question

colin avatar image

multiplus SoC not syncing to 100% (mains backup system)

Hello,

I have a system with:

1 x MultiPlus 24/3000/70-50 firmware 476 (but problem predates this firmware too)

4 x Trojan SAGM 06 375

for mains backup.

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Edit 2020-11-24

Having clarified a few things since first opening this post I'm listing here the key points as I see them:

The system is not triggering a bulk/abs cycle after anything except a very deep discharge. I.e. it returns straight to float after small to medium discharges.

This means that over time, with several small/medium discharges the SoC indication gradually drops off, when really it should return to 100% once mains returns after such discharges.

I have been told that, by design, the bulk/abs cycle is triggered when the battery voltage drops below 26.2V (2.6V, for 24V systems, below the set absorption voltage of 28.8V). This is happening here, yet the bulk/abs cycle is not activating.

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Original post:

The SoC indicated by the system will not sync to full for long periods of time, which results in the system always showing well less than 100% even when in float. Please see the VRM screenshot below.

I raised this issue somewhere on Victron community/disqus a good while back, but can't find it anymore! We never got a resolution, but I recall it was discussed that the reason was:

i) the battery voltage doesn't drop low enough under the brief discharges/small loads to trigger a full bulk/abs cycle (the SoC does reset to 100% on the rare occasions this does happen).

ii) the repeated absorption feature (which is set at every 7 days on this system) does not trigger a SoC reset on completion

I'd be very grateful for any ideas on how to resolve this, especially now as the system is using the new "shut-down on SOC" feature, which means the batteries will be cut off sooner than necessary due to starting with an already low SoC.

Many thanks

Edit 2020-11-02

I've added vrm screenshot to compare charts for the same mains failure event for this system and another very similar system (only differences I can think of are 48Vnom/375Ah vs 24Vnom/375Ah and CCGX vs venus GX). This system goes straight back to float, other system goes through bulk/abs cycle.

1604294982946.png

Edit 2020-11-24

Just adding another screenshot here as we've had some interesting events over the last three days. Of note is how a bulk/abs cycle was triggered by the full discharge to 40%. However, the most recent discharge to 80% didn't trigger a bulk/abs cycle. I'm wondering if perhaps somehow this system has the bulk/abs cycle voltage level trigger of a 48V system even though it is a 24V system?

1606220203663.png

MultiPlus Quattro Inverter ChargerSOC
1601472667908.png (88.6 KiB)
1604294982946.png (142.0 KiB)
1606220203663.png (171.2 KiB)
2 comments
2 |3000

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I’ve seen Thai probl m with easysolars (I and II) I think it’s calculated and there is some leakage in the calculations... I fixed it by putting in a BVM-712.. instantly showed 100% SoC and hasn’t changed except where there was actual change. FWIW discharging by 5-10% by removing the grid and then reconnecting the grid will cause it to reset and charge back to 100% on the graph as well as in reality. Other interesting thing is allowing it to drop to 20% SoC the inverter doesn’t shutdown.. but if you go off grid when it is that low the UPS functionality doesn’t work.. it will drop the inverter, work out the battery voltage is high enough, try to start and realise it can run... go figure..

thanks misullivan

that's just it - the system doesn't go back to 100% after a brief grid interruption. It does not trigger a bulk/abs cycle like other systems. I've added a screenshot to my op showing a comparison. This bulk/abs cycle is what permits a recalibration to 100%. I don't know how the inverter decides to do a 100% recalibration, but it only seems to happen after a bulk/abs cycle (doesn't happen on repeated absorption or prolonged float AFAIK).

I don't feel that adding a BMV should be necessary, nor am I convinced it would help in this case (admittedly also don't want to spend the funds and effort! XD)

2 Answers
colin avatar image
colin answered ·

(Edit - just to note that I am the poster of the question and have accepted my own answer!)

It was eventually discovered that the re-bulk voltage is tied to the float voltage, not the absorption voltage as I was previously advised. Based on this, the inverter has been performing as designed (that is - only triggering a rebulk whenever the batt voltage dropped below 24.4V (2.6V below float voltage of 27V)

So the issue apparently is that the Trojan SAGM batteries have a lower than typical recommended float voltage and this does not play well with Victron's re-bulk mechanism. It was only at very deep discharge levels that the voltage would drop below the re-bulk level. Smaller, yet still significant discharges would not drop the voltage below re-bulk level.

Setting the float voltage to Victron's default of 27.6V has fixed the problem. Though there is some concern that Trojan's recommended float setting (27V) is not being used.

Edit: The reason that the system SoC was drifting below 100% was because it was not going to bulk after most discharges. As far as I am aware, the Multiplus only syncs to 100% after having completed a full bulk-absorption-float cycle.

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Your answer does not match the original question, or opening post.

I can not find a re-bulk in the "charger" settings in either Victron Connect, or VE config, for a multiplus...


@klim8skeptic

Thanks for your comment. I am both the poster and answerer. I see what you mean about it not matching. I have updated it to explain the link!

The re-bulk setting is not a user/installer changeable setting - it is hard-coded in to the system.

There are in fact quite a few posts on the community about this (search for "re-bulk"), like this one:
https://community.victronenergy.com/questions/52921/multiplus-not-bulk-charging.html

Alexandra avatar image
Alexandra answered ·

@Colin

Have you every cycled the batteries down to about 60% then back up again? We have just done a new install and day 2 was when we had a good VE bus SOC sync after about a 60% discharge overnight.

Also what affects the SOC are the settings for efficiency and bulk percentages under the internal battery monitor. Are you batteries reaching the correctly set voltages in your charger settings?

Another idea may be to check the storage setting since it is a backup system once you have sorted out the SOC issue.

https://docs.victronenergy.com/veconfigure.html#charger-settings

Unless you have told your system specifically to switch off on SOC then it will use battery voltage instead, so you can set the cut off voltage higher since they are AGM and you do not want to run them dead flat. Although I must admit that the SOC is easier!

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Hi Alexandra, very grateful that you took some time to look at this.

It did go down to 32% once back in June I see from VRM and did not change the behaviour.

The batteries are reaching the correctly set voltages according to the bulk/abs/float state indicated in vrm.

Since posting I found and revived an earlier email thread I had with Victron about the issue. It appears to be that the inverter is not triggering a bulk/abs cycle after short mains interruptions. Apparently it should do so when the battery voltage drops 1.3V per nom 12V below the set abs voltage. The voltage does drop below this in this system, yet the charger returns straight to float when mains returns and therefore does not trigger a SoC 100% re-sync. I haven't yet had a response from Victron to my latest update on the issue.

I should add that there's a little more to situation (didn't want to overcomplicate my opening post!):

I initially installed ESS, solely to make use of the dynamic low voltage cut-off feature. So I wonder if there is some setting automatically added by ESS which is now left over that I'm missing... (I have since updated the firmware to make use of the SoC cut off feature that was not available when I first installed - ESS is no longer installed) E.g. After posting the initial question, I discovered that DVCC was enabled on the GX - I presume that it was a "left over" setting from when ESS was originally installed. Anyway I've now disabled that, but the behaviour is the unchanged as far as I can see.

I have another similar system - same batteries but 48Vnom inverter rather than 24Vnom but that one works fine.

ooh, just saw in one of my old messages in the email thread to Victron my comment: "I seem to remember that the system had the same SoC behaviour even before installing ESS"

Another difference between this system and the other is that this is using Venus GX, the other is CCGX. Can't think how that would be an issue though...

It is an interesting one. Looking at your screenshots I see storage is triggered occasionally. Yet the soc drift? It may be a good idea to put in a bmv as was suggested. Especially if soc is an important feature of your design

Yes, that's the repeated absorption feature enabled for every 7 days (storage mode is not enabled).

The soc drift I feel is accurate given the settings - it drops off with each discharge and the float current and repeated absorption isn't enough to get it back up (0.8 charge efficiency set).

I'm stumped that this system doesn't bulk/abs after a grid interruption like I see with other systems (I've edited my op with another screenshot in regard to this).