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Thank you for the great forum, Safe Driving over the weekend. Sincerely Jason
Chris Hobson

Karoo Farm Installation

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A big thanks to members of this forum for all the guidance. My installation is 99% complete. I just have cosmetic things still to do.

 

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The panels are 250W Yingli panels on a frame facing true North and I am able to alter the tilt to suit the season.

 

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Still need to use cable wrap to tidy this up and fence it off.

 

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Although weatherproof the combiner box and wiring to be covered to protect it from the sun. Plastic becomes very brittle in the Karoo and that insulation will perish in the sun.

 

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Inverter DB and batteries. Was pleased as managed to mount the DC circuit breaker over the armoured cable as it enters the house. I think it looks neater. Need to get flexible conduit big enough to take the 16mm cables from the PVs. The batteries are 4x12V 260 Ah VRLA batteries. The battery box is made from scaffold planking.

 

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My mother-in-law and wife punishing the system on day 1 with microwave and hair-dryer on at the same time. Need to do solar 101 with missus and her mother again since they have just failed. Still managing to get 1.7 kW from the panels mid -afternoon. I was getting 2.24kW at midday.

 

Very chuffed with the system. 

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Thanks Mike. If you like it then I have done all right!

 

We have had a generator up until now so energy management should be second nature but apparently not. 

 

Thank-you Alistair. I see the 3kVA can only handle 900w that must be frustrating. I am totally off-grid and was only going to put up 2kW of panels but either Mike or SuperDIY recommended I go the whole hog.

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Hi Chris,

 

It looks nice. Tell a bit more about your battery box? Is the second set of batteries below those that are visible? Can you access them to take voltage measurements for instance?

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Battery box is 3 sided and the batteries are stacked on top of each other. The cable visible connecting the batteries is quite long but is the same length as the two coming up from the bottom two batteries. The batteries weigh 74 kg so there is a wooden partition between the batteries on the bottom to support the batteries on top. I will attach a thermocouple when I Install a Victron BMV 702 battery monitor. Waiting for it but I think the supplier is pissed I did not use him to do the build. The Victron will allow me to monitor mid-point voltage which will help me determine when a remedial equalisation charge is necessary. The batteries are cool to the touch at the moment but admittedly I cannot really feel the 2 batteries below hence the thermocouple and midpoint voltage measurements.

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The rubber boots came with the batteries CB Solar and having looked around I think I am going to get a digital aquarium thermometer rather than a thermocouple. The model I am looking at looks neater than the cheaper thermocouples and I do not need to know that my batteries are at 1000oC. At that temperature I will have other things on my mind.;)

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Well done, Chris.

 

Must be a joy not hearing the genny full time, and seeing the reduction in your energy costs.

 

One question: Are you comfortable with your pv-mount and the wind? I moved to the WC recently, and learned some expensive lessons about the power of the wind.

 

I put a pallet with 18 new panels next to the battery house. They were only stacked, not tied down. A week ago the wind lifted 3 of them from the stack, put 1 next to the stack, 1 about 4m away and 1 about 10m away from the stack.

 

Someone came here last week telling a story of an acquaintice who installed R350k worth of panels and the wind flipped the whole installation over shortly after it was cemented into the ground.

 

If I listen to you I think you've gone through a very steep learning curve in a short time.

 

Congratulations.

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Hi Jakes

 

Thank-you.

 

You right I have gone through a very steep learning curve which has been enjoyable. My PV mounting is copied from a mounting here on the farm for a Lorenz submersible. I am concerned about the wind and the winter tilt is not as steep as it could be but I did not want it catching the wind. The frame is securely mounted to the old tennis court so I don't think the wind will lift the frame. I was concerned about the wind blowing from the south side and getting in "under" the panels. The house and tree should provide some shelter from the south and we had strong winds before the weekend (north) that did not do any damage. Each panel is attached by 8 Tekscrews so hopefully we will not be in for any surprises. I think if I lived in the WC I would have chopped down the tree and done a roof mount.

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post-822-0-93522400-1437120337_thumb.jpg

 

9:45am -Time for a cup of coffee. So I put the kettle on and it is a good time to see what the panels are doing.

 

P = V x I (91.8V x 36A) = 3304.8 W. But I only have 3kW of panels and it is the middle of winter (OK today's temperature could be making the panels more efficient).

 

The power being consumed is only 2415W  so where is the unaccounted for 800 odd Watts (plus we are drawing 600 odd Watts from the battery - even worse - now we are looking for 1400W).

 

Does anyone have a reasonable explanation?

 

I do not think the figures on the Axpert are accurate and will do the same experiment once I have my battery monitor set up.

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That does seem a bit optimistic. My 1800W of panels only get up to 1200W at midday (measured on my Microcare MPPT). Its a fixed installation though at 30 degrees and not pointed directly at the sun.  

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Just checked my MPPT and is says this:

 

Battery : 55.9V

Boost at 20.7A

Panel : 83.7V

Power :1141W

 

So 83.7V x 20.7A gives 1733W, but 55.9V x 20.7A gives 1157W which is close to the power reported.  So you should multiply the panel amps by the battery V to get the power produced. In your case is then 36A x 52.1V = 1875W. Still doesn't make sense? Missing 600W. Discharge = 13A * 230V = 2990W? Now I've confused myself.

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Hi Chris

 

It is quite possible to get more than 3KW from your panels, if it is cold and if you are getting almost direct sunlight.  My calc to the above would be:

P = V x I (91.8V x 36A) = 3304.8 W from panels

Power consumed = 2415W

Difference: 3305 - 2415  => 890W

Battery is charging (not discharging - this might be a cosmetic bug) at 13A => 13A @ 52.1V => 677 W

890W - 677W  => 212W in losses?

 

Maybe the 3305W from the panels are not spot on either...

 

Does this make more sense?  :)

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The odds of getting more that 3KW out of the panels is slim to non existent I would say. I have also Yingli  panels and if I could get 2kW from 1.8Kw of panels I would be very pleased. Unfortunately that is not the case.

 

2.4kW from 3kW of panels seems quite reasonable for this time of year..

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Hi DeepBass9

 

I have been in contact with someone who has fiddled with the Axpert settings more than I have. If you set the battery cut-off voltage to the default of 42V then the SOC miraculously come right (and a whole lot of other readings). So I have rerun the test and got about 2.54kW which makes sense. Amazing that one small thing can throw the whole system awry. The 42V is not good enough as a battery cut-off so I will definitely be using the Victron battery monitor's relay to remove the load at a 70% SOC.

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Ja, 42V is just about dead! Glad to hear you came right. So you are getting about 84% of max rated power, and I am getting 65-70%. The angle obviously makes quite a difference!

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The odds of getting more that 3KW out of the panels is slim to non existent I would say. I have also Yingli  panels and if I could get 2kW from 1.8Kw of panels I would be very pleased. Unfortunately that is not the case.

 

2.4kW from 3kW of panels seems quite reasonable for this time of year..

 

DeepBass

 

Currently, almost a month after the shortest day, my inverter reports on average 2.75KW from my 3KW panels at midday on a sunny day. When I still had only 11 panels installed (2.75KW) I once got over 3KW at midday on a cold day in April. Remember that most of the panels are also rated at -0 +5% output.

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Hi DeepBass9

 

I have been in contact with someone who has fiddled with the Axpert settings more than I have. If you set the battery cut-off voltage to the default of 42V then the SOC miraculously come right (and a whole lot of other readings). So I have rerun the test and got about 2.54kW which makes sense. Amazing that one small thing can throw the whole system awry. The 42V is not good enough as a battery cut-off so I will definitely be using the Victron battery monitor's relay to remove the load at a 70% SOC.

 

That is a pity, but practically it is not an option to set cut-off to 42V - that would ruin any battery in no time. At 46V they are already about 85% discharged. :(

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That is a pity, but practically it is not an option to set cut-off to 42V - that would ruin any battery in no time. At 46V they are already about 85% discharged. :(

The setting at 48V is also near useless as that is a SOC of between 40 and 50% so I was not really using it. I am due to receive a Victron battery monitor and I will use its relay to control a contactor to control load and so prevent a DOD greater than 30%.

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Hi Chris.

I noticed the same on my 3kva Axpert. I changed low dc cutoff from default 21v to 23.5 and since then when running on battery almost straight away, the 4 line battery indicator dropped to 2 lines and i could not understand why as multimeter readings still showed plenty battery.

I then changed setting back to 21v default as test and battery indicator went back to 4 lines.

I reverted then to 23.5v so as not to drain batteries 100%.

My question now is whether this setting 29 has any influence over axpert charging behaviour if anyone has any advice to strike right balance between low dc cutoff and charging.

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My question now is whether this setting 29 has any influence over axpert charging behaviour if anyone has any advice to strike right balance between low dc cutoff and charging.

 

 I do think that this may be the root of the trouble you (and others) have been having.  The maximum low DC cut-off (24.0 V) is a DOD of between 50-60% so this in my opinion is a little too late in terms of making sure you batteries are not discharged too deeply and so last. So my decision to buy a Victron battery monitor has merit. I will be using the DOD from the Victron BM to effect a low DC cut-off using the built-in relay. I wanted to use a mini contactor to reduce load, that would fit on a DIN rail. I was told that 20A mini contactor are not available but I found some 2P 20A mini contactors on e-Bay and I will see if they work.

 

Essentially the built in battery cut-off is too low to be practical (even at 24V) so set it to 21V and buy a proper battery monitor. For R12k (for the 5kVA model anyway) you end up with quite a good system. My system has behaved impeccably since I set the battery cut-off to 42V, and now the readings I am getting make sense.  

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DeepBass

 

Currently, almost a month after the shortest day, my inverter reports on average 2.75KW from my 3KW panels at midday on a sunny day. When I still had only 11 panels installed (2.75KW) I once got over 3KW at midday on a cold day in April. Remember that most of the panels are also rated at -0 +5% output.

Aren't you using an Axpert as well? Is it set to 42V cutoff?

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Having chatted to an Aussie PIP user (Aussie version of Axpert) on one of their forums. I have found another idiosyncrasy. PV voltage is measured before the MPPT. PV current is measured after the MPPT so it has been modified already. So production is roughly PV current multiplied by battery voltage not PV voltage. This gives an answer within 50W of what is displayed on the LCD display.

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