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UPDATE: Some progress -- Mecer 5kva inverters - want to run in 'SUB' mode


Radar
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Greetings all.

I'm very new to the topic of inverters, solar and backup power systems for homes.  I'm also not very technically minded.

I recently had a system installed which consists of 2 x Mecer 5kva inverters, a solar panel array and batteries, with the latter having a very small capacity (worked out to about 6hrs standby only).

It is currently running in SBU mode, and we're experiencing the following challenges:

1.  When switching from solar to battery in the evening, its almost as if the system is doubting whether to switch over permanently or not.  E.g. as the sun is setting, we switch on the oven.  The oven's load results in load exceeding what solar can supply, so the system switches to batteries.  We then finish with the oven, and it switches back to solar.  This happens a few times during sunset period as we switch on and off high load devices/applicances.

2.  In the morning, before sunrise, the system is running on battery and switches to utility when the batteries reach 90% of their capacity.  When we switch on the central aircon system to heat the house, an additional load is placed on the batteries, and obviously drains them faster.  That means it gets to the 90% quickly.  This in itself is not a problem.  The problem is that when the system switches from battery to utilities, its as if the aircon compressor senses a very small load fluctation and goes dormant for a few minutes before starting up (perhaps some sort of protection mechanism?).  As the load requirement changes, batteries charge up and the system switches back to battery, and the cycle continues to repeat.  In summary, the aircon is never on long enough to build up 'pressure' and heat in order to work properly.  The 90% cannot be changed, as then I wouldn't have enough battery backup during the night should utilities fail completely.

 

Based on the above, do you have any suggestions on settings on the inverter that I should rather be using? 

 

One of my thoughts was to run in it SUB mode (I know there isn't really such a mode, but was hoping I could emulate it with a combination of settings).  Therefore there would only be one cut over at sunset and sunrise each, and it would be utilities.  Then battery would be a true backup.  I have looked into SOL mode, and believe it may help, but my installer reckons it won't (and I haven't been able to meet up again to talk through why he believes so)

Another option was to somehow switch to utilities from solar on a time basis rather.  I see I can do that with ICC, was wondering if there is an inherent way to do it in the settings of the inverters?

Any thoughts and comments appreciated, let me know if you need more info to assist.

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40 minutes ago, Radar said:

... senses a very small load fluctuation ...

Is that due to the <20ms changeover break on the inverter, which if it is, is normal as far as the inverter is concerned.

And I agree 100% with Seant and Pilotfish ... unless off course you installed the system to handle it all ito inverter capacity (2 x 5kva should do that) with panels fro daytime use and battery bank sized for night use, which may not be economical ito kh/h cost for batteries vs utilities.

Do you have a BMV and software to switch the inverter based on BMV's SOC?

 

21 minutes ago, seant said:

Like your oven and stove ...

I am so happy ...

12 minutes ago, pilotfish said:

MUCH bigger on everything

... to see I'm not the only one. :D

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50 minutes ago, Radar said:

The problem is that when the system switches from battery to utilities,

Make sure that [MENU 03] = [UPS] which has a faster switching time than the default [APL], if currently set to [APL] then changing this setting may help.

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1 hour ago, Radar said:

1.  When switching from solar to battery in the evening, its almost as if the system is doubting whether to switch over permanently or not.  E.g. as the sun is setting, we switch on the oven.  The oven's load results in load exceeding what solar can supply, so the system switches to batteries.  We then finish with the oven, and it switches back to solar.  This happens a few times during sunset period as we switch on and off high load devices/applicances.

The inverter does not switch from solar to battery. Battery power augments solar power depending on load. As other have already mentioned running large loads off battery power (especially a small battery bank) does not make any sense. Your batteries are not going to last long. The heat from the rapid discharge is going to prematurely age your batteries (Karoo farmers' skin undergoes a similar process in the sun).

Are you not on SOL perhaps?

1 hour ago, Radar said:

In the morning, before sunrise, the system is running on battery and switches to utility when the batteries reach 90% of their capacity.  When we switch on the central aircon system to heat the house, an additional load is placed on the batteries, and obviously drains them faster.  That means it gets to the 90% quickly.  This in itself is not a problem.  The problem is that when the system switches from battery to utilities, its as if the aircon compressor senses a very small load fluctation and goes dormant for a few minutes before starting up (perhaps some sort of protection mechanism?).  As the load requirement changes, batteries charge up and the system switches back to battery, and the cycle continues to repeat.  In summary, the aircon is never on long enough to build up 'pressure' and heat in order to work properly.  The 90% cannot be changed, as then I wouldn't have enough battery backup during the night should utilities fail completely.

Perhaps you should get a copy of Manie's ICC software and you could then schedule your switching from utility to solar and ensure that you are on utility when you run the oven in the evening and also when you run the aircon in the morning. With the small battery bank I would want to keep it charged in case of power interuption. So solar during the day and utility at night.

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1 hour ago, seant said:

Just a thought , why don't you have the high current devices on seperate cuirts which are fed by Eskom. Like your oven and stove , or geyser , etc. To me it makes no sence to run your oven on battery power .

I'm busy looking at rebalancing it, to take this into account.  Geyser is already off, and the aircon seems to have been mislabelled at the DB previously, so we thought it was off but is actually still on.

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1 hour ago, pilotfish said:

Oven/Hob/Geyser/Aircon should be directly on grid, if you want to keep those items on the PV system then you will need to go MUCH bigger on everything - like 6 Axperts, 20kW of panel and 1200Ah battery bank, that should do it.:huh:

Yip, busy looking at that today.

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1 hour ago, The Terrible Triplett said:

Is that due to the <20ms changeover break on the inverter, which if it is, is normal as far as the inverter is concerned.

And I agree 100% with Seant and Pilotfish ... unless off course you installed the system to handle it all ito inverter capacity (2 x 5kva should do that) with panels fro daytime use and battery bank sized for night use, which may not be economical ito kh/h cost for batteries vs utilities.

Do you have a BMV and software to switch the inverter based on BMV's SOC?

 

I am so happy ...

... to see I'm not the only one. :D

I'm not sure its that changeover break, might well be.

 

We are relooking load setup today.

 

I don't have a BMV or software, what would you recommend I use?  ICC and ?

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1 hour ago, pilotfish said:

Make sure that [MENU 03] = [UPS] which has a faster switching time than the default [APL], if currently set to [APL] then changing this setting may help.

Thank you very much, will check this out.

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3 minutes ago, Radar said:

I don't have a BMV or software, what would you recommend I use?  ICC and ?

Axpert in conjunction with a BMV to correctly monitor battery condition and ICC to offer quality control and monitoring by utilizing BMV information is excellent value - without these items it is junk.

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36 minutes ago, Chris Hobson said:

The inverter does not switch from solar to battery. Battery power augments solar power depending on load. As other have already mentioned running large loads off battery power (especially a small battery bank) does not make any sense. Your batteries are not going to last long. The heat from the rapid discharge is going to prematurely age your batteries (Karoo farmers' skin undergoes a similar process in the sun).

Are you not on SOL perhaps?

Perhaps you should get a copy of Manie's ICC software and you could then schedule your switching from utility to solar and ensure that you are on utility when you run the oven in the evening and also when you run the aircon in the morning. With the small battery bank I would want to keep it charged in case of power interuption. So solar during the day and utility at night.

Ii believe it does switch from solar to battery when there is insufficient solar energy e.g. at sunset.  The inverter is set in SBU mode, which by definition is Solar -> Battery -> Utilities in order of diminishing priority.  Or do I understand it incorrectly?

The Inverter is not on SOL, its on SBU.

Yes, if I can't use the existing feature set to achieve what I ultimately want, the ICC route seems to be the best option.  I do want to keep the small battery bank charged, so I want to amend settings to do exactly that.

Ultimately I do want solar during the day and utility at night.  Hence my tongue in cheek reference to an SUB mode being ideal.  Just not sure how to simulate it without ICC, but what I'm picking up on this forum is that it seems the only way to go.

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1 minute ago, pilotfish said:

Axpert in conjunction with a BMV to correctly monitor battery condition and ICC to offer quality control and monitoring by utilizing BMV information is excellent value - without these items it is junk.

Thank you.  I don't even know where to start to source a BMV, any suggestions ?

I did a bit of reading up on ICC and will be ok with sourcing that.

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1 hour ago, pilotfish said:

There are 2 modes [Solar/Battery] and [Utility].

When in [Solar/Battery] mode it will use whatever Solar is available (which may be zero at night) and augment that with battery if required.

the manual and installer both refer to this inverter having three modes which determine the order of priority:

 

SOL = Solar first.  Utility provides power to the loads when one condition happens: a) Solar energy unavailable, OR b) battery voltage drops below setting level.

UTI = Utility first

SBU = Solar, Battery, Utility, in that order. 

I'm currently running in SBU mode.

I was considering trying out SOL mode, as it may solve problem 1 i.e. once solar is unavailable it should switch direct to utility (instead of battery).   But I'm not sure if I'm reading the definition correctly.

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You are correct, what I was referring to is that while in SBU there are effectively 2 operating modes - a [solar and/or battery mode] and a [utility] mode.

I have never used [SOL] mode but my understanding is that it would operate in a very similar way during the day when PV is available, the difference being that at night when no PV is available it would switch to grid irrespective of battery state. This mode may well be a good option for you on a small bank.

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7 minutes ago, pilotfish said:

You are correct, what I was referring to is that while in SBU there are effectively 2 operating modes - a [solar and/or battery mode] and a [utility] mode.

I have never used [SOL] mode but my understanding is that it would operate in a very similar way during the day when PV is available, the difference being that at night when no PV is available it would switch to grid irrespective of battery state. This mode may well be a good option for you on a small bank.

Ok I get what you were saying, thanks.

 

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4 hours ago, Chris Hobson said:

Your batteries are not going to last long. The heat from the rapid discharge is going to prematurely age your batteries

what do you regard as a safe maximum discharge rate? The highest battery discharge current I have is 100A on a 400Ah bank. This kind of load usually occurs for only very short periods of time (<1 minute) and only a few times a week. Do you think this is safe or should I change my system to reduce the maximum load on the batteries? The batteries are Lead Carbon VRLA AGM's.

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22 minutes ago, Chris Hobson said:

Any discharge greater than 1C is thought of as "fast discharge" . Anything below 0.5C is considered "slow" so I would try stay below 0.5C.

Depends on the kind of batteries.

For lead acid, it depends on the battery construction. Starter batteries typically have CCA (cold cranking amps) which could be as high as 5C (5 times capacity), but obviously only for really short times. For house batteries C/5 is a high discharge rate and you generally want to be between C/10 and C/20. Some forklift/golf cart batteries can handle C/5 for longer periods.

For LFP, again depends on the type. The high energy type batteries can do 2C continuous (ie full to empty in 30 minutes), but generally they are rated at 1C or 0.5C, and Pylontech for example has a maximum continuous discharge rate of 0.5C (or C/2).

100A on a 400Ah bank would be (naively calculated) C/4, but actually it's higher because of the Peukert effect (under high discharge rates the available capacity is less). But since it is only for a minute, you should be fine.

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  • Radar changed the title to UPDATE: Some progress -- Mecer 5kva inverters - want to run in 'SUB' mode

Thanks for all the replies.

 

We did some 'balancing' of items going through the inverter, so now geyser, stove, oven and aircon are directly on utility power only.  The system is also working fine on SOL mode.  All other issues that were experienced are now resolved.

 

Next questions:

 

1)  how do I ensure the health of the 4 x 100ah batteries (Vision is the make/model as far as I know), without a BMV and ICC ?  I'm currently working on the assumption that a forced switch over every 2 to 3 weeks to run directly from batteries until inverter warns that it is reaching the limit is acceptable?

2) what settings (I'm an electrical ignoramus) for the batteries DOD/SOC should I be using on the inverter panel directly, assuming I will continue without a BMV and ICC ?  The electrician has setup voltage related values/limits on the applicable inverter menu settings, I just don't have them with me at the moment.  I will add them later to this post.

3) the electrician suggested that if I put a solar based heating element into the geyser, it may reduce the load on the inverters and can actually be placed back on on the system.  I'm not exactly sure what device/make/model he is referring to and want to do some research first.  What should I be googling?  I know this will place a strain on the batteries if they are used, but in 'normal' conditions I'm running on solar in the daytime and have headroom for the geyser to run off the panels too.

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On 2018/07/06 at 8:09 AM, pilotfish said:

Oven/Hob/Geyser/Aircon should be directly on grid, if you want to keep those items on the PV system then you will need to go MUCH bigger on everything - like 6 Axperts, 20kW of panel and 1200Ah battery bank, that should do it.:huh:

Nonsense! That is taking a wild stab in the dark. You can very well run all those of a much smaller, but properly designed PV system.

A modern aircon will draw about 1.7Kw, a over/hob about 2Kw and if you replace the geyser element with a 2Kw element, it will still reach 65 degrees if you let it run longer. I changed mine to 1.5Kw and it runs for about 3-4 hours and we have 65 degrees water every night. 

 

@Radar change open 1 on your inverter to SOL, it will ease the switchover between solar and Eskom a bit. Your battery bank is too small and thus more expensive than Eskom (well, depending on what you pay per KW) and keep the batteries for backup / loadshedding - which kicked in last night. 

This topic will explain what happened / why it happens, when changing from solar to Eskom or batteries. But I found that in SOL mode (I also have a small bank) the "flickering" as explained in that topic is much less - generally once per evening, and sometimes once per morning. 

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23 minutes ago, SilverNodashi said:

Nonsense! That is taking a wild stab in the dark.

Haha - you are a real sweatheart @SilverNodashi, of course it was a stab because I have no idea of the actual equipment ratings - I was just making the point that putting that type of equipment onto a small solar system is not a great idea, but is possible with a seriously big installation.

But while we are nitpicking at each others posts lets take a glance at a couple of your own howlers;

26 minutes ago, SilverNodashi said:

A modern aircon will draw about 1.7Kw

@Radar never mentioned what aircon he has and what era it stems from - a small split type modern inverter aircon (9 to 12 BTU) may draw the type of numbers you are talking about - but there are lots of others types of aircon (non-inverter, bigger model, central etc) that will draw MUCH more than that!

33 minutes ago, SilverNodashi said:

a over/hob about 2Kw

A small solid plate is about 1kW a large plate about 1.5kW, so a common type of hob with 4 plates may draw up to 5kW will all plates on the go. Then you have a grill element and top and bottom bake elements of around 2kW/1.5kW/1.5kW depending on the model - so the oven alone would draw up to 3kW depending on bake (heating or holding) or grill mode. So you could be drawing up to 8kW preparing for Christmas dinner. We normally install a 32amp CB (about 7kW) to oven/hob combo, and on the odd rare occasion will need to upgrade to 40amp to accommodate the homeowner.

39 minutes ago, SilverNodashi said:

if you replace the geyser element with a 2Kw element, it will still reach 65 degrees if you let it run longer.

Are you talking about a 100L, 150L, 200L, 250L....? A 2kW is standard in a 100L geyser element is designed to reheat the water from 15' to 55' in 2.5 hours, so 65' will probably take closer to 4 hours as losses increase with higher temp gradient - in a 250L geyser that should increase to about 10hours. So put a 2kW in a 100/150L by all means, above that is a bit dodgy.

...otherwise your post is right on the money:D

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