternal absorption.
But these are workarounds. Real solutions would address the cause of te problem, namely the strong ripple that causes the algorithms mis their timers.
Also, I believe it is not really nice to have installers keep tracking the behaviour of their systems in order to adjust the settings on a continuous basis.
How do we get Victrons attention to these matters?
I will let people know. I have not participated much more as I am not having the issue not being a true ESS system.
Limit managed battery voltage.
See the manual Section 11.4.2.
Here is the documentation I could not find. Thank you!
It has some important remarks:
This is what Limit managed battery charge voltage is designed for. Enabling this makes it possible to lower the maximum charge voltage until cells have balanced.
Do not use it for any other purpose as it may have unwanted side effects, for example, balancing can fail or not start at all if the charge voltage is set too low, causing the battery cells to become severely imbalanced over time. It is also not possible to set the value above the charge voltage limit (CVL) sent by the battery.