SOC and MPPT float

DIY
, , ,
1

I am having difficulty determining an accurate SOC. I have installed a Cebro connected to 4 MPPTs, a 300a SmartShunt to measure wind output and a 500a SmartSmart shunt to measure the battery bank. Today all my MPPTs went to float at 2:30 and SOC shows 88%. The battery draw at this time is only averaging around 7a and there is lots of sun. The SOC showed 87% at the start of the day. The MPPTs floating well below 100% SOC has been happening for a while.

Questions:

-Is the SOC incorrect or did the MPPTs float too soon. I don’t see any way to prevent them from floating, so I assume the SOC is actually close to 100% .

-Best information I could find for solar setup was to set charged voltage 0.2v below absorption. I also charge with generator but mostly solar. Should I just sync my battery to 100% .

Setup on my boat in the Caribbean:

-4 225Ah Mastervolt AGM batteries for total of 9 00Ah

-3 150w solar panels with 3 MPPTs

-1 175w solar panels with 1 MPPT

- 400w wind generator

MPPT settings:

Battery Vol tage12v

Charge Cu rrent15a

Battery Preset User Defined

Expert Mode Off

Absorption Voltage 14.25v

Float voltage13.80v

Equalization Voltage 16.20

Automatic Equalization Disabled

Temp Compensation -16.20mv/degC

Low temp Cutoff Disabled

500a SmartShunt settings:

Battery Capacity 900aH

Charged Voltage 14.00v

Current Measurment Direction Normal

Discharge Floor 50%

Tail Current 2% ( I had it at 1% earlier)

Charge Detection Time 5 min

Peuket Exponent 1 .2

Charge Efficiency 88 %

Current Threshold 0.10a

Tim e to Go Averaging Period 3min

Battery set on Reset Keep SOC

2

Settings look ok to me. So if the battery hits 14.0V and the current is below 18A for over five minutes, then the SOC will be reset to 100%.

Did you connect the main shunt correctly, meaning is there only one cable going from the battery negative to the main shunt battery terminal, no other connection?

Have you checked if the MPPT can actually stay in absorption for those five minutes? Meaning, is the weather good enough to keep the voltage up for that time?

Did you check how much of that MPPT output voltage made it to the battery, i.e. how big the voltage loss is across the cables? Check the battery voltage while the MPPTs are in Bulk charge, its very likely that you will see 14.0V at the MPPTs battery outputs but only 13.7V for example at the battery terminals.

3

You make a good point on the output voltages. The MPPTs say they reached over 14v , but when I look back at shunt measurements, I didn’t see anything higher than 13.9v. This is probably when the MPPTs go to float as the voltage drops after these peaks. What is the solution to this? Even with this being the case, I’m surprised that the SOC raised so little over the day. The shunt is correctly installed.

4

Use a multimeter to find where exactly that voltage drop is occurring, checking fuses, MCBs, crimp connections, screw terminals. Its possibly also just a culmination of many small drops, so possibly not much you could do apart from changing the supply cables for bigger gauge.

Depends on the weather. What were the peak power figures of the MPPTs/Wind generator today? I would also expect a bit more, but thats why i asked if the shunt is wired correctly

5

If the MPPS change to float before the current deceases to the tail curent, syncronisation is impossible. The “charget voltage” has to be lower than float voltage.

6

Charged detection voltage is generally set 0.2V lower than absorption voltage. Chargers will stay in absorption until the tail current is low enough, 1A for small models, 2A for bigger ones, plus an adaptive or fixed max absorption time.

A reset to 100% happens when both the voltage is above charged detection level and the current is below the tail current setting. Which usually is no issue to achieve regularly through MPPT charging.

If the reset to 100% happens too early, adjust the tail current on the shunt down.

If it happens too late, or not at all, adjust the tail current on the shunt up.

If your chargers are having trouble maintaining absorption voltage level due to weather/low irradiation, adjust the charged detection timer down on the shunt.

Setting charged detection voltage under float level could trigger a sync to 100% even if theres no charge at all. Applying a load pulls the voltage down, removing a load will let the voltage rise again. This could be enough to trigger a reset, which would be the wrong time.

7

I haven’t seen any Victron docs which detail this - it that 5 minutes continuous or cumulative? i.e. if a cloud comes by and the voltage drops briefly, does the 5 minute counter reset to 0 or just pause counting?

8

Good question, i dont know a factual answer.

I would expect it to reset just from a gut feeling.

9

But the battery is not fully charged. And if 18A are the result af a load a SOC of 90% or less is possible.

10

If the battery voltage is at absorption voltage level, then it can be considered near fully charged.

The tail current when it comes to a SmartShunt is a charging current, so a current flowing into the battery. A load current, so flowing out of the battery will not trigger an SOC synchronisation.

But, of course its possible that a load can lower the net charge current. For example, a 20A charge current coming out of an MPPT and a 15A load will result in only a net charging current of 5A. This is well below the tail current setting. So you are correct, this could trigger a false reset to 100% SOC. But if the battery also stays above the charged detection voltage then i would not worry that the SOC is too far off from 100%.

However this situation should be rare. If it occures more often, then the only thing you could do to counteract it is to lower the tail current setting.

11

I am the original poster. Today, three of my 4 MPPts have gone to float, so there is no chance of my shunt seeing over 13.8v meaning it will never sync to 100% SOC it was getting up to 13.9v and 14v momentarily. My charge current is down to 10a on a 900ah bank, but voltage is only around 13.5. SOC is reading only 84% which I believe should be close to 100%. Is there anything I can do to rationalize this?

12

That is another problem. Use VE.Smart Networking. So all MPPs will go in float.

There was a mistake: The “charged voltage” has to be a little (0.2V) higher then float voltage. The same problem in another installation:

image
image1064×648 46.2 KB

The Mpps changen to float when SOC was about 92%. Because Absorbtion duration ist limited. SOC jumped to 100% at 5.9A and 27.14V

image
image1048×590 38.7 KB

13

My problem seems to be that my charge voltage won’t stay above 14v for more than a few seconds. Not enough time to sync the shunt, but enough time to put the MPPTs into float.

14

What’s the battery temperature? The base is 25C, if they’re around 40C, the absorption voltage would get adjusted down to ~14 V. Even though the shunts can be configured to measure temperature, I don’t think they do any voltage compensation (there’s no setting for that).

If the MPPTs aren’t receiving temperature info from some other source, they’ll use their internal temperature sensors, which will probably read higher than the actual temperature of the battery - they warm up when operating.

You say you have 4 MPPTs connected to the Cerbo. How? A Cerbo only has 3 VE.direct ports (and you also have 2 shunts, which only do VE.direct). Do you have VE.CAN MPPTs? There are lots of options with a Cerbo - are you using DVCC, and if so how are those options set?

15

They use the internal temparature in the mornin when the MPP wakes up. When there is no Current for an hour the use the temparatur from this moment.

16

No temperature sensor at present. Ambient temp in battery area is ~30. Three MPPT connected by VEdirect, one Mppt and two shunts are connected by VEdirect to USB cables. The MPPTs are not smart so are not connected by Bluetooth. Today I did enabled the DVCC.