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Time to purchase some solar panels


Noobie

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So I mounted the PV panels on the roof, and we have had a sunny day today. Because I love looking at pics of other members installs I have attached a couple of pics of mine.

 

A couple of things I would still like to do, and explanations why, I would appreciate any ideas or opinions here please:

 

1) As suggested by Chris Hobson: Purchase and install a 24V DC contactor to disconnect the inverter output when the BMV702 battery monitor states that the batteries are at 50% DOD. Currently the inverters low DC cutoff is set to 24V, so when you turn on the microwave the battery voltage drops to below 24V and the inverter shuts down. Once this 24V DC contractor is installed I will set the Axperts low DC cut off to 21V and I will rely on the battery monitor to shut down the inverter through the contactor when the batteries are at 50% DOD. Thanks Chris

 

2) I would like to install a timer and contactor to the PV panel output before it feeds into the inverter. My thinking here is that in the late afternoon when the panels are not producing enough watts the inverter draws power from the batteries until the batteries reach 25,5V, then it will get the battreis back up to 27V and run off of the PV panels again. If PV is not available it will switch back to council supply (bypassing the inverter). Essentially it would be discharging the batteries very often, even if its only a 10% DOD it will eventually add up and this would shorten the battery life. If I install a timer and only supply PV power to the inverter between 8am and 1pm it will turn off PV supply to the unit after 1pm and then bypass the inverter.

 

3) During the day today I pulled a maximum of 300watts from the inverter and this was fed from the PV panels. The panels are 250w and there are 6 in total, giving me 1500w. What I would like to do is connect my fridge onto the inverter so that during the day the fridge is being powered by the panels. The problem is, when I have load shedding I dont want the fridge to run :huh: I was thinking of running the fridge AC supply through my inverter DB and then it would be controlled by a contactor which senses council supply, any other ideas or gadgets which could achieve this?

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2) I would like to install a timer and contactor to the PV panel output before it feeds into the inverter. My thinking here is that in the late afternoon when the panels are not producing enough watts the inverter draws power from the batteries until the batteries reach 25,5V, then it will get the battreis back up to 27V and run off of the PV panels again. If PV is not available it will switch back to council supply (bypassing the inverter). Essentially it would be discharging the batteries very often, even if its only a 10% DOD it will eventually add up and this would shorten the battery life. If I install a timer and only supply PV power to the inverter between 8am and 1pm it will turn off PV supply to the unit after 1pm and then bypass the inverter.

 

Ed or Jaco's software would solve your problems. Ed's software even works with the USB port common on the newer inverters.It makes sense to control one's inverter with software. 1pm might be a bit early to switch over. At 4pm here I am still often generating over 1kW from 3kW of panels so you should be generating more than 500w. But that is the beauty of the two programs available you can build up a bit of history and know that in most cases that there is insufficient sunlight say by 15h45 and then go to grid. Obviously you would need to tweak this as the days shorten. Interestingly here in the Karoo it seems we get better production during spring and autumn as temperatures are lower and panels respond favourably.

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Noobie ... if I read correctly, why not use the BMV's relay to swap between Eskom and Inverter power.

 

If SOC is +-80%, swap back the loads to Eskom, SOC 100%, swap loads back to inverter. If Eskom is off, keep running off inverter. Simplest way in my opinion.

 

Re. the fridge. How much power does it really use in 24 hours? My fridges runs 24/7 off the inverter, because it is A+, small and efficient, uses 150kwh per year.

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@ Chris: I have had a look at the axpert software but have decided to get all the hardware done before going down that route. I was generating about 200w yesterday at 17:00. You're right, turning off the solar input at 1pm would have been an absolute waste!

 

@ TTT: I was hoping to use the BMV's relay to shut down the inverter at 50% DOD as I have read that the axperts DOD is wildly inaccurate? I have 2 fridges, one large one in the garage and a small under counter bar fridge in the kitchen. I was hoping to run the under counter one only, its got an A star rating, not sure of the power consumption though. I will do the sums and take it from there.

 

The only thing which I don't like about the inverter (so far) is the following:

When I'm generating enough PV power the loads are fed from PV only

When there is not enough PV power the inverter pulls from the batteries and PV at the same time

When the batteries drop to 25,5v (90%) the inverter PV charges them up again to 27v

Once the batteries are at 27v if there still isn't enough PV power the inverter pulls from the batteries again, and so on and so on. This means I am cycling the batteries very often, even if its only to 90%. Yesterday evening at around 18:30 (twilight) I watched the inverter switch between battery and PV about 4 times until it eventually switched over to council supply as there was no power coming from the panels.

This also happens in the morning when there isn't enough wattage coming from the PV panels.

I know that during partly cloudy conditions this would happen a lot, but I would prefer not to cycle the batteries that often if I can help it?

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Noobie, Chris, using the BMV to change circuits is done outside the inverter, on the power that is being fed to the devices.

 

My 4 year old design looks like this, today you can do it 10 times simpler.

post-122-0-22739200-1454306951_thumb.jpg

 

 

Edit: It was built back when this was a completely novel idea, low budget and was done over cautiously and with extreme care, for back then there where no such devices available, nor the expertise ito having built it for solar was relatively new concept.

Groundbreaking comes to mind.  :D

 

Today I would go for electronic relays.

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The only thing which I don't like about the inverter (so far) is the following:

When I'm generating enough PV power the loads are fed from PV only

When there is not enough PV power the inverter pulls from the batteries and PV at the same time

When the batteries drop to 25,5v (90%) the inverter PV charges them up again to 27v

Once the batteries are at 27v if there still isn't enough PV power the inverter pulls from the batteries again, and so on and so on. This means I am cycling the batteries very often, even if its only to 90%. Yesterday evening at around 18:30 (twilight) I watched the inverter switch between battery and PV about 4 times until it eventually switched over to council supply as there was no power coming from the panels.

This also happens in the morning when there isn't enough wattage coming from the PV panels.

I know that during partly cloudy conditions this would happen a lot, but I would prefer not to cycle the batteries that often if I can help it?

 

You lucky there a couple of other things I don't like about an Axpert. Jaco and Ed's software addresses the very issue you are having.

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Mark, would love to, but I had it built by an engineer. I can answer questions, if someone else can do the drawing?

 

Design was such that the relays and timers are easily unplugged and replaced if they break. Going on 5 years, still as new. Only flaw is that the timers cannot go below 1 second. You do not get -5-8ms timers that I can find.

 

More detail: Jaco's 'Changeover Box'.pdf

 

Newer simpler design we started, but never finished, that does not have a 1 sec break between power sources, more like 20ms or less:

post-122-0-90480800-1454307603_thumb.jpg

 

 

So it is quite simple. 

The core fact I based my plan on, was that every single UPS out there (bar online versions), have a break between inverter and Eskom power.

The fastest ones are 5-8ms, and the ones with 20ms is too slow.

 

So the BMV detects the SOC of the batteries and based on the state of its internal relay (OFF or ON) triggers a 220v AC relay that does the swapping between Inverter / Eskom, with the 5-8ms break.

 

Now, do not miss this little part: If there is no Eskom, batts must be used, so a relay linked to Eskom power is required to override the system to stay on inverter power.

 

As simple as that. Batts are protected, until there is no Eskom.

 

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You lucky there a couple of other things I don't like about an Axpert. Jaco and Ed's software addresses the very issue you are having.

Pray tell Chris?

I rather know what is not so lekker and still buy it than to buy it having to find it out I cannot live with it.

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Pray tell Chris?

I rather know what is not so lekker and still buy it than to buy it having to find it out I cannot live with it.

Perhaps I am being too harsh. If I had to buy an inverter tomorrow I probably would buy an Axpert. Mine has run faultlessly for 6 months and at a third of the price of its competitors and that in my books counts hugely in the Axpert's favour. In line with randomness of the TV programme QI the Axpert scores an unassailable 1000 points on that score alone.

 

Things that I don't like about the Axpert

  1. The software is useless.
  2. The SOC is useless. They could Coulomb count and come up with a better SOC. They have all the information and yet don't use it. Hence the fact that most folk add a battery monitor of some sort.
  3. The 115 VDC through to 145VDC limit on the MPPT. I am not sure how it behaves between 115 VDC and 145 VDC. The tend is higher and higher voltages on the input side of inverters and I am hoping they (MPP Solar) follow suit.
  4. 10mm2 terminal for the PV (fine if you could run your PV input at 300-400V DC).
  5. The airflow is all wrong. Fans blow down against natural airflow of warm air.
  6. Parts of the manual are ambiguous. 
  7. The maximum low battery cut-off is 48V on a 4kW version. This is too low and has been solved by Coulomb but it should not be the customers job to make things work properly.
  8. I would like more control of the charging algorithm. Yes have a default but at least let the customer have some control. If he stuffs things up you can always have a "I told you so clause".
  9. I am sure the Trojan users would like higher charge voltages.

A nice to have would be a 2nd AC output that can be only PV/grid powered so geyser for instance could not drain your batteries.

Last on my wishlist would be the ability to boost the capacity of grid like Victron and Imeon. I seriously considered the Imeon because of this capability but being a cheapskate the Axpert's price won me over. Three Axpert's in parallel would still come in cheaper than an Imeon.

 

Edit: reworded to make it clearer (I hope)

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Hi Noobie,

 

If you having problems with battery cycling, perhaps you need to look more closely at your battery settings?

25V at 50 amp and 25V at 10amp is totally diffrent.

2nd one might be battery size? The bigger your storage capacity, the less likely it would be to cycle so many times.

 

I split my loads after the inverter. Those I would like backed up during loadshedding (lights + TV) and those just looking to run on PV, like the fridge.

I used a 220V contactor powerd from Eskom power. So if Eskom falls out, the contactor also falls out and thereby disconnecting the loads I would not like to run on UPS power.

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Thanks Chris.

 

Ja, I gathered that the Axperts are really good value for money, but the SOC and non-negotiable battery charging, eish, that is seriously not cool.

What shocked me was that even the Imeon uses batt volts as SOC. 

(Fixed.) Victron's Multiplus and Quattro's have SOC built in.
In my opinion, batts are most probably the most expensive part over 20 years of your system. Yet it is overlooked by most. SOC based on volts is absolutely nonsense, unless you rest the batts in between to properly measure it.

 

The run of the mill brand name MPPT controllers, the ones real solar guys go for (sorry I just had to!!!), also tend to max out on +-150v generally speaking.

Then there are really big ones that go up to 600v ...  enter the realm of stuff to dream of at a price to shrink from. 

 

Most of us buy panels based on price, instead of buying panels that are optimal for the MPPT controller you have. To solve this issue, Morningstar's and Outback calculators give you the best panel and wiring combination based on which controller of theirs you want to use:

http://string-calculator.morningstarcorp.com/

http://www.outbackpower.com/outback-support/string-sizing-tool

 

Axpert would be cool to have similar calculator.

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Two notes from me today on this:

 

1. The 145VDC limit on the MPPT? That's not bad as far as MPPTs go. Even the high end Victron's (150/70 and 150/85) only goes up to 150. I think it makes complete sense: When charging batteries, much better not to make that ratio too big. If you're going directly to AC, then it makes sense to go for the super high voltage. So I don't think this particular limitation of the Axpert is a problem.

 

2. The Victron inverter DOES do proper SoC accounting, but only if you're using the AC charger. When you have a GTI in the system (ie a hub2 or hub3 system in victron lingo) it works perfectly. The Victron inverters cannot however do SoC accounting if they don't do the charging, ie if you have an external MPPT.

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Errata: Victron Multiplus and Quatro's have proper SOC. Apologies.

 

But if you want to see what is going on, you need to spend a LOT of money on additional hub systems.  :P

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Errata: Victron Multiplus and Quatro's have proper SOC. Apologies.

 

But if you want to see what is going on, you need to spend a LOT of money on additional hub systems.  :P

Yup, I remember jdp lamenting the lack of an LCD on his weekend away with a Victron :)

 

Having to buy the CCGX to see what's going on AND the BMV to get SoC monitoring is rather unfortunate, absolutely agreed. If my plans work out though, I will hopefully have something that can do SoC simply by monitoring the inverter (which can tell me DC volts and DC amps going out) and watching the messages sent by the MPPT. Add in a bit of newtonian integration and a Peukert efficiency factor and Bob's your uncle. Seriously, if you have one of the ve-direct MPPT models (ie not canbus) and the mk2 dongle, there is no reason you cannot already do this with a bit of spoeg-en-plak python code :-)

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Thanks Chris.

 

Ja, I gathered that the Axperts are really good value for money, but the SOC and non-negotiable battery charging, eish, that is seriously not cool.

 

Hi you can set bulk voltage and float voltage. But it has a crazy algorithm where the absorb phase is 10 X the time of the bulk phase with a minimum of 10 min and a maximum of 8 hours (according to the manual). Coulomb says this is a flight of fancy so no one really knows so a bit more control here would be nice.

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So heres something interesting thats been happening since I installed my PV panels...

 

The inverter is set to SOL mode, every day at around 14:30 the earth leakage which is fed from the inverter trips.

I bypass the inverter output and feed the earth leakage straight from council supply and it stops tripping? In the evening I switch back over to the inverter output and the earth leakage doesn't trip anymore?

I disconnected the solar panels from the inverter this morning and will see if this would stop the dreaded 14:30 trip.

I will see if this has any effect.

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Now this is really interesting.

The PV panel input was disconnected and the E/L in my inverter DB didn't trip today

BUT

At 14:30 I heard a knocking coming from my council DB, turns out the geyser timer pulls in a contactor at the exact time.

Seems like as soon as the geyser timer is on and the PV panels are connected the inverter E/L trips.

The geyser, timer and contactor are all in my council DB and are fed from before the council E/L.

Tomorrow I will connect the PV panels again and at 14:30 when the inverter DB E/L trips I will turn off the geyser timer and see if the inverter DB E/L will stay up.

Fingers crossed!

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