Solar pv batteries distance to CU

[binky]

strings are rarely exactly the same. No idea about dropping to 0Amp for 10 secs - I've never seen that. It would suggest a possible issue with the MPPT, maybe a dodgy connecton? I would be inclined to contact the technical helpline. As this occurs when charging batteries at around 80% capacity, it's possible that it's limiting power to the battery. At around 80% battery charging tends to slow down to avoid cooking the batteries, slows down further at around 90%. Likewise at under 10% charge, the battery gets trickle charged until around 15% when it goes to full tilt.

 

[revor]

Thanks Binky,  What you state about the charging is what I have observed I have been making observations of what is happening to increase my understanding. Could do with a dummies guide to BMS. I had been in touch with the manufacturers via the distributors and they have checked the inverter remotely and say it is working properly but that would be the case as 99%+ of the time it is. It is when it got to the  80% plus point it started to drop out it was also when solar was highest as well so that may be putting a strain on a connection somewhere. In checking the MPPT 2 how would you go about it?. I ferruled all my connections to avoid the stray strand issue used all the proper tooling for the MC4 leads all the panel MC4 were ready made. So where do I start. 

 

[binky]

I would try one other thing first, when it gets upto the 80% mark, try putting on the washing machine and tumble dryer and see how it behaves then. I'm interested if having a load on the system stops string 2 from shutting down whilst still charging the battery? I'm thinking if this is the case, then it's a power control function of the inverter. 

The trouble with PV is checking continuity of a cct, which would show a dodgy joint, isn't very feasible as we don't have any info on the resistance of the panels - it's bugged me for years as this is a basic check for electrical ccts. The only thing I can suggest is powering down that string and a physical inspection of each connection ie disconnect them all and check for any signs of thermal damage on the connector pins, also look to see if the pins are fully located in place and not slightly short of the joining ends of the connectors. Also check the cable nuts are fully done up. Obviously be very careful when doing this as all pins are live so no poking fingers in the ends. 

 

[revor]

Thanks for the things to try. One thought I had was to swap the strings around that would be easy to do. Today been a dullish day brightening up later on. Battery was charging slowly got past the 80% and nothing happening but washing machine was running at about this time. When it got to 94% started seeing the issue so measured volts going into inverter on string 2 and it goes up to about 330V from about 280 when it happens. If monitoring volts on the inverter you can see it happening there as well.  Generating about 5.5 Kw when sun came out most going to grid battery volts about 60.2 charging at about 1kw decides to discharge at about .2/.3 kw but what was strange the system wanted to push juice into the house at around 1KW there was nothing on to draw the load but it kept fluctuating between 0.3 the normal background load and 1Kw.  When got around to putting the immersion on it had settled down but sun had gone behind clouds  as well.

The spec sheet for the inverter includes this bit appended showing there is a difference between the 2 MPPT's  which  I do not understand the significance of but could be the explanation and along the lines you are thinking.

 

[binky]

so it's turning one string off and going to 'stalled voltage' which is higher than running voltage as the MPPT drops voltage to gain ampage / power. 

Is the inverter 16A limited to meet DNO requirements? That may explain why it tries to shove leccy into the house then fails.

Swopping strings over is an easy check. 

 

[revor]

Don't think it has anything to do with limitation as it is up to 6Kwp although never is that, was putting up to 4.7 yesterday.  It can generate up  to over 6 on occasion but on a prolonged sunny day as the inverter does more work and gets closer to 60 deg c throttles back and will settle around 5.5 generated and will do that happily all day. The impression I am getting is that this issue has cropped up recently otherwise think would have noticed it sooner but could have been there from start. Most of the summer have hardly used the battery fully just the top end.  As we have had duller days often only generating 0.3 to 0.8 Kw solar the battery has got used more to the point of empty. All summer  not had that  situation, at end of day, often much sooner was 100% full. Now it is starting from a much lower point close to 1% on some days the last week or so and does not get to 100% . At about 80% it can be 60+ volts then it start discharging and charging working its way up but in recent days not got to 100%. Shorter cloudier days. Yesterday got up to 94 and 60.2 ended up at 19.45hrs  92% and 59V. This morning ended up at 52% 54.2V. Promises to be very sunny today so will see what happens. I am struggling to see the relationship between SOC and V but in the last couple of week have seen the highest and lowest V since been up and running. 61v at 86% and 51.8V at 24% I am wondering if a contribution is that  the battery has not been used enough and now it is catch up time. 

 

[binky]

what I was getting at, is if the battery is charged up or nearly full, there's nothing on in the house or very little, then perhaps it's trying to limit waht is being exported back to the grid to 16A or 3.6Kw.

To be honest, the working of batteries is a bit of a mystery to me as I havn't got one myself, and can't sit around all day watching those I've fitted for customers. BUt if it's working and generally behaving itself it's probably best left alone. My one point of concern is that you say the battery depleted itself to 1%, I would be looking to set a minimum retained charge of around 10%. My reasons for this is the battery charging tends be very choked until around the 10%-15% level to protect the battery cells, so can be very slow charging until it can push more juice into the cells. One system I fitted took 2 days to get past this low start level, and mid winter we had to set it to take some charge from the grid because if the sun did come out, it failed to put much into the battery.

 

[revor]

Yes I think you are right system is limiting what goes to the grid I wonder if it is to doing it by meeting resistance from the grid to take it. The normal background grid volt here is about 245 V but at peak production it goes up to 255/256V  soon as load on or producing less solar drops to about 250V .There is a function that stops export all together have not tried that recently but that controls by throttling back the solar output don't know how it does it.

I too have been concerned about the battery "collapsing" to a low percentage but I do not know what that means in terms of acceptable volts and do not know the relationship between SOC and volts. Been taking a lot of readings to see if I can find the correlation between the two  best at moment I think that it exhibits a hysteresis  it is different on way up (charging) to when it is discharging. All summer we have never really used all the battery as I was told that the BMS has not "learned", the analogy I was told was "how do you know how much a bucket holds unless you empty it and fill it". My understanding is as yours it should not go below a set minimum charge mine is without looking up 20%. during the recent period I have recorded the highest and lowest voltages since commissioning  61V and 51.9 but do not know the expected range for a 54 v battery. What have been led to understand is 60 V is considered 100%. When it  was low it was taking charge from the grid until a certain point then solar took over. 

 

[revor]

There was something I wanted to add to the last posting but is not appropriate to be in the public domain. Is there a way can communicate direct to a member without it being viewable to all? Another forum I am on there is a facility called whispering where you can communicate to another without the post being available to all. Is there a similar facility on this forum?

 

[Sharpend]

There was something I wanted to add to the last posting but is not appropriate to be in the public domain. Is there a way can communicate direct to a member without it being viewable to all? Another forum I am on there is a facility called whispering where you can communicate to another without the post being available to all. Is there a similar facility on this forum?

If you click on an avatar you will see an envelope which is a Private Message. 

 

[revor]

Thank you Sharpend much appreciated. Learn something every day. 

 

[binky]

Yes I think you are right system is limiting what goes to the grid I wonder if it is to doing it by meeting resistance from the grid to take it. The normal background grid volt here is about 245 V but at peak production it goes up to 255/256V  soon as load on or producing less solar drops to about 250V .There is a function that stops export all together have not tried that recently but that controls by throttling back the solar output don't know how it does it.

I too have been concerned about the battery "collapsing" to a low percentage but I do not know what that means in terms of acceptable volts and do not know the relationship between SOC and volts. Been taking a lot of readings to see if I can find the correlation between the two  best at moment I think that it exhibits a hysteresis  it is different on way up (charging) to when it is discharging. All summer we have never really used all the battery as I was told that the BMS has not "learned", the analogy I was told was "how do you know how much a bucket holds unless you empty it and fill it". My understanding is as yours it should not go below a set minimum charge mine is without looking up 20%. during the recent period I have recorded the highest and lowest voltages since commissioning  61V and 51.9 but do not know the expected range for a 54 v battery. What have been led to understand is 60 V is considered 100%. When it  was low it was taking charge from the grid until a certain point then solar took over. 

as far as I'm aware, system limiting is by making the MPPTs less efficient, but it sounds like your inverter targets one string to achieve this. I guess each manufacturer will have different means of controling power. The inverter is designed to protect the battery either way, so I wouldn't worry too much about that. Not sure about battery 'learning', they tend to work to preset programming, but the inverter and BMS do talk to each other. To be frank, it sounds like it's working properly, especially if it is topping up the battery from the grid occasionally. You will see grid voltage rise, the inverter will be loking at the grid, then exporting a higher voltage, so that like water at a higher level, energy from a higher source, ie your panels gets used in the house first. I've seen inverters put out 260V before and I think many are set to max at 263V.