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Reduce cycling of battery bank


Noobie
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Need some advice here guys and girls.

During dusk and dawn my axpert inverter switches between battery and solar very often, cycling the batteries more often than I would like.

I know that there is software available but I want to keep PC's or Pi's out of the control circuit if I can help it.

Would this work: Installing a relay which will connect/disconnect the solar panel input to my inverter and have this connected to a timer switch.

I would then tell program the timer to turn off the relay (disconnecting the PV input to the inverter) before dusk, lets say 16:00 and then turn it back on after dawn the next day, lets say 08:00

My hope is that this will stop the battery bank charging and discharging so often during dusk and dawn?

I understand that during the day if there happens to be some cloud cover over the PV panels this will cause the batteries to cycle, but I dont think there is much I can do about this.

Would this setup work or am I on the wrong track?

Also what setting would be best to set it to in Program 1, SOL or SBU? I think that once the relay and timer is installed setting it to SOL would be best?

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Your problem, I think, is due to SOC calculated on the battery volts.

No, I would not disconnect between panels and inverter.
From inverter to load yes. You get plugs with timers on.

Just a thought: The reason for the software (Pc or later Rpi) with a BMV monitor is to get a more accurate SOC guess to protect the batteries with less strain on the system. 

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My thinking on this is you haven't got your voltage levels right. I don't know the Axpert so I don't know how that works, just that presently I use voltage as a make-shift method of doing the same kind of switching. I have carefully tuned mine so I know roughly at what voltage I have to switch to get a certain DoD. After having done this, I use (on my 24V system) 26.5V for switching to solar and 24.7V for going back to grid. When it switches to solar there is usually not enough solar (yet) for my modest morning loads (rarely more than 300W), but the sun catches up pretty soon. So I believe this situation is the same as yours: I have that early-morning window. My voltage levels provides a large enough voltage window that it only switches once.
 

So my thinking is that either your battery bank is too small/old, your loads are too high, or your voltage window is too small. Or some combination thereof.

 

The ideal of course is to switch when the solar is sufficient for the loads, thereby not cycling at all, but you're going to need at least some kind of microcontroller and a hall-effect current sensor for that ;-)

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I also experience the same on my Axpert system where the PV is disconnected and reconnected to the MPPT multiple times during dawn/dusk.

This doesn't have anything to do with the battery voltage, but more with the voltage provided by the panels.

The Axpert MPPT charger has a minimum voltage at which it will accept power from PV. If the PV drops below this voltage, the MPPT will switch to the next source of charging, if any. During the dawn/dusk period, the voltage fluctuates around this cut off, and the inverter constantly switches sources, so I can understand Noobies frustrations.

Unfortunately, there is no setting in the inverters to stop this, and short of his idea of a relay the cut power from the PV and a point before the situation occurs, using software is the only way.

I got to a point last year of manually disconnecting the PV from the inverters each afternoon around 16:30.

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Yeah I don't really see a problem as at that time I usually let the unit swap to utility mode then the batteries just keep charging each time the panels do have enough voltage.

Every minute I can keep them charging/float is more important I think so don't disconnect them :)

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I think all Axperts does this. Not much concern to me though. It happens at such low power states that I think it barely creates a relay spark on the MPPT "switch".

The inverter will only change source priority though if it is set to SOL mode instead of SBU or UTI, i.e. the mode that uses Solar during PV voltage times (day) and when the PV drops away switches to Grid. To avoid the to and fro - you will have to set it manually to UTI each night and back to SOL in the morning....

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I know you said you don't want to run software. But sorry to say if you want the Axpert to work the way you want it then you will need software. That is why it was build. To solve that problem you have. AICC will switch the modes for you so you dont have to if you run SBU and UTI modes. Or else you will have the problem explained.

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7 hours ago, viceroy said:

I also experience the same on my Axpert system where the PV is disconnected and reconnected to the MPPT multiple times during dawn/dusk.

This doesn't have anything to do with the battery voltage, but more with the voltage provided by the panels.

The Axpert MPPT charger has a minimum voltage at which it will accept power from PV. If the PV drops below this voltage, the MPPT will switch to the next source of charging, if any. During the dawn/dusk period, the voltage fluctuates around this cut off, and the inverter constantly switches sources, so I can understand Noobies frustrations.

Unfortunately, there is no setting in the inverters to stop this, and short of his idea of a relay the cut power from the PV and a point before the situation occurs, using software is the only way.

I got to a point last year of manually disconnecting the PV from the inverters each afternoon around 16:30.

Viceroy, you have hit the nail on the head. Every afternoon before dusk I turn off the circuit breaker that connects my PV panels to the inverter and then turn this breaker back on after dawn the next day. I have no doubt the software will achieve what I need to accomplish but I want to keep the solution as simple as possible. If I am away from home for extended periods and my Mrs needs to start fiddling with software to get the inverter to work, that's a less than ideal situation.

In the end, what did you wind up doing to sort this out?

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This is what happens currently:

1) Sun is shining, 12:00 midday, PV panels supply power to load and charge battery

2) Sun starts going down, 16:30, solar panels can't generate enough power to power the connected load

3) Inverter compensates low PV input power by running off of battery and PV power combined 

4) Once battery voltage hits low level alarm (25,5v) inverter goes to bypass and supplies grid power to load 

5) Now the PV panels are charging the batteries up again while grid is supplying the load 

6) Once the battery reaches fully charged (27v), the inverter disconnects the grid and then the load is again supplied by battery and PV combined

7) Once again, once the battery low level reaches 25,5v it switches the load to grid and solar charges the batteries up again.

 

Its a vicious circle, and every time the batteries charge up and discharge its shortening their life expectancy 

JDP, if you wouldn't mind explaining to me in lay mans terms what your software will do to overcome this situation, I would be greatly appreciative 

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His software will force the inverter to swap to utility mode on the time you configure it. (bypass mode)
Then in the mornings you configure the time when you want the solar to power the load.

You can also control the swaps by using the BMV's SOC data with the use of the software if you prefer that

In Utility mode the batts gets charged by PV as long as they get enough sun.

It's a win win situation

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1 hour ago, Noobie said:

This is what happens currently:

1) Sun is shining, 12:00 midday, PV panels supply power to load and charge battery

2) Sun starts going down, 16:30, solar panels can't generate enough power to power the connected load

3) Inverter compensates low PV input power by running off of battery and PV power combined 

4) Once battery voltage hits low level alarm (25,5v) inverter goes to bypass and supplies grid power to load 

5) Now the PV panels are charging the batteries up again while grid is supplying the load 

6) Once the battery reaches fully charged (27v), the inverter disconnects the grid and then the load is again supplied by battery and PV combined

7) Once again, once the battery low level reaches 25,5v it switches the load to grid and solar charges the batteries up again.

My inverter also hits 25.1V around 17:00, but I think the extra 0.4V I give it makes the difference. It takes until well after 19:00 before it switches to grid, and at that point there is just no way it's going back up again :-) Of course it means that I cycle them around 20% DoD on a daily basis, but my recipe is to use cheap second-hand UPS batteries which I get cheap enough that I don't care if I kill them in a year... at least until my ship comes in and I buy the big bank :-) If you look at the cycle life charts, there really isn't a big difference between permanent floating and such light cycling.

 

In fact, it's 19:45 now. I'm still running on inverter power, 24.8V, running at 300 watt. It will switch in a few minutes, and then bounce back to 25.2V.

 

I take it the Axpert cannot be adjusted? :-)

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18 hours ago, Noobie said:

This is what happens currently:

1) Sun is shining, 12:00 midday, PV panels supply power to load and charge battery

2) Sun starts going down, 16:30, solar panels can't generate enough power to power the connected load

3) Inverter compensates low PV input power by running off of battery and PV power combined 

4) Once battery voltage hits low level alarm (25,5v) inverter goes to bypass and supplies grid power to load 

5) Now the PV panels are charging the batteries up again while grid is supplying the load 

6) Once the battery reaches fully charged (27v), the inverter disconnects the grid and then the load is again supplied by battery and PV combined

7) Once again, once the battery low level reaches 25,5v it switches the load to grid and solar charges the batteries up again.

 

Its a vicious circle, and every time the batteries charge up and discharge its shortening their life expectancy 

JDP, if you wouldn't mind explaining to me in lay mans terms what your software will do to overcome this situation, I would be greatly appreciative 

I switch between SBU and Grid.

At 4:30 pm my inverters switch to grid. What ever the panels are still producing still goes to the bank. Reason I switch at 4:30 is as you can see at 1) on the image, that is the time my panels stop producing enough power to carry my house load. It will probably change over the seasons, that is wy AICC keeps the stats so I can go check. Then when I have time I can expand the software to run a full year in full auto mode once I have the stats and info I need. At 2) the panels drop out totally, but until that time you will see the purple line is just above it, that tells you the power is going into the bank. As soon as the panels fall away then Eskom takes over the float. That is why the purple line then picks up and flat lines on its own.

 

modes.JPG.530f9a3b543dc276a6b6a4c365b961

 

At 11:00pm its switches back to SBU mode and stays there. The next morning when the sun comes up it will charge the bank and carry the load.

If it is a cloudy day I will switch back to grid mode manually at this stage. I am working on a way to do that automatically.  But its has not been a problem to switch manually as I can see how much the panels are producing when I Teamviewer the PC AICC runs on.

But then again this is not 100% fool proof as the bank still discharges if there is not enough comming from the panels, as you can see the purple line drops below the 0 mark from time to time depending on a cloud. But at least my Invertes are not jumping between modes all the time letting my DVR skip a beat and wearing out the relays.

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11 hours ago, The Terrible Triplett said:

Plonster, by what time does your system get the batts fully charged?

That depends on what you mean by "fully". When it switches from absorb to float, your batteries are technically not "full" yet. It depends on house load, but it will go from absorb to float anywhere between 11AM and 2PM. The BlueSolar charge controller has counters that tell me how many minutes it spent in each phase, so what I do is check that it spends at least two hours or so in float, and then every now and then I'll have a "grid day", usually one of those semi-cloudy days, where I'll stay on grid the whole day and just let it charge. This also happens to some extent on Saturdays, when we use less electricity for some strange reason...

Edit: Of course this is why I need to either buy a BMV or finish my monitoring system... so this sort of guesswork can stop.

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56 minutes ago, The Terrible Triplett said:

Let me rephrase: At what time do you switch your load from Eskom to solar in the mornings?

At the moment, approximately 8:00AM. We had a bit of cloud on the horizon today, so it didn't switch until 08:30 (ish). I have an override switch that forces it to stay on grid. If it's a particularly cloudy day, I hit the switch.

Edit: Wait, I think I get it. You're asking because you only switch to solar once the batteries are full. I don't do that. I have enough solar to drive my loads and charge the batteries, and I get better self-consumption if I switch earlier. Otherwise the batteries fill up "too quickly" and I end up throttling the panels to half capacity for the rest of the day.

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Can you maybe share how you manage that, to go over at 8ish after using up to 20% of the batts at night?

In other words, how is your system set to work currently?

I am asking because I am adjusting my parameters, triggered because of what all that the forumites have said on the forum to date. :D

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1 hour ago, The Terrible Triplett said:

Can you maybe share how you manage that, to go over at 8ish after using up to 20% of the batts at night?

In other words, how is your system set to work currently?

I am asking because I am adjusting my parameters, triggered because of what all that the forumites have said on the forum to date. :D

So I have 200Ah at 24V, or around 4.8kwh storage. I go about 1kwh into that storage at night before I switch. I also make between 4.5kwh and 5kwh using my solar panels when running flat out, total potential. I tested this on an older bank, I added loads to permanently keep the bank below absorption voltage specifically to check what it can do.

It's a small array, I probably have the largest volume of writing vs solar setup ratio on this forum!.

So the aim is to switch to solar round about the time when solar equals loads, and depending on the time of year, that might be anything between 7 and 9AM, so you power the loads from very early on while you use the excess to charge the batteries. Doing it this way I still lose about 30% some days (ie make only 3.5kwh out of the possible 5kwh) because the batteries fill up too quickly and I don't have enough loads to dump the rest into.

So it is not scientific at all, but if the charge controller indicates that I made less than 4kwh on a particularly sunny day, I know the batteries had a good charge. I can also look at the float-charge timer on the CC.

But I really need better management... I keep saying that... for my own benefit mostly :-)

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I think I may challenge you on your claim re volume of writing vs solar setup. :D
225ah batts, only about 4.35kWh per day from panels using but a plain 1600va inverter.

Ok, you win, I have a BMV also. :P

Yet, it has to be us, the smallest outjies, who make our systems sing like canaries early in the mornings, sommer lekka cheap, manually if we have to. :D

 

Okay, I like what you are doing, swap over when solar matches load versus me who flavored SOC to be X before system swaps over.

So how are you going to automate that? For I am very interested. 

In the mean time I think I am going to set my changeover back to Eskom @ SOC of 85% and back to solar on 86% SOC (it will bottom at about 80% SOC) so that I can grab the early morning light a bit, after running a bit longer at night.

Also going to start a weekend regime where on Friday / Sat evenings I go for broke down to 50% SOC, let the batts charge the whole of Sat en Sun, as we do not use a lot then.

 

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1 hour ago, The Terrible Triplett said:

So how are you going to automate that? For I am very interested. 

It's already somewhat automated, based on voltage, which we all know isn't perfect. As a first stop, I just want better charge/discharge numbers, or in other words, I want to build my own BMV :-) After that, the idea is to do cool things with the data. I'm thinking of pulling in data from the solar geyser too. And someone gave me a 1-wire weather station. Combining all that, some kind of better self-consumption. I'll worry about that later :-)

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Plonkster, like the real developer you are ... where on earth are you going to find the time to build your own BMV, having waited 9 months for parts? :D

Sorry ... could not stop that one. :D

And once it is built, will you / can you sell in on to a select group of users?

Seriously though, what else can be used, off the shelf, to replicate your idea?

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4 hours ago, The Terrible Triplett said:

Plonkster, like the real developer you are ... where on earth are you going to find the time to build your own BMV, having waited 9 months for parts? :D

Sorry ... could not stop that one. :D

And once it is built, will you / can you sell in on to a select group of users?

Seriously though, what else can be used, off the shelf, to replicate your idea?

Look, if you have charge numbers coming in, and you have load numbers going out... It is a matter of Newtonian integration to change that into Watt-hour numbers. You want 5-10% more coming in than is going out. Done: You have a BMV. The fact is that all this info is available from most charge controllers, and it's downright trivial if you have a Victron mk2-able inverter and a BlueSolar (ve.direct) charge controller. Replicating it would be 1) Buy raspberry pi, 2) load the software, 3) plug in the mk2 and the ve-direct cable, 4) profit.

Once I'm done anyway... :-)

This damn Pi is resisting me every damn step of the way. I rigged the cp2103 dev board into the circuit today. Same. Bloddy. Problem. The moment I plug in the USB cable into the Pi, everything stops working. Works perfectly on the laptop. It's as if the code on the Arduino stops running, because the can link icon on the bluesolar's LCD display turns off at this point. This is even when running everything with separate power supplies.

I think it's still a power supply problem. Time to get a real arduino and forget this hare-brained idea of making my own? Perhaps.

Edit: I think it's noise on the power rails. The arduino is connected to the cp2103 only by two wires, ground, and rx (via resistor divider because the cp2103 is 3.3V). If I disconnect the rx connection, the strange instability remains. If I disconnect the ground, stability returns. Some kind of noise makes it over that shared ground. I'll get the scope on later tonight to confirm.

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