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SOC 55% and battery balancing


Chris Hobson
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Over the weekend I had  the power from the genny trip in the evening and we had the rotisserie on. We use it during the day or on the genny but we had an accidental fairly rapid discharge of the batteries to a SOC of 55%. I was sitting outside with guests and did not notice a problem until the BMV started moaning. On Sunday and today for the first time in more than a year  I have had midpoint deviation alarms during late absorb. I treat my batteries relatively benignly, in an off grid situation, with an average DOD of 30%. My battery bank is 600 days old so this may be an indication that the battery bank is starting to show its age.

I will monitor and report my findings 

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3 hours ago, Don said:

Chris, just out of interest sake, what is your charge efficiency on the batteries at this stage? Comparing your discharge energy to the charge energy.

I thought this would be easy and looked at the BMV history logs.

Cumulative discharge 1109 kWh

Cumulative charge 1827 kWh

No of synchronisations 371 (These are auto-synchronisations and it happens most days. There are probably less than 10 days in the last year when it has not synchronised).

No of cycles 67 (Number of times it has dropped below 65%).

That is efficiency of 60% which makes no sense.

 

So I extracted data for Sunday/Monday from one SOC of 100% to the next one the following day for an efficiency of 87.2% which makes more sense

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Thanks @Chris Hobson.

8 hours ago, Chris Hobson said:

That is efficiency of 60% which makes no sense.

I think it actually does make sense. I have relative new batteries that has only been working for about 3 months. My cumulative discharge = 59.4 kWh and my cumulative charge energy = 65.7 kWh. My charge efficiency = 90.4%. Your batteries have a lot of kWh on the clock and I am sure have worked hard over the last couple of years on the farm. This is the chart I am keeping my eye on. With time and as I abuse my batteries, this figure will drop. 

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13 hours ago, Chris Hobson said:

I have had midpoint deviation alarms during late absorb.

Obviously I don't have a problem with by new batteries. My midpoint sits at 0.001 and at times it is 0.

- What I am trying to establish is at what stage I can expect the batteries to show signs of wear and tear. 

- At what charge efficiency it would be more economical to replace the batteries. 

Obviously with age the batteries will discharge at a faster rate, which might be the only consideration. I am sure you will not be impressed if the battery SOC alarm wakes you up at 3h00 every morning to go start the genny. 

Nobody can tell you or me when to replace the batteries. It depends how many spare cows we have to sell. But at some stage I think a person need to draw the line. Let us assume a charge efficiency of say 40%, because I am now putting in 60% more energy into the batteries than I have taken out of them, to get them fully charged again. That is wasted energy I could have used to run the washing machine. Or am I looking at it incorrectly?

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48 minutes ago, Don said:

Nobody can tell you or me when to replace the batteries. It depends how many spare cows we have to sell. But at some stage I think a person need to draw the line. Let us assume a charge efficiency of say 40%, because I am now putting in 60% more energy into the batteries than I have taken out of them, to get them fully charged again. That is wasted energy I could have used to run the washing machine. Or am I looking at it incorrectly?

The energy I put into the batteries is nearly free.What I cannot use I need to store in the batteries. Since I am in the Karoo I have no real need for more than about 16 hours autonomy. As my batteries age so the battery voltage at a given SOC will drop and I will have less and less capacity. At the end of their life I will probably pull the ring out of them and discharge to 42-46V.

59 minutes ago, Don said:

 I am sure you will not be impressed if the battery SOC alarm wakes you up at 3h00 every morning to go start the genny. 

I think the crunch will come when we no longer can go overnight on the batteries. If it is during winter I might just switch off the inverter as we go to bed. If it is summer I will have to bite the bullet and buy more batteries. The batteries I have now cost R15k ex. So I have had good service for not much money. I know Plonky has done costing of battery kWh compared to Eskom/COC kWh. In an off grid situation I am more worried about the number of cycles. If my batteries last 800 cycles and they should since  the bulk of our cycles have been a DOD of about 30%) then my battery cost is R19 per day. If my batteries fail tomorrow then I am sitting at a slightly higher R25 per cycle. This is all very acceptable considering the alternatives. I am fortunate that since the install I have had no inverter or PV trouble and additional costs. So this means next time around I can spoil myself with a bigger bank, maybe Trojans or even Lithium batteries.

Don what happened to your old batteries? Your usage is very light about 600Wh a day (assuming you had a BMV from nearly day 1).

 

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9 hours ago, Chris Hobson said:

At the end of their life I will probably pull the ring out of them and discharge to 42-46V.

So there is actually no "economical" time to replace the batteries. You look after them for to get the most cycle out of them as possible. Once they reach the required cycles at end of life, you squeeze them even more and hammer them to death. Once they fail, you replace them. Ok, that makes sense. 

9 hours ago, Chris Hobson said:

Don what happened to your old batteries? Your usage is very light about 600Wh a day (assuming you had a BMV from nearly day 1).

 

Chris, this is my first set of batteries. I bought inverters, batteries, panels, etc October/November 2015. Never installed it. I only installed the inverters and batteries end of October 2016 and the panels end of December 2016. My system has only been running since October/November 2016. Yes, the BMV has been running since day 1. Initially I never used the batteries. I would switch to batteries to discharge them a bit and switch to grid again to let them charge again. 

I had no idea what I was doing at the time, I nearly hammered my batteries to death, because I entered the wrong information into the BMV. I have 8 x 250 ah batteries. I entered the correct ah in the BMV of 500 ah. 2 Strings of 250 ah. The first time I switched to batteries, the BMV said I have something like 2-3 hours Time To Go. I thought that can't be right. I have 8 batteries that weigh 75 kg each, surely I must have more time than that.  (Obviously not even looking at my load at that point in time). I thought it must be my calculation of ah that is the problem. I multiplied 250 x 8 and got 2000 ah (obviously wrong) and entered that in the BMV. Now I had 27 hours Time to Go. That looked much better and suited me better, but it still bothered me that something was not right. Then I spoke to @PaulF007 about 2 weeks later and he asked me what my battery capacity was. When I told him 2000 ah, he was very impressed, until I told him I only have 8 batteries, lol. I then reluctantly had to change it back again to 500 ah. If it was not for that I probably would have blown my batteries by now.

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Since then I have learnt a lot from you guys on the forum. Paul has also helped me a lot getting my base load down from about 1400 watt to around 600 watt. I did a trail run for 2 day completely off grid, then the clouds started to hamper me. I could not get my batteries fully charged before the sun set. So if I have to, I can do it, but the wife got irritated with me as well telling her when she can do the washing and when she could run the tumble dryer. 

What I am doing now is I switch to Batteries/Solar at 24h00 and run through to 18h00 when the sun sets, then go to grid for 6 hours and start the cycle over again. I try and get my batteries fully charged before before I change to grid. When my panels kick in early in the morning, my SOC is round about 90-92%. On a good day my batteries SOC reaches 100% anything from 10h00 -1200, depending on my wife's load. I can therefore actually go to Batteries/Solar earlier if I want. I might change it to 22h00 and see how it goes. 

I think my batteries are still recovering from the shock treatment I gave them some time ago with the wrong BMV settings.

 

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11 minutes ago, Don said:

I had no idea what I was doing at the time, .....

We all start there because of what we are lead to believe.. I was going to get 2-3 hours of 3000W from my panels and it would not matter if the missus used the oven as 90% of the  power would come from the panels.Hmm now that I know better it does not work like that. Another 3 panels and it will. Prices look so good I am tempted but the bank manager says NO!

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@Chris Hobson you had better have a word with the bank manager again and explain forex to him. Earlier this morning the ZAR/US$ rate was just above 13.01 and now barely two hours later (11:30) it is 13.0557. The US will surely prosper when Pres. Trump drops company tax from 35% to 15% and then there is zero doubt that the ZAR will be back on its Zuma induced slippery slide to oblivion, ably assisted by the phenomenal economics in in destroying a currency that he learned from his well informed pal, Mad Bob Mugabe. Time to jump in will probably not get better than now.

@Don your discussion on using and charging the your batteries is most interesting. I ended up with very much the same procedure although being in the Western Cape, the sun comes up a little later so my charging starts at about 9:30 am even though solar generation starts at around 7 am. I have set my charge off grid to 2A to make sure that I don't use the inefficient grid charging route. As soon as I finish details my often mentioned idea to have ICC switch to grid for the time when the wife (mine is not a technical Einstein either), switches on heavy loads while the system is on solar/batteries. We really have to find a shorter term for solar/batteries, I would suggest S/B or S/Bat just to minimise the typing for self taught typists like me.

At what amps are your grid charging set as a matter of interest.

 

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4 hours ago, ebrsa said:

At what amps are your grid charging set as a matter of interest

Hi ebrsa,

I have setting 11  (Maximum utility charging current) set to 20 amps on each inverter. The charge current going into my batteries with NO SOLAR and only grid available, runs at about 38 amps. 

I have setting 16 (Charger source priority) set to SNU - Solar energy and utility will charge battery at the same time. Therefore you need to have a look at setting 2 (Maximum charging current: To configure total charging current for solar and utility chargers) I have this set to 30 Amps on each inverter. 

The reason I do that is when I change to grid, I don't lose whatever solar is still available. When I change to grid, the house load comes from grid and whatever solar is left will go into the batteries. Only when solar is insufficient to charge the batteries will it use grid. Although I have the Change to Grid set at a certain time, I watch what is happening late afternoon. If I see there are too many clouds around and the load is too high and I am now seriously dipping into the batteries, I would change to grid before the set time, because I want my batteries full charged when the sun sets without making use of grid to charge my batteries. 

I switched to grid just to illustrate to you what happens with the above settings. Load goes to grid and solar to charging the batteries.

grid10.thumb.JPG.12eb4c4644d99e79742c9b66c46fbfd3.JPG

 

This is a typical day. Everything running along fine and from 15h15 the clouds move in spoiling what was a great day. At 17h00 I decided to go back to grid. I want my batteries back to 100% SOC before the sun sets. Although there was not enough sun available to handle the load, but it was sufficient to charge my batteries without making use the grid. 

58a441ef7cf9d_typicalday..thumb.JPG.0f42df1df4750f41a0e2ae31275f1dab.JPG

 

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2 hours ago, ebrsa said:

As soon as I finish details my often mentioned idea to have ICC switch to grid for the time when the wife (mine is not a technical Einstein either), switches on heavy loads while the system is on solar/batteries.

Ok boys - has anyone got a wife who knows her way around solar? We had the girl with the blender and another Cape Town based woman who both demonstrated a fundamental understanding of the intricacies of solar systems but solar girls are far too few on the ground. 

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2 hours ago, Chris Hobson said:

Ok boys - has anyone got a wife who knows her way around solar? We had the girl with the blender and another Cape Town based woman who both demonstrated a fundamental understanding of the intricacies of solar systems but solar girls are far too few on the ground. 

It was a MOERSE fight in the beginning but I will have to admit that the SWAMBO is quite well trained here..  Put it to you this way when ever she picks up any thing electrical in the shop she first turns it around to look at the watts. Then at the price and then at me.. So far so good.

But please don't tell her about this quote , net nou raak sy mak..

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OK today we went over 1% deviation for about ½ hour. So the batteries did not enjoy their deep discharge but the balancers are beating them back into shape.

1 hour ago, PaulF007 said:

as a MOERSE fight in the beginning but I will have to admit that the SWAMBO is quite well trained here..  Put it to you this way when ever she picks up any thing electrical in the shop she first turns it around to look at the watts. Then at the price and then at me.. So far so good.

My missus has got as far as knowing that if it heats something up then use one at a time except the kettle and toaster they can go at the same time. The other heavy draw item is the vacuum cleaner. The motor on a vacuum cleaner works hard. We have wooden floors and a polisher and it uses a quarter of the power of the vacuum..

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@Don my settings are very similar to yours. I will give you my understanding of how ICC-Pi works based on information gained on this and the AICC forum for the time it existed. I once asked Jaco Fourie whether AICC change setting setting 1 to Uti when you switched to grid and he confirmed that. I would expect that ICC-Pi works the same way and perhaps @Manie would confirm if that is the case. My setting 16 is at CSO as I do not want the grid to charge the batteries and this gives solar as first priority. The reason why I have set setting 11 to 2A is just to counteract the minimal draw from the batteries when on grid. My setting 2 is set to 20A as that gives me a total of 40A maximum, with a 2xAxpert 5Kw cluster, which is near C/10 for my 450AH bank and I don't want to fry the batteries. My panels in any event mostly only supply the house load and around  20A maximum during the day Of course you can afford to charge at a higher rate as you have a larger AH capacity. Fortunately for me the batteries are, with my loads, at 100% by around 15:00. In the mornings the batteries are at 75% SOC and I could possibly do something more to minimise load during the night, like turning off the TV completely as well as some computer equipment not in use.

So the optimal setup as far as I can see is to ensure that the grid is not charging the batteries when solar is available and to switch to grid when loads above a settable load occurs. That is the requirements I am trying to formulate in detail to submit to @Manie for inclusion if possible, should develop a Version 2 and aims to overcome this problem. It would optimise on grid usage and ensure that batteries are charged by solar as well as enable one to run on SOC which would be preferred in my case and I would venture probably is true in most cases. Of course when SOC drops below the minimum SOC value ICC-Pi should switch to grid but that is what I hope will be prevented with a maximum load setting before switching to grid. The other option is to exchange the Axperts for an Infini and that will involve significant costs. So in essence I am trying to see if the Axperts which we both already own, could be turned into a quasi Ifini with the modification to ICC-Pi and avoiding changing costs. It is interesting to note that @Manie started off with a 3Kw Axpert but now runs and Infini.

Thanks for your views. I hope I could contribute something of value. It is only through discussion of this nature and some I had with other forum members that I have arrived at settings, that at this stage, work fairly satisfactorily for me. Of course I would welcome any suggestions or even counter arguments.

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42 minutes ago, ebrsa said:

I once asked Jaco Fourie whether AICC change setting setting 1 to Uti when you switched to grid and he confirmed that. I would expect that ICC-Pi works the same way

Yes, it does , I can confirm that as well. Setting 1 is your load priority only, which excludes battery charging. Therefore when selecting Uti in setting 1, your load will be powered by Utility/Grid as first priority. If Utility /Grid is not available, load will go to solar or battery, whichever is available.

52 minutes ago, ebrsa said:

My setting 16 is at CSO as I do not want the grid to charge the batteries and this gives solar as first priority.

Setting 16: I am not so sure there is a big difference between CSO and SNU. If you look at the first picture I posted and tell me with my setting at SNU, how much is coming from grid? Nothing. What I have experienced is with my setting on SNU, it behaves exactly as I would it to behave with the setting at CSO. The only difference is that with CSO setting, when your panels only produce 50 watt, that is the only charging that will go into your battery, until the sun sets and solar dies completely. With the SNU setting, if your solar panels are only producing 50 watts, it would add grid charging at the same time. That is where setting 2 comes in. You tell it what the combined charging rate should be between solar and grid. 

1 hour ago, ebrsa said:

So the optimal setup as far as I can see is to ensure that the grid is not charging the batteries when solar is available and to switch to grid when loads above a settable load occurs.

I agree 100%. Currently I have a set time to change over. That only works when I am not at home. I override this setting every day when I am home, depending on weather conditions, as I tried to illustrate with my second picture above. Yes, it would be nice if that function could be automated. 

1 hour ago, ebrsa said:

The other option is to exchange the Axperts for an Infini and that will involve significant costs.

Yes, If I can do it all over again, that would be probably the way I would do it. As you said, it comes at a cost. The Infini is 3 times the cost of the Axpert, but comes with all the features I want in an inverter.

1 hour ago, ebrsa said:

I hope I could contribute something of value.

I find any contribution of value. Not one of us thinks the same or have the same priorities when it comes to solar. Arguments and counter arguments make you think about different aspects you might not have thought about or considered.

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2 hours ago, ebrsa said:

Of course when SOC drops below the minimum SOC value ICC-Pi should switch to grid but that is what I hope will be prevented with a maximum load setting before switching to grid.

You can always use Setting 12 to switch back to grid. The difference is this is a voltage setting and not a SOC setting. Setting 12 will switch the inverters back to grid at the set voltage. The problem I have with this setting is that if you are running close to the set voltage and you say switch on something like a kettle that would draw 2000 watts for a few minutes. That sudden load would cause the battery voltage to drop rapidly and pass that set point and switch you back to grid. Once the kettle switches off, the battery voltage would recover, but probably not enough to reach your setting 13 value to go back to batteries. You also don't want to switch back and forth between grid and batteries in any case. 

For me the most important setting is setting 29. The DC cut off voltage. I still have an electrical problem I need to sort out. At times when the inverter goes back to grid, the earth leakage would trip. There is no grid and the inverters would immediately go back to batteries. Now I am on a run away train, because the inverters can't go back to grid, as there is no grid. You might not have an electrical fault, but that can happen when lightning strikes your house or for whatever reason tripping the earth leakage. Murphy's law, are not home to reset it. 

You get home to a dark house and eventually get you system up and running just to find out you ran your batteries down to 42 volts, which is the default setting. I have mine set at 48 Volts which is the maximum setting. I have no reason to run my batteries past 48 volts whatsoever. 

 

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5 hours ago, Don said:

For me the most important setting is setting 29. The DC cut off voltage. I still have an electrical problem I need to sort out. At times when the inverter goes back to grid, the earth leakage would trip. There is no grid and the inverters would immediately go back to batteries. Now I am on a run away train, because the inverters can't go back to grid, as there is no grid. You might not have an electrical fault, but that can happen when lightning strikes your house or for whatever reason tripping the earth leakage. Murphy's law, are not home to reset it. 

Hi Don 

If I remember correctly your inverter should be getting clean power (i.e before the ELB). The inverter's output should have its own ELB and the inverter itself should be earthed to your AC supply. With program 38 using a relay one should be able to maintain an alternative pathway to source (either Eskom or inverter depending on which mode you're in). I know you have been asking  about it but I only truly understood your setup with the post above.

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40 minutes ago, Don said:

@Chris Hobson, I need to sort this out fast. That earth leakage just tripped again. No grid available.

Hi Don 

You need something like this. 

Don.pdf

The relay needs to open when you are running off the inverter and close when running off grid. The earth wires need to be bridged at the inverter. 

2 minutes ago, Don said:

Data comes from here.

Ok I was beginning to wonder with two arrays producing exactly the same power but I see there is some variation.

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