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Introduction and Axpert (Mecer) 3Kva


Riaanh
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Good day to all.

I have been searching and reading a huge bulk of this forum during the past week, and registered this week as I eventually got as far as buying the Inverter.

I run a 24V setup, and the bulk of my lights have been converted to 12VDC. (Yes I did read the thread on the DC lights, and SANS regulations etc).(I Comply).

Started off with a 12V battery feeding lights for those dreaded Load Sh!tting days, and progressed from there and currently step down 24V - 12V for lights, rest of the DC cct's running on 24V.

Current setup had 1 X 300W solar panel 24V, 4 X 12/105Ah batteries, 2 banks of 2 and a 50A PWM Solar charge controller.

Last week I bought the Axpert/Mecer 3Kva and installed over the weekend.

Transferred setup to the Inverter.

Herewith  my question:

The PV Array MPPT Voltage range is 30 - 66Vdc. with a max PV Array OC Voltage = 75Vdc.

I found the charging values to be much lower than the PWM cct, and on logging with the current weather I peak at ~34Vdc and 4A max somewhere between 12:00 and 14:00.

I also built a "complex" Arduino setup that monitors all 4 batteries individually, Current draws, temp and feeds etc.

The Axpert only log PV current as 1A, 2A, 3A and 4A respectively.

The Arduino log to 3 digits and take the the PV Voltage on the PWM to calculate Watt and WH.

On the PV side I also measure the Voltage without the drop on the controller side.

The voltages seems to match, but with the PWM I peaked at 7-8A.

From another thread on this forum by Adding another panel in series with a net Voc result of 64V this should bump the MPPT into proper action?

Rather this than adding a second panel in parallel and still record PVin voltage around the 30V mark?

Suggestions and knowledge sharing would be appreciated.

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Welcome Riaan.

I presume you have 10 x 300w panels?

Your PWM controller, if I understood, should ideally have been a MPPT controller.

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Thanks TTT.

Nope I currently have one singular 300W24V panel charging the batteries (Trying anyway)

Had the PWM in all the time so far, and part of why I bought the Axpert was for the MPPT portion on Solar.

Yet I believe with my panel V hovering around the 30 - 34V mark, the MPPT prove pretty useless in function of why MPPT is better.

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Sjoe ... I think 1 x 300w panel for a 3kva Axpert is a wee bit too little, and borderline for charging the batteries, depending on how much you take out of them.

On arrays below 1kw the jury is out on the benefit of a MPPT over a PWM, leaning towards PWM being more cost effective versus benefit derived.

Over 1kw array the MPPT definitely adds a lot more value especially ito higher volts, less losses on cables.

EDIT: so you are right, on one 300w panel, you will see next to no value.

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Oookay. So you had a system with a PWM controller. You now have an Axpert with a MPPT. The MPPT performs poorly. Got it :-)

A 300W panel is likely a 72-cell panel, so open circuit it will make around 40V, and Vmp (Voltage at max power) should be around 36V. A 24V bank charges to about 29V (cold mornings), but as high as 31V (Trojan golf cart batteries on equalize). You need some room for the buck converter to work, so the Vmp must be at least a few volts above battery voltage, usually around 5V.

So your setup has a very narrow range for the mppt to work with, and you're probably right about the voltage being 30V-34V being responsible for this. The bad news is that with that panel there is little you can do. Adding another in series will exceed the Voc of the MPPT.

You could move it back to the PWM controller. You could look at a different MPPT, though I doubt that would work. Best option, I think, would be to get a couple of 36-cell solar modules (aka 12V panels). That limits you to 150W per panel, but price per watt is pretty constant so it isn't really a problem. Wire them 3 in series, that should give you a Voc of around 68V, and a Vmp of 52V-54V. I'm basing these numbers off my own Tenesol panels, they do 22V open circuit, peak at around 18.1V (per module). Just do the math on these before you buy and make sure you have some room at the top.

The problem here, really, is that the 24V axpert with the smaller MPPT isn't particularly flexible. I remember when the first ones came on the market... I did the math and it was literally like one (72-cell) panel is too little, two is too much... :-)

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Thanks for the prompt responses TTT & Plonkster.

I agree with the statements, and will respond tomorrow with all the detail from PC. Phone not the best long txt device. 

In a nutshell my plan is to add a 12V 36 cell module to the existing 72 cell 24V module. This would give me 450W 36V panel. In series I ensure the current is equal, but get the added benefit of the Voltage increase. This"should" bang the mppt into action. Agreed the setup is not ideal and keep in mind I scale this setup as money become available. (Unfortunately).

I know the pitfalls of panels in series, and got the price for a 150W unit from the same manufacturer to minimise panel degration issues etc. Current (Short and Max) match on both units. 

For the Voc 30-66 on the Axpert, the 300W and 150W sum total on Voc to 64V, so just within limits.

I know this is not ideal, but for the next 6 months I hope this will proof some consepts on my side.

Thanks again for all inputs.

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Thanks again for the responses.

TTT agreed, but I am scaling as possible, and the plan is to add more panels with time, As the panel count grow, the Axpert need to be replaced with either a Plus or eg 5Kva.

All is not lost, as this system components move on to other functions, so yes not a total loss of investment. PWM served me well so far, was just under the impression that the mppt would perform way better, but the limitations was not considered hence the attempt to get the mppt to perform way better.

Plonkster agreed, The panels are to small to really add value to the system from an operational point. With the current/historical setup, PWM generated ~650Wh per day and my load is ~1200Wh per day. The shortfall was supplemented by utility from 16:30 to 18:00. This is/was however sufficient to enable some comfort during those load shedding days. The mppt manages only ~450Wh per day. My supplement from utility is still picking up the slack here.

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Riaan, you are doing same as me. bit longer, but much more fun. :D

EDIT: One day you can tell others lots of stories of what NOT to do. Those experiences are worth their weight in gold. ;)

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Yup, my plan was similar. Buy enough big stuff so that some expansion is possible. Add on over time. I could not quite foresee how permanent some things became, because kids, doctors and therapists (all part of the same exercise really) does not come cheap, so things have been quite stationary for a long time now. When the last windfall came in, I opted to paint the house and not buy the big battery bank I wanted, so that has been postponed at least another year. In the mean time however, I actually don't have too small a system. Even at a low 4kwh/day or so, that is enough to live off. The fridge and freezer is sorted, and even a little but of television and computer use.

The reason why I bought a Victron Multiplus back in the day, was because it was the only inverter that could be programmed to do grid-fallback, and it was the only one that could be paralleled. In the mean time, those Voltronics became available of course, and the second inverter never materialised. Then our government alchemists turned our currency into a different substance, doubling the cost of a second inverter... and well... we're still there.

Next step, however, remains a larger battery bank.

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So herewith some feedback.

I ended up over the weekend on quickly building a frame for the new panel, and ended up finishing the install with the new combiner box etc. at around noon on Saturday.

Initially since the install the following was found:

From my Arduino monitoring system: PV Volt hovered around the 30V mark with a PV Current just over 4A. On the Axpert, PV Volt matched the Arduino, but the PV Current displayed as 3A (WTF). This followed pretty consistent, meaning should the Arduino report 5A, the Axpert went to 4A etc.

After the install, the following was observed:

From my Arduino monitoring system: PV Volt hovered around the 50V mark with a PV Current just over 4A. On the Axpert, PV Volt matched the Arduino, but the PV Current displayed as 6A. This followed pretty consistent, meaning should the Arduino report 5A, the Axpert went to 8A etc.

This in my opinion state that the concept is working well, and that once the Axpert was “biased” enough, the mppt could function as designed.

Sunday was the first full day that I could work with some stats provided by the mppt tracker/Watch Power application. Problem with both these is that PV Current is simply displayed in increments of 1A. This makes it difficult to determine accurate PV Watts and thus make the calculation of daily WH produced ineffective. The PV Amps on the Arduino has then in effect also become useless for calculations.

But all things considered I gained approximately 46% increase in production vs the statistics from the previous week. Plan was to monitor this for the week and get some better averaging results.

Unfortunately someone here from Cape Town decided to visit Gauteng and bring the sh1tty weather with him/her.

As soon as this person returns home and take his/her weather with them, I hope to gain some valuable results.

So the setup currently contain one 24V 300W panel combined with a 12V 150W panel in series.

Maximum V(oc) sum for the two panels = 66.0V

Voltage (Vmp) sum for the two panels = 53.4V

Current Imp for the two stays the same, as 8.38A and 8.52 respectively, meaning that the best that can be expected will be the lower of the two, 8.38A.

Should conditions become “perfect” then 53,4V at 8,38A will produce ~447,5Watt that closely matches the string of 450Watt.

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Hold on ...  are these 2 panels connected together in series, the 1 x 24V 300W panel and 1 x 12V 150W panel?

EDIT: And that visitting person from CPT ... they have invited some more friends by the way. :P

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32 minutes ago, The Terrible Triplett said:

Nope, 24v 150v panel.

You cannot connect panels together that are not identical. the MAX they should differ, is about 10%.

I'm sure you can, voltage will add and current will be limited to the lowest in serie. Not efficient but should still work

If you start looking at parallel strings yes then you should definitely try and keep them the same (or at least the strings the same)

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TTT I tend to differ in opinion.

But I stand to be corrected, and are open to criticism, corrections and suggestions here.

The theory of Electronic components, in this case Solar Panels in Series:

The voltage in series will add up to a new total. The current would stay the same for a series circuit, or at least be limited to the lower of the two currents.

EG. 24V panel capable of 10A (resulting in 240W panel) in series with a 24V panel capable of 10A in series should provide a 48V panel (String) capable of 10A resulting in 48V 10 A 480W panel.

I.E. my decision to add a 150W 12 V panel to the 300W 24V panel, to try and match the current as close as possible.

The theory of Electronic components, in this case Solar Panels in Parallel:

The voltage in parallel will remain constant, provided that panels are of same voltage, thus two 24V panels in parallel will still only produce 24V. The current would be the result of an addition of the two panel’s capability of producing current.

EG. 24V panel capable of 10A (resulting in 240W panel) in parallel with a 24V panel capable of 10A in series should provide a 24V panel (Array) capable of 20A resulting in 24V 20 A 480W panel.

This is obviously all very theoretical and there are numerous factors that can influence the end result. (Panel degration, equal sunlight exposure but to name a few.)

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Just now, jdp said:

I agree with you, if you run things in series it all adds up, you can use different sizes. As long as you stay inside the specs of the inverter you can mix and match as you like.

Thanks, To confirm, I went for the exact same brand of panel as I would assume the degradation of panels should be close to similar.

The Voltage VS Current characteristics for both are also pretty close.

The Max Voc of the inverter is 75V with mppt range of 30V - 66V.

Current panels in series result in Voc of 67V.

Odds for me reaching 66V+ on a nice sunshine day is small IMO.

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Guys, I agree, you CAN do it and we know the 300w panel will just be limited.

But it is a risk should something go wrong. The larger panel can cause some serious damage, even fire, should something go wrong on the smaller panel. So if you have to do it then fuse each panel according to its Isc.

As a temp setup it is fine but on a more permanent setup, it is a such a waste of a 300w panel.

Riaan, send me the 300w, I send you a 200w one. :P

 

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See the connection box, I sadly did not take pictures of the wires.

At the time when I accidentally discovered the problem, the panel was so hot it burned your hand.

EDIT: Solar panels can burn your house down.

Tenesol panel problem.jpg

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1 hour ago, viper_za said:

I'm sure you can, voltage will add and current will be limited to the lowest in serie. Not efficient but should still work

Of course you can do that. You'd simply limit the current to whatever the smallest panel can do, and obviously there are certain stupid things you should not do (like putting a 10W panel in series with a 300W panel). A 150W 36-cell panel in series with a 300W 72-cell panel is perfectly acceptable. If you think about it, each cell is 0.5V anyway, so the 150W panel has them all in series to make around 18V, and likewise the 300W panel also have them all in series to make around 36V, and by combining them you made a 108 cell panel of similarly-sized cells.

Also... this :-)

https://www.victronenergy.com/blog/2015/11/10/connecting-different-sized-victron-energy-pv-modules-series-or-parallel/

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3 minutes ago, The Terrible Triplett said:

The larger panel can cause some serious damage, even fire, should something go wrong on the smaller panel. So if you have to do it then fuse each panel according to its Isc.

As a temp setup it is fine but on a more permanent setup, it is a such a waste of a 300w panel.

Riaan, send me the 300w, I send you a 200w one. :P

My Junction box I built at the top run 3X 10 A fuses 32V. One for each panel and one on the output. Then another one at the bottom that feed the Axpert. Overkill I think, but rather safe than sorry.

300W swop for 200W Yeah right:unsure:

 

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On the flip side ... today, here ... I have learned something new! ;)

What was a set rule 5-10 years back, has changed with MPPT's and new found experiences and knowledge.

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Just now, The Terrible Triplett said:

On the flip side ... today, here ... I have learned something new! ;)

What was a set rule 5-10 years back, has changed with MPPT's and new found experiences and knowledge.

That's most likely why we join Forums. To learn new things and share experiences. If we knew it all we would just steam ahead and do what we knew.

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Some of it is common sense, but of course common sense is also learned. I referred earlier to not doing stupid things. For example, if you have a small 36-cell module in series with several larger modules, and say you accidentally short the output. That's the scenario.

Solar cells acts like diodes. When you reverse bias them, they simply block the current. If you push the voltage high enough, it will start to break down and pass current in reverse, and if you push it higher still (above 9V per cell) they will begin to destruct. So of course you need over 100V in a 36-cell module to pass current in reverse and over 300V in the same module to destroy it, but I think it illustrates the point: At least think about what you are doing :-)

Edit: Of course I completely ignored the fact that panels have bypass diodes to avoid precisely this, but with a suitably mismatched setup you can blow those up too!

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Common sense differs based on who you talk to, their experiences / knowledge / expertise and most importantly, their gender.

Don't believe me? Go and ask your wife. :P:D

EDIT: Plonkster, none of what you said was common sense to me. But now it is.

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