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on grid but battery still looses power


Gabriël
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1 hour ago, gabriel said:

how would i determine that?

Morning Gabriel.

At night shutdown your inverter. You could do it during the day but you then also have to switch your PV off. With no load you disconnect your batteries from the inverter. There should now be no current flowing across the BMV shunt and the BMV should report as such. If your BMV registers current flow then it needs to be re-calibrated (setting 9).

Regards

Chris

 

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29 minutes ago, gabriel said:

would a disconnect at the shunt side not be sufficient?

Flipping the disconnect (as long as it is between the inverter and the shunt would have the desired effect. However one should not switch a disconnect off while it is under load, except in an emergency (that is what it is for). Most disconnects are purely physical so one separates the two contact far enough apart to prevent arcing. Thus the recommendation is not to switch under load. So switching off the inverter is a good idea. I have seen an inverter go because of the lack of a disconnect and the direct re-connection of the batteries seemed to cause a surge. If however your disconnect has arc extinguishing capability - then go for it switch it under load. I have a DC circuit breaker on my PV* which doubles as a disconnect which I happily switch under load while testing.

*Just in case someone decides to go cheap my PV has both fuses (outside) and a CB (inside).

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On ‎04‎/‎05‎/‎2018 at 7:00 AM, gabriel said:

how would i determine that?

EDIT: I see others have responded.

Start with by checking that the BMV show same data as the software.

If BMV differs, then come back here OR take the steps as per Chris ito BMV settings.

If BMV shows identical value, then go to V in the alfabet. :-) 

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On 5/4/2018 at 7:37 AM, pilotfish said:

it is not big enough of an issue for me to worry about.

the viking in the video doesn't share your perspective :P

3 hours ago, SilverNodashi said:

Check this out: 

i might be to stupid for this, but this looks like my problem, in fact my batteries were at 100 soc when i switched to solar this morning and now they are at 88 soc, see pic

outflow2.JPG.78e380c4e06f8f4b7b9cd71a2f64b3de.JPG

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5 hours ago, gabriel said:

the viking in the video doesn't share your perspective :P

i might be to stupid for this, but this looks like my problem, in fact my batteries were at 100 soc when i switched to solar this morning and now they are at 88 soc, see pic

outflow2.JPG.78e380c4e06f8f4b7b9cd71a2f64b3de.JPG

Hi  Gabriel 

An Axpert always has a small DC draw even when it is being powered by grid. What is of greater concern to me is the situation where I would expect your batteries to be in float and have a minimal charging rate. Instead your batteries are discharging I think there are calibration issues  with the BMV which is giving us a invalid picture. Solar/battery is 200W higher than our draw. Yes the Axpert uses self consumes about 50W but this does not account for the disparity. The BMV is a very accurate device but is reliant on its initial parameter setup to report correctly. You can compare a BMV to a blind bouncer at the door of a nightclub who determines how many people are in the club by the sound of the foot traffic. There is some background noise so you need to tell the bouncer when there is no one coming or go so that he can memorise what that sounds like. Once he knows that and is told ok the club is full he then accurately keeps tabs on patrons coming and going. 

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Hi Gabriel 

What does the graph for your soc vs batt watts look like? 

My system uses about 40w once I switch to grid can you post a Image of the graph?

2018-05-06_075251.thumb.png.6a55c592c9aa21da31bc6b0c5dda381c.png

As you can see my SOC still drops when I am on grid but very slowly.

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hi @PaulF007, i attach an image, but i believe @Chris Hobson's comment regarding the synchronization of the bmv702 is the way to go.

that's why i am now looking for a charger to top up all 4 batteries to the same float voltage and get the settings regarding charge/trail etc voltage on the axpert and bmv set correctly. as we will most probably have a few cloudy days early in the week it wont interrupt power harvesting too much.

thanks for your assistance guys, ill keep you posted on this!

5aeeef0790179_socvsbattwatt.thumb.JPG.8cf9bd52bd5c5e46a6ed5db07d48d9ce.JPG

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1 hour ago, gabriel said:

but i believe @Chris Hobson's comment regarding the synchronization of the bmv702 is the way to go.

Well maybe and maybe not, I decided to replicate your situation and take some measurements...

20180506_144442.thumb.jpg.c8cb422b6a244e96bc6b68f7f3af35c0.jpg

So in the above image I have switched off PV input and switched to grid mode, and as you can see the BMV is showing a parasitic draw of 1.5amps off the battery.

20180506_143752.jpg.95778374f3a656741dd5e2127c7c0143.jpg

The above meter is my favorite muliti-meter, it has 2 channels so can measure V and A at the same time, Vin against Vout at the same time etc. It is deadly accurate and it can record - I love it! 

In this instance I am only measuring 1 thing because I only have 1 current clamp, which is a pity because I need to measure amps into each of my 2 parallel Axperts. (The current probe cant fit over both cables at the same time, so I measured each inverter individually).

The current probe is set so that 100mV = 1amp.

20180506_144638.jpg.b909ec90fceb028cc0ed9b02c55b34fc.jpg20180506_144648.jpg.05a149918338556659e598ae639f00cd.jpg

The probe was 1st calibrated to zero, as you can see the one Axpert is drawing 0.75amps and the other is drawing 0.72amps, which when added is close enough to the 1.53amps reported by the BMV that I am not going to spend any more time looking for it.

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Thats what I was thinking as well. Just looking at the first screen shot , the total between solar , batts and house seemed to be close enough. It wont hurt to sync the BMV. Mine was out by quite a lot and I only realized it after 6 or 8 months!  So still do that. 

EDIT:

One other thing , I like to give my batts a "proper" charge from time to time so what I do is not to use that batts for two days in a row , by switching to grid at night , and then it will have plenty time to get a "proper" charge.  I use the Batt charge watts as a gauge as if they will only accept 30 to 40 w at flout I know they are as full as Ill ever get them.

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The first of Gabriel's images is easily explained by the Axpert's DC draw the second image is more troublesome for me. After years of working correctly I discovered my BMV was out by about 140W. Not a big number but enough to make a mockery of SOC on my small Lithium bank.

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9 hours ago, gabriel said:

how do you do that, with the axpert or a third party charger? and if with the axpert, how?

By not using the batteries through the night they will be fairly "full" in the morning ( I refuse to use Eskom to charge the batts but you also have that option)  Since the batts might have only discharged about 3% through the night there will be plenty time for them to get fully charged. The second "check" that I now do is to compare that BMV's amps with that of the Axpert's. The values wont be the same but the general trend should be same.

Hope it helps.

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On 2018/05/05 at 10:48 PM, Chris Hobson said:

An Axpert always has a small DC draw

Let me get a little technical again. It is pretty common for Hybrid inverters (and even the Axpert which isn't hybrid) not to have a current shunt on their DC side. What they do is measure power on the input, again on the output, and then they infer the battery current from there. This means it will hardly ever hit exactly zero (unless you give it no choice by disconnecting). Now if you've done any customer support at all you already know why this is a problem: Half of them complain that you're using grid power and they hate Eskom's guts, the other half complain that you're draining the battery unnecessarily over night.

The answer of course is to ignore both of them because they will never be satisfied :-P

With a Blue setup running ESS you can also increase the grid setpoint (a value used to offset the zero point it calculates) and pick a point that works for you.

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On 2018/05/05 at 10:48 PM, Chris Hobson said:

What is of greater concern to me is the situation where I would expect your batteries to be in float and have a minimal charging rate. Instead your batteries are discharging I think there are calibration issues  with the BMV which is giving us a invalid picture. Solar/battery is 200W higher than our draw. Yes the Axpert uses self consumes about 50W but this does not account for the disparity.

If I look at @gabriel 2nd pic then he has [PV 629W] + [BATT 96W] - [LOAD 526W] = Loss 199W

I took a few pics of my own setup this afternoon while both PV + Batt where required to supply the load with the following results;

  1. [PV 1121] + [BAT 1033] - [LOAD 1904] = LOSS 250
  2. [PV 1172] + [BAT 994] - [LOAD 2000] = LOSS 166
  3. [PV 1123] + [BAT 916] - [LOAD 1896] = LOSS 143

I already know from my previous test a few comments up that my system has a parasitic battery loss of 1.5amps or 75W - this is not a BMV error which I have shown to be very accurate but an actual battery current drain while the system is in grid mode with the battery theoretically idle.

In the condition where both PV and BATT are supplying the load I would expect MPPT loss and INV loss. As can be seen from my 3 tests and Gabriels 1 result this TOTAL LOSS can swing between 150W and 250W. I expect some of this variation has to do with ICC update delays so not all numbers refer to the same moment in time. 

I think @gabriel result is within normal range for the Axpert.

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