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iops

Another Axpert SOC Thread

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Another thing, mounting the panels flat will not give you that bad results. Only in wintertime it will reduce a little.

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Posted (edited)
12 hours ago, wolfandy said:

Your panels are 72 cells and 44.8V VOC. If they are connected 3S, then my guess would be that you are getting too close to your max operating voltage of 115VDC and your Axpert starts to derate

Actually, the 24 V 145 V max SCCs seem to behave the same as 48 V models, i.e. the derating actually starts at 130 V. [ Edit: but who knows what happens in work-alikes such as Synapses. ] The worst case when the panels are coldest, e.g. on a clear winter morning. Using a factor of 1.07 (not checking the panels' actual temperature coefficient), I calculate 144 V at 0°C, which should barely be enough to get things started (perhaps with a 15 minute delay for the panels to warm up a bit). Once they draw current, the voltage will pull down, allowing more current, and soon they would be operating as well as they can be at that time in the morning. Until a cloud, when it has to start again.

So I think 3S of these 72-cell panels will just squeak it in.

[ Edit: it seems that @RikH has looked it up, and states "and with that temp coeff. of 0,37% it means a Voc of 49,13 at 0 degree C per panel". Indeed, that's a little too much, if the correct temperature coefficient has been used (there are several, and they vary a lot). I would not worry about re-configuring immediately, but it would be a good idea before winter. ]

Quote

But somehow I do not think that this by itself will explain that you get so little out of your panels

I agree, especially not at this time of year. Perhaps in a few months it might be more significant.

The flat orientation will be a large part of it I suspect. In winter, there will be some 45° to the sun at noon, and obviously more at other times of the day. [ Edit: that's about a 1 - cos(45°) = 30% hit. ] Tilt frames at some 40° will help dramatically in winter.

Edit: I have such frames for 6 panels on my carport, though they are smaller (215 W) panels. They're wired 2S3P.

Edited by Coulomb
Added a little more text about starting in cold winter mornings.

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3 hours ago, Coulomb said:

it seems that @RikH has looked it up, and states "and with that temp coeff. of 0,37% it means a Voc of 49,13 at 0 degree C per panel". Indeed, that's a little too much, if the correct temperature coefficient has been used (there are several, and they vary a lot). I would not worry about re-configuring immediately, but it would be a good idea before winter.

Took the temp coeff from his datasheet. But I made a mistake by assuming his system was 115V but it is 145 max, so actually no reason for the panic I may have caused.

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Tested the batteries last night after a 24 hour charge, the results weren't great at all. 24V cut-off was 4 hours in under ~500W of load.

I've mailed the installer to come and double check things, seems like there's a lot wrong with my system.

5 hours ago, Coulomb said:

Edit: I have such frames for 6 panels on my carport, though they are smaller (215 W) panels. They're wired 2S3P.

Can I ask what kind of output you're getting?

Screenshot 2020-05-04 at 08.20.44.png

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1 hour ago, iops said:

Can I ask what kind of output you're getting?

At this time of year, I get nearly 4000 W near noon, from 5650 W nominal. Brief peaks above 4500 W, very rare peaks over 5000 W. So that's ~70% of nominal, with brief peaks of ~80%. But only 3 of 14 strings (about 23% by nominal power) are these panels tilted at 45° on the carport. They definitely outperform the rest, especially at this time of year and into winter. In summer, they are at a poor angle, but in summer I usually have plenty power from the other panels. So the tilted panels are probably doing about 80% of nominal at present.

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1 hour ago, Coulomb said:

At this time of year, I get nearly 4000 W near noon, from 5650 W nominal. Brief peaks above 4500 W, very rare peaks over 5000 W. So that's ~70% of nominal, with brief peaks of ~80%. But only 3 of 14 strings (about 23% by nominal power) are these panels tilted at 45° on the carport. They definitely outperform the rest, especially at this time of year and into winter. In summer, they are at a poor angle, but in summer I usually have plenty power from the other panels. So the tilted panels are probably doing about 80% of nominal at present.

Wow OK, so your panels are 85W less for each panel but you generate that much? I'm no where near that.

I need to read up more on solar panels... I'm totally lost.

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1 hour ago, iops said:

Wow OK, so your panels are 85W less for each panel but you generate that much?

To be clear, I don't have a separate measurement of what those tilted panels produce. But my guess is something like 80% of the nominal 6 x 215 = 0.8 × 1290 = 1032 W. And that's near noon with no shading, this time of year. So yes, if they were 300 W panels, I'd expect around 80% × 1800 = 1440 W. But even flat to the horizontal roof, I'd expect around 70% of that, or about 1000 W.

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Quick update, I'm using the opportunity to slowly move things over to Victron. I have a 150/70 MPPT controller on the way along with a BMV-712. I have a spare Pi that'll be used to run VenusOS. A replacement inverter for the Axpert will happen a little down the line.

For now the priority is getting the full potential out of these panels and proper battery monitoring in place. Unfortunately it seems like the full month (plus a few days) of running the system incorrectly seems to have shot the batteries entirely. I can barely squeeze 2 hours out of a 200aH bank and that's with the inverter screaming about low voltage.

 

Fingers crossed the batteries can be replaced.

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Finally, Victron 150/70 MPPT controller installed with one of the batteries replaced.

PV output has already touched 950W under load and has the batteries at float, will testing the battery charge tonight - fingers crossed there. Overall I'm happy with the setup, BMV is on back order.

 

I realised when playing around with the settings there's no way to set the Axpert to use the batteries, and therefore PV, without having PV wired into the Axpert. @wolfandy I see you mentioned earlier in the thread that you've got this setup, how have you achieved switching between PV and using Eskom?

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6 hours ago, iops said:

PV output has already touched 950W under load and has the batteries at float, will testing the battery charge tonight - fingers crossed there. Overall I'm happy with the setup, BMV is on back order.

Cool - happy to hear that it's working 🙂

6 hours ago, iops said:

I realised when playing around with the settings there's no way to set the Axpert to use the batteries, and therefore PV, without having PV wired into the Axpert. @wolfandy I see you mentioned earlier in the thread that you've got this setup, how have you achieved switching between PV and using Eskom?

I have my system running in SBU (Solar - Battery - Utility). As there is no PV available for the Axpert, it will use batteries - which is where your Victron MPPT is also connected to. Once your MPPT production drops below your load, the batteries will automatically provide the difference. If you do not have sufficient battery capacity to get you through the night, Axpert will change over to Utility based on your "Back to Grid" setting

On 2020/05/10 at 8:44 PM, iops said:

I have a spare Pi that'll be used to run VenusOS

One question about this: I do not think that VenusOS will give you any benefits at this point as it cannot control the Axpert. I am running ICC on my Pi, which by now is also able to communicate also with a Victron BMV and MPPT. ICC can then switch your Axpert between SBU and Utility based on the SOC calculated by your BMV rather than the Axpert's guesstimate. If you already have a Pi, then the cost of the software is relatively low. It works great for me

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So this kind of went down a rabbit hole, and with lock down hasn't given me much else to obsess about. With the batteries now finally sorted out and performing as they should, it gave me a good few weeks to look at the system as a whole.

 

What started out as a perfectly decent loadshedding backup solution has turned into a bit of a pet project. While I think the Axpert has been great for the loadshedding solution it hasn't performed very well (and I admit that the installation, battery and solar issues from the installers point of view have soured things a bit) I looked at the longer terms goals and wasn't comfortable with what the Axpert offered:

12 month warranty

Limited expandability

 

After a lot of thought and being very impressed with the 150V/70A MPPT from Victron and BMV-712 on the way, I've decided to take the plunge to the Blue team and get the MultiPlus II 3kVa inverter. I'm hoping it'll be installed some time this weekend - new thread with some pictures coming soon :D

I decided to upgrade from a 24V system for better lithium battery support in the future, which is the next to upgrade on the list. I'm keeping my 8 AGMs for now and just wiring them up into 2x 48V banks.

Huge thanks again for everyone involved in this thread, it's been hugely helpful. Excited to be part of the forum!

On 2020/05/23 at 9:15 PM, wolfandy said:

I have my system running in SBU (Solar - Battery - Utility). As there is no PV available for the Axpert, it will use batteries - which is where your Victron MPPT is also connected to. Once your MPPT production drops below your load, the batteries will automatically provide the difference. If you do not have sufficient battery capacity to get you through the night, Axpert will change over to Utility based on your "Back to Grid" setting

Thanks again for your help on this, it's nudged me in the right direct to get things setup.

On 2020/05/23 at 9:15 PM, wolfandy said:

One question about this: I do not think that VenusOS will give you any benefits at this point as it cannot control the Axpert. I am running ICC on my Pi, which by now is also able to communicate also with a Victron BMV and MPPT. ICC can then switch your Axpert between SBU and Utility based on the SOC calculated by your BMV rather than the Axpert's guesstimate. If you already have a Pi, then the cost of the software is relatively low. It works great for me

You're right, it wouldn't but the VRM monitoring from Victron appealed to me. And I like tinkering with Raspberry Pi's to see what they can achieve :) Ultimately that, and this thread, has cost me a lot of money. I'll PM everyone invoices who contributed to the Blue movement ;)

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Victron inverter, MPPT and BMV all installed and humming along nicely. Taken a few days to test things out and I'm happy with everything, with the exception of the batteries.

After having 3 of the 8 switched out (turns out two were AGMs and the other six sealed lead acid, same brand but different model) I'm moving on from the installer. The BMV reports 90% SOC on, what should be a 200AH 48V bank, but the voltages are way down at 47V. I honestly don't know what to look for on the wiring side, but I'm at the end of my tether having battled with this for the last 6 months.

 

I'm not sure how to fix this, or if I should just cut my losses and buy new batteries. What do you guys think?

Weighing up the options, and finances, I can't afford to get a Lithium bank the size that I want, and I probably can't afford the recommended two Pylontech's on my Multiplus II. Should I purchase 4x new AGMs to wire up while I save another 9-12 months for a proper Lithium setup?

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19 minutes ago, iops said:

90% SOC on, what should be a 200AH 48V bank, but the voltages are way down at 47V

Remember that the SOC on the BMV can be wrong. To use just one example, when you first power it up it always starts at 100%. The BMV has to be programmed correctly with the capacity of the battery (the current capacity... not the capacity it had when it was new), and then you have to fully charge the battery so that the BMV's 100% coincides with an actual 100%.

Now if you do all that, and then find that the voltage still drops out to below 48V at what should be a good SOC, then you have one or more weak batteries. Then you go in with a volt meter and you go find that battery.

This is actually what it means when we say a battery can be taken down to 50% DoD for 1000 cycles. It means that after 1000 cycles, the battery has half its capacity left. A 200Ah battery turns into a 100h battery after 1000 cycles (possibly less, I'm just using nice round numbers). A BMV programmed for the original 200Ah will report 50%, but in reality that thing is empty man...

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5 minutes ago, plonkster said:

Remember that the SOC on the BMV can be wrong. To use just one example, when you first power it up it always starts at 100%. The BMV has to be programmed correctly with the capacity of the battery (the current capacity... not the capacity it had when it was new), and then you have to fully charge the battery so that the BMV's 100% coincides with an actual 100%.

Now if you do all that, and then find that the voltage still drops out to below 48V at what should be a good SOC, then you have one or more weak batteries. Then you go in with a volt meter and you go find that battery.

This is actually what it means when we say a battery can be taken down to 50% DoD for 1000 cycles. It means that after 1000 cycles, the battery has half its capacity left. A 200Ah battery turns into a 100h battery after 1000 cycles (possibly less, I'm just using nice round numbers). A BMV programmed for the original 200Ah will report 50%, but in reality that thing is empty man...

You've been beyond helpful, thank you so much!

The BMV currently has default settings outside of the banks capacity, granted it's at new which should be 210AH according to Deltec. I didn't sync it correctly the first time, did a factory reset and think I got it right. Voltage is still on the floor. These batteries have probably been through hell, I can try and set the BMV capacity to 100AH, but I'd prefer to have a working system for a few years.

A volt meter has been on my "to buy" list for a while, but it'll probably just frustrated me when I inevitably find a weak/dying battery :(

Maybe the idea of getting new batteries is burning a hole in my pocket, should I get replacement AGM batteries and worry about this all again in a few years or buy a small Lithium battery and build that up?

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1 minute ago, gooseberry said:

So how are you supposed to know what the current capacity of the battery is? Shouldn't the BMV be able to infer it from the first couple of cycles by looking at a discharge curve?

Infer? With a lead acid? Well that's a good joke 🙂

The BMV simply counts amp hours, and then depending on the efficiency value you programmed into it, it ensures that slightly more Ah is returned to the battery before it considers it full again. It can do rudimentary Peukert calculations too, where it derates the usable capacity if you work the battery hard. But it relies on a programmed capacity and yes, I do see how that's a chicken and egg thing... how are you supposed to know?

Well, with a new bank, you know the capacity. And from the spec sheet, you can adjust the number depending on the cycles. Some installers may even program the initial capacity to 10% less to compensate for this somewhat. But I think the long and the short of it is that once the battery is beyond a certain point, the only way to find out is to do a full discharge and see how much you get.

Lithium BMSes can do a little better, because along with Ah counting it can rely on the voltage. If the lowest cell drops to 2.8V... we're done here mate, battery is empty (I have BYD charts somewhere of SOCs dropping through the floor... the current sensor is not very accurate below 2A, so it totally underestimates the Ah bit of it... and then when the voltage measures low it catches up).

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