SoC drops about 1% with each recharge cycle

DIY
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1

Folks, my solar power system has been working well but for one issue. Since I installed a MultiPlus II, when the system charges up it does not reach 100% The voltages are correct, I have the charge voltage set to 29.2 vdc (28v system) as indicated by the manufacturer LiTime. The float voltage is set to 27.4 vdc IAW LiTime recommendations.

I have allowed the system to come up to full charge then set it to 100% and it works just fine, the next day it gets light starts charging, reaches 29.2 vdc and after a bit it goes into absorption and at some point has settled down to 27.4vdc. But it never shows 100% SoC.

Prior to installing the MultiPlus, it work exactly as it should, reaching 29.2 vdc and after some amount of time and current flow it would then go into absorption, and repeat the same the next day. I have been trying to figure out why the SoC indicate less than 100% even though the voltages and current all seem to show that the batteries have reached full charge. Here is the screen shot of the Smart Shunt which meters the batteries

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Here is the settings view of the same, the voltage settings are visible here.
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This is the 3rd day since I synchronized the SoC after allowing it to reach full charge. As you can see it has dropped down too 97% even though the voltages and charge data indicate that all is full.
The next images are from the MPPT controller, a Victron 150/70.
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This is what I expect to see from it after having a nice clear morning to suck down the solar power. The next screen is the settings screen for the controller.

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I just cannot figure out why it never quite reaches 100% SoC even though all the numbers indicate it is there.

Just as I was about the close this, the system went into float, here is the screen shot:

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I use the charger on the MultiPlus II just when the solar is not going to give enough to keep things powered, but it is not that common that I use the charger.

2

You could try decreasing the tail current and increasing the charge efficiency to 100%. What is your Sync count value?

3

If you have an absorption vpltage of 29.2V, then you should set the charged voltage on the SmartShunt to 29.0V, below your absorption voltage to make sure it synchronises to 100% every day. Maybe before the Multiplus was installed it was peaking just over 29.2V but not anymore.
Failure to synchronise is a regular report if the charged voltage on the shunt is the same as absorption.

I would try this before you try the suggestions above, especially reducing the tail current which makes it harder to synchronise. Increasing the charge efficiency may well help BUT it does not answer the basic problem that you are no longer synchronising at 100%.

4

Thank you I shall do so, I am not sure I made it clear but each day it drops by about 1% Today it had charged to 97%, yesterday was 98% and the day prior was 99%. The day before that I had reset it again to 100%

5

The other thing is to make sure that the synchronisation settings in the shunt are achieved BEFORE the MPPT goes into float. If you have a tail current setting in your MPPT to end absorption (this is found in expert settings), then make sure this is lower than the tail current in the shunt settings, say 2% in the MPPT vs 4% in the shunt. The shunt needs 3 minutes after getting to 4% tsil current to synchronise, so stopping the MPPT going to float in that 3 minutes is important. You can refuce the 3 minute time, but you then risk passing cloud causing a premature synchronisation.

6

Shall do.

7

Yes, I am awsre that this is what you mesn. Changing the charged efficiency to 100% can alleviate some of this once you have proper synchronisation again. Also the Peukert exponent can have some impact on the mismatch. When you have reliable synchronisation you may find once full the SOC jumps from 99% to 100%. Then start tweaking these 2 settings if that bothers you.

8

OK, tail current is disabled on the MPPT settings screen. I set the SmartShunt to 29.0 vdc.I also re-synchronized while it was still in absorption. So now I guess just wait and see. Thank you for the quick response.
Oh how do I reach that level of controls remotely? I am going out of town and would like to be able to continue to make small adjustments but I do not see it on the remote screens.

9

You can only do settings changes remotely if you have the equipment connected to a GX device such as a Cerbo. Then you can use VictronConnect through VRM.

10

OK, I did the settings (not the tail yet) setting the charged voltage to 29.0 vdc and kept an eye on it today. lt got p to 29.2 and stayed around that voltage until it went to flow where it dropped down to 27.4 and stayed there. I observed it several times to see if it would drop down to 29.0 but it never did, it dropped down to 29.1 and above. But it still came back as being at 99% after it was all said and done. I looked at the tail current it is in percentage on the SS and it is set to 4.00%. I went to check it on the MPPT and it is disabled. I enabled it to check and it is given in amperes, the default it 1 amp. So not sure about doing that setting. I am getting ready to go out of town to do some service work will be away for several days. I am going to try again to access the system remotely to that level, I can see a lot of things to control but have not been able to see the settings I see when I am connected over bluetooth and Victron Connect.

11

The 29.0V charged voltage setting IS NOT a control voltage, it is a monitoring voltage so the voltage does not go yo that. For the shunt to synchronise to 100% the voltage has to be above 29.0V AND the current below 4% of your battery capacity (16A) FOR 3 minutes. Either your tail current does not get low enough or it goes yo float before the 3 mins.

The tail current on the mppt should be set to something like 8A, or the absorption time to 15 mins.

Post a screenshot of your battery boltage and current on the Smartshunt as it reaches 29.2V until it goes into float.

12

I am going to try to access that level remotely I am out of town (8 hours west of home in Albuquerque NM) for
the next week or so. I will try to set those things remotely.

13

I keep on trying to catch the state of charge at the critical time but every time, I am away. Working on it, this is my busy time as I service medical lab equipment and they are all up for inspections and of course they want the gear PM’d just before they all get inspected. Such is life…