question

Is the MPPT connected to the Multiplus II GX via a VE.direct cable?
Only with this connection the GX device can count the current coming from the PV and use it to increase the SOC.

Hi M. Lange,

Yes, the MPPT is connected to the Multiplus II GX via a VE.direct cable, and the MPPT shows up as one of the connected devices on VRM.

Here a screenshot from VRM. The system was connected to VRM on the 13th of January. The SOC started at 85% and its has reached a max of 87%, even though we have had plenty of solar generation in excess of consumption. I am just puzzled by this.

Hi,

I've stumbled on the same issue. As far as I could understand, you have at least two ways to make it 100%. The trick is that what controls this is what you have set in the Multiplus / VE Config in the Charger section.

a) If you have AC available, let the Multiplus charge the bank (at night for example) one time and reach the Float phase. It will reset to 100% SOC.

b) If you lower a little (100mV) the absorption voltage set in VEconfig/ Multiplus charger, it will help the system recognise absorption phase when power is provided by MPPT. And more importantly: lower a little (try to make it still sensible) the absorption time set in Charger setting in VEconfig. In my experience, once the total absorption time (provided by MPPTs) exceeds this time set in VEconfig, it also resets to 100% SOC.

Hi Elimac,

Thank you very much for your suggestions. We don't have AC available yet, so I will try the other options you suggested. In a past post I saw the suggestion to slightly lower the absorption voltage in the Multiplus charger vs. the MPPT, and I did that already. The absorption voltage in the Multiplus is set at 56,4 Volt vs. 56,6 Volt in the MPPT charger. But that hasn't solved the issue. Maybe the difference should be greater.

I will therefore try your other suggestion, and shorten the absorption time in the Multiplus menu. I will let you know if ithat solves my issue. Would be great. Thanks again.

Attached is an image of the MPPT voltage and current curve widget. One can see from these that every day there has been a very short bulk and absorption period, followed by the float stage (on Jan 19th I lowered the absorption voltage to 54 Volt from the preset of 55,2 Volt, based upon the feedback on this issue from the battery distributor). On the Multiplus settings (general tab), I have 85% set as the "SOC when bulk is finished". So the SOC should have always finished the day above 85%, but it hasn't. I guess there is still something in the setting or algorithm that doesn't recognize the right SOC.

Hi Edgar. Some general observations..

1/ With 8x 150Ah batts configured 4S2P, you should have 300Ah set as your batt capacity. Have you?

2/ You have a short Absorb time because your batts are starting the day in good condition (probably better than 90 %). If you're using an Adaptive algorithm, that's to be expected, so all good there.

3/ In Float, the charge current 'tail' is quite low (<1% of batt capacity), so yes, they're effectively 100% charged when you see it taper down to constant A. You seem to have a small fluctuating load, like maybe a fridge, but doesn't matter.

3/ If your base 'tail' is just 1A, then it would take 3 hours to show a 1% SOC rise. If you have a Charge Efficiency setting available, you could lower that too, it will calculate a faster rise in SOC.

4/ Don't forget temp compensation when setting your V targets. Your mppt will do that for you unless turned off, so no need to adjust for it. Your batt maker/supplier should be aware too when recommending V's.

Maybe some of this can help..

JohnC,,

Thank you for your comments. yes, my battery bank capacity is set at 300 Ah in the Multi menu. regarding the V´s, I have set them according to the battery manufacturers specs. I got an email from the manufacturer, stating the specific V´s, so I have no doubts there. I have also set the V´s slightly higher in the MPPT vs. the Multiplus, something I read wrecommended in the manual.

From the various blogs, I see that many are having very similar issues as I am having regarding the SOC.

Next possible solutions I will try are:

a) lowering the parameter on charge efficiency, as you suggested (thank you);

b) raising the setting under "% charged when bulk is finished" from 85% to 90%, and see what happens then.

I am still puzzled by all this....Can´t seem to figure out what is causing the incorrect SOC reading. Since the system goes through a short bulk cycle each day and reaches the absorption voltage, the SOC reading should at least jump at that point to the level set under "% charged when bulk is finished", but it doesn't.

@Edgar Graadt van Roggen

Try setting the maximum absorption time in Multiplus as LESS when compared to what you usually have with the MPPT.

Hi Elimac,

Thank you for your recommendation. I will try that.

Looking a my DC charge voltages and current graphs, I think I am on to something. And it is related to the adaptive absorption time algorithm.

From the DC graphs I see that my battery bank voltage never drops below 51 Volt at night. Under the adaptive absorption time algorithm, if the battery voltage at solar start-up is higher than 12,6 Volt (i.e. 50,4 Volt in a 48 Volt set-up like mine), then the maximum absorption time is reduced to 1 hour. And I see that that is exactly what is happening in my system.

When the system was installed and connected to VRM about a week ago, the SOC started at 85%, as this was the level set in the "% SOC when bulk finished" parameter in the Multiplus. Since the batteries where full and there has been little AC consumption, the voltage of the batteries has not dropped below 51 Volt and the adaptive absorption time algorithm has kept the absorption stage to 1 hour, and then switches to float.

I think that could be the reason why the SOC has slowly been dropping, as the absorption fase has been too short to raise the SOC (and float of course doesn't raise it either).

The only other thing I am still puzzled by is why the SOC has not returned to 85% automatically, every time the bulk stage finishes. It seems like the SOC algorithm is not doing an ´SOC reset¨ every time the absorption voltage is reached. I do have a higher absorption voltage set in the MPPT compared to the one set in the Multiplus (56,6 Volt vs. 56,4 Volt), but maybe the difference should be greater. I.e. maybe the Multiplus doesn't recognize that the absorption voltage has been reached and therefore does not do the "SOC reset" to 85%.

This reasoning sounds plausible?

Hi Elimac,

Mi problem is fixed!!! Today after finishing bulk, the SOC jumped to 90% and then to 100%. Last night I increased slightly the difference in the absorption voltage set in the MPPT vs. the Multiplus. Originally My system is a 48 Volt system, and I had originally set the absorption voltage parameters as 56,6 Volt in the MPPT and 56,4 Volt in the Multiplus. Last night I lowered the absorption voltage in the Multiplus a bit more to 56 Volt, in order to increase the difference. I think this change finally triggered that the Multiplus recognized that the bulk stage in the morning hours had finished, as the SOC first jumped to 90% (as I set in the Multiplus in the setting "% SOC when bulk is finished"). During the absorption stage the SOC then jumped further to 100%.

The manual indicates that one needs to set the absorption voltage in the MPPT a bit higher than in the Multiplus, and I guess it is a matter of trial and error to see how big this difference needs to be in your system in order for the SOC algorithm to work correctly. The MPPT and Multiplus measure the voltages at their own cable connection points, not a the battery conector itself, and I guess because of voltage losses in the cabling, the voltages seen by the MPPT and the Multiplus will be slightly different (but always higher at the MPPT contact).