Calling all Multi Skybox users, External CTs and battery discharge

True Hybrid Energy System
Post Reply
SHunsader
Forum Expert
Posts: 52
Joined: Mon Feb 15, 2021 2:54 pm
My RE system: Pair of Outback Skybox Systems each with 12 - LG 340 panels, one with 4 - 14kWh Lishen batteries, and the other with a pair of Discovery 6.5kwh batteries, both running as Masters. External CT's, one using L1 CT input and the other using L2 CT input so as to help share load more evenly (Called 'Split Mode')
Location: SF Bay, California
Contact:

Calling all Multi Skybox users, External CTs and battery discharge

Post by SHunsader »

Everyone,

It's been more than a month now that I've been working with a pair of Skyboxes, running external CTs, and WISHING that both Skyboxes would 'help' share the grid load our complex has been using. Unfortunately, regardless of configuration, (at least until now), I have been unsuccessful getting both boxes to discharge batteries in parallel during our peak pricing window (from sundown to 9p). I have tried numerous configurations, without success. My current configuration is running both Skyboxes as Leaders, and have the L1 CT plugged into 'Sky 1', and the L2 CT plugged into the L2 input of 'Sky 2'. Although I have received another pair of CTs available to install, so that each Skybox can sense the total power used/provided at the main panel, I'm considering trying something different first. Installing the 2nd set of CTs is painful, and I believe that I'll be trading one problem for another (both Skyboxes thinking they need to 'extinguish' the same load at the same time will probably leave to overshoot, perhaps creating a sinusoidal feedback demand/supply loop, or worse yet, a divergent sinusoid, leading to a grid power draw or system shutdown from an overloading condition. Until I find a way to accurately dial down the load to 50% of the real load to each device, I'm contemplating something else instead.

Before going there, I should point out that I've been having a second problem, arising from batteries failing to completely charge before sundown. Initially this might not seem like an issue, BUT, as it turns out, batteries CANNOT be discharged as long as the system is trying to charge them. It appears that systems continue to 'keep' the system in that charge state even when the sun goes down. Thus, if one is grid tied and the utility charges Peak Rates after sundown (to 9pm for me), if the batteries are stuck in charge mode, I can't recoup some of the battery cost by offsetting that peak usage by using those batteries (is supposed to be possible in Self Consumption mode). Since there isn't a "Stop Charging After Sundown" checkbox, or any other method of stopping the charge after sundown, below is a method that I'll be trying instead:

Here's an email I sent to a fellow multi Skybox owner:

I MAY have figured out how to deal with partial battery charge....

Of course, this will depend on how well having dedicated CTs for each box works out.

WEEKS ago I considered taking each of the outputs of the 2 CTs into a double throw, double pole switch. The outputs of these switches would each go to the Sky 1 and Sky 2 boxes. I could split the output, or direct the entire output to a single Skybox. The goal here initially was to automatically flip the switch every 15 minutes, providing a more even sharing of the loads presented to each of the skyboxes. The initial plan was to flip my current L1 and L2 from skybox to skybox, so even if there were imbalanced legs (L1 and L2), they'd be flipped every 15 minutes, averaging the load for each system.

Today I'm seeing that my Sky 1 box will NOT get completely charged today, because it's been very overcast again. BUT, because Sky 2 uses the L2 input, and L2 has a very little load to carry, that battery will get completely charged.

It would seem that Sky 2 SHOULD unload this evening, but not completely sure. If the load is minimal, then not much discharging would occur during the Peak window. BUT, if I flipped the L1/L2 with the switches, then the Sky 2 box would see the significant load, and unload completely during the peak window.

On the following day, should it be another cloudy day, I could make sure that the switch is NOT thrown back to the original condition, and allow the battery from Sky 1 to get completely charged, and Sky 2 would probably end up partially charged. That evening, I'd need to flip the switch once again, to match the high load with the system that has a fully charged battery.

Keep in mind that the only time this switching would need to be done is whenever one of the batteries fails to charge completely (or until Outback fixes the incomplete charge issue).

Once I have a larger battery array (2x what I currently have), I can unload the batteries at a much higher rate to cover the entire Peak window, and still have battery capacity remaining to get a complete charge the following day.

What's more, during sunny days, if I choose to automatically flip the switch every 15 minutes, the load will be spread out more evenly amongst the batteries, prolonging their cycle life.

As I said, a lot of how this is done will depend on whether/how Outback solves the incomplete charge issue, possibly in the next version of firmware. (Currently running V 1.5.23)

I'd like some feedback from those reading who have multiple Skybox installations.

ALSO, it would be AWESOME knowing what configurations you're all running whether you have single or multiple Skyboxes. Outback hasn't been very forthcoming with what the 'general' configuration for its Skybox users are running.

Steve
SHunsader
Forum Expert
Posts: 52
Joined: Mon Feb 15, 2021 2:54 pm
My RE system: Pair of Outback Skybox Systems each with 12 - LG 340 panels, one with 4 - 14kWh Lishen batteries, and the other with a pair of Discovery 6.5kwh batteries, both running as Masters. External CT's, one using L1 CT input and the other using L2 CT input so as to help share load more evenly (Called 'Split Mode')
Location: SF Bay, California
Contact:

Re: Calling all Multi Skybox users, External CTs and battery discharge

Post by SHunsader »

Solutions found tonight:

To turn off "Absorb": Edit the Absorb Time beneath the battery tile to a time less than the current "remaining run time". It was at 00:01:49, so I changed the absorb time to 00:01:00, and after applying, the Charger Status indicated "Charger Off". DO NOT make a mistake and INCREASE the time by accident. Doing so will change the charger state to BULK.

To turn off "BULK"....not liking it, but it works 100% of the time for me: HARD SWITCH REBOOT

Once either of these are executed, the battery should begin IF using Self Consumption Mode (and probably Non Export as well as Maximum Independence) , the battery is above the Minimum SOC percentage, and PV doesn't meet demand.

If reducing Absorb time, just remember to PUT IT BACK once the battery has been discharged to your liking so that the battery fully charges the next day.

- Steve
Brian_B
Forum Member
Posts: 23
Joined: Fri Jan 29, 2021 8:33 am
My RE system: Skybox
3.25kW Canadian Solar
40kWh Samsung SDI
Predator 8.25kW Gas Backup

Re: Calling all Multi Skybox users, External CTs and battery discharge

Post by Brian_B »

I'll start out by saying that I don't have 2 Skyboxes.

I can say that putting the CTs on a switch is not a good idea. They will be generating current if there is any load, and in the moment the switch is flipping and it's open circuited - yeah it's a very short time, but open circuited CTs are bad and voltages can spike really really high (it's basically an inductor). Usually CTs get installed with shorting connectors so that you can do work around them without ever having to open circuit them under load.

That said, seems like you should just have to plug the CTs into your lead unit, rather than splitting L1/L2 out. The master/slave configuration should take care of it from there. If you have them both configured as Master and CTs split, yes, I can imagine that you could end up with some weird power oscillations, especially since each inverter is going to be blind to half of the situation. But that's the point of Master/Slave - so they communicate and that (hopefully) won't occur, and at least according to the programming manual, you should only need CTs on main going to the master unit.

I will also state - it's not uncommon to run multiple devices in series from one set of CTs, since it's current and not voltage. It depends on burden (resistance), as it will load down the CT (ie: a 100:5 CT 100A on the line won't equal 5A CT any more, it would only get up to something lower because of excessive burden) - but you can sort of account for that by calibrating the turns ratio to get it back in line. I don't know what the burden is on Skybox CT inputs, but I know with protection relays and metering in industrial settings it isn't uncommon to drive two or three devices from the same CT. That said - consumer CTs that go into residential equipment usually aren't all that robust, so depending on what CTs you have installed, that may not work well, if at all: YMMV. You shouldn't need to do that, at least according to the Programming Manual, but if you wanted to try it - that would be easier than installing a second set of CTs just for the sake of trying it (and safer than a switch).

From what I've read from other users - it does sound like the second unit is only picking up if the load exceeds the capacity of the first unit. From an inverter standpoint, that makes sense - they tend to be more efficient higher in the load band. From a battery use standpoint, I don't know if Skybox will switch over to the Slave unit once the batteries on the Master are down low and I don't see anything about it in the Programming manual. I don't have any first hand experience with two units though, so I'm just going off what I've seen others say and that could be 100% wrong.

I agree that you would want to keep the battery banks roughly equal: just because you have a low load that fits on one inverter, if you drain one battery bank, then your load picks up to exceed the capacity of one inverter later on, your dead in the water. Also the issue of cycling and battery life, you'd have to replace the Master battery bank much more often just because it would get cycled more often.

Just looking through settings, I would assume if the preferred behavior isn't implemented, that at least once you hit the Low SOC setpoint on the main unit, it would swap over to the slave unit if needed.

For stopping charge - I know there is a switch on Optics to stop a charge in progress without having to go into settings (System Tools - the gear icon in the upper right). But I don't know if that will work to interrupt a charge from Solar, if solar is available.
Last edited by Brian_B on Tue Mar 23, 2021 7:47 am, edited 2 times in total.
Brian_B
Forum Member
Posts: 23
Joined: Fri Jan 29, 2021 8:33 am
My RE system: Skybox
3.25kW Canadian Solar
40kWh Samsung SDI
Predator 8.25kW Gas Backup

Re: Calling all Multi Skybox users, External CTs and battery discharge

Post by Brian_B »

I will also post this here:

https://www.outbackpower.com/downloads/ ... p_note.pdf

According to this note, if you have the Load terminals of both Skyboxes wired in parallel to a common Critical Load subpanel, off grid at least it should balance out the battery loading.
The master unit takes control of the output voltage and modifies the current it is sending to the loads to maintain a
120V/240VAC output. The master then communicates this value to the slave unit so it contributes to maintaining the 120V/240VAC power. If either unit is not harvesting enough PV to cover the power demanded by the loads, it will begin to discharge its battery. Once EITHER unit reaches the minimum state-of-charge (SOC) set point, the entire stack will stop providing power to the loads. The smallest of the independent battery banks determines the backup capacity for the stack.
Now, that only deals with offgrid specifically - it doesn't talk about grid connected discharging for TOU energy rates: they may act differently there.
Last edited by Brian_B on Tue Mar 23, 2021 8:04 am, edited 1 time in total.
sbrownian
Forum Czar
Posts: 562
Joined: Mon Nov 30, 2020 9:23 am
My RE system: GS8048 FM80 FNDC Mate3
4kw solar
7kw Kubota propane gen
Had 1200 Ahr FLA Locomotive batts
Just replaced with
2 x 300 Ah ReLion Lithium packs
MUCH more efficient!!
Location: Central Washington State

Re: Calling all Multi Skybox users, External CTs and battery discharge

Post by sbrownian »

"I will also state - it's not uncommon to run multiple devices in series from one set of CTs, since it's current and not voltage. It depends on burden (resistance), as it will load down the CT (ie: a 100:5 CT 100A on the line won't equal 5A CT any more, it would only get up to something lower because of excessive burden) -"

Right, but CT circuits in series don't generally have the burden issue, as they are in series and the CT is more than capable of producing enough voltage to overcome the small amount of resistance in those current sensing inputs. (Unless you go totally nuts and try to run that loop through a whole bunch of devices, which I suppose could shift the knee / saturation point.)

Just as a FYI, Kim also mentioned that Outback has two different CTs available, one with a calibrated internal burden for devices with high impedance voltage inputs, and one that is a regular CT, designed for a low impedance current input.

Edit:
I second the 'It's a bad idea to put CTs on a switch.'
Plain, unburdened CTs, can THEORETICALLY output infinite voltage. Practically, no, but it can easily be several thousands of volts, which will arc across pretty much any low voltage rated (600v) switch you may install.

I've always known the relay current inputs were low impedance, but I've never bothered to check with a meter..

Ok, wandered into the test shop and found three different SEL relay models, a Bitronics meter, and several different flavors of JEM watt meters, and ALL of the CT inputs basically read the same as shorting the DVM leads together. Maybe a tenth of an ohm, if that.

Did a bit more digging, and internally, they basically have a shunt across the terminals with a CT around it that outputs a voltage to the sensing circuitry...
Information, you get not; if incorrect question, you ask..
mhspohr
Forum Virtuoso
Posts: 86
Joined: Sun Jun 30, 2019 8:56 am
My RE system: 6.6 kW SolarWorld/Enphase and 2.6 kW Sunpower/SMA in Main house
4.4 kW REC string into second Skybox in detached Cottage
Skybox with 10 kWh SLA (used for power outage backup only)
Tesla 2015 Model S 85D charged from the sun.
Skybox name "Bristlecone"

Re: Calling all Multi Skybox users, External CTs and battery discharge

Post by mhspohr »

Brian_B wrote: Tue Mar 23, 2021 7:44 am I will also post this here:

https://www.outbackpower.com/downloads/ ... p_note.pdf

According to this note, if you have the Load terminals of both Skyboxes wired in parallel to a common Critical Load subpanel, off grid at least it should balance out the battery loading.
The master unit takes control of the output voltage and modifies the current it is sending to the loads to maintain a
120V/240VAC output. The master then communicates this value to the slave unit so it contributes to maintaining the 120V/240VAC power. If either unit is not harvesting enough PV to cover the power demanded by the loads, it will begin to discharge its battery. Once EITHER unit reaches the minimum state-of-charge (SOC) set point, the entire stack will stop providing power to the loads. The smallest of the independent battery banks determines the backup capacity for the stack.
Now, that only deals with offgrid specifically - it doesn't talk about grid connected discharging for TOU energy rates: they may act differently there.
Just a few suggestions.
For off-grid, it would be better if the system was smart enough to draw down both batteries equally so you wouldn't get to the point where the system shuts down even when one battery has extra capacity.
My observation of on-grid operation is that the "follower" unit never contributes anything to the load. It is constantly is "waiting" mode. Furthermore, it runs on the battery and runs down the battery (and won't recharge the battery) even when the grid is connected. This needs to be fixed.
SHunsader
Forum Expert
Posts: 52
Joined: Mon Feb 15, 2021 2:54 pm
My RE system: Pair of Outback Skybox Systems each with 12 - LG 340 panels, one with 4 - 14kWh Lishen batteries, and the other with a pair of Discovery 6.5kwh batteries, both running as Masters. External CT's, one using L1 CT input and the other using L2 CT input so as to help share load more evenly (Called 'Split Mode')
Location: SF Bay, California
Contact:

Re: Calling all Multi Skybox users, External CTs and battery discharge

Post by SHunsader »

Everyone,

Thanks for all the thoughtful responses. Over the past couple of weeks, I've learned that using CT for L1 on Sky 1 and CT for L2 on Sky 2 has worked quite well. The only time both batteries discharge at the same time is when PV doesn't quite cover the load in the late afternoon. Once a significant amount of battery needs to be used to power the load, it makes enough power on the opposite leg (either L1 or L2) to 'swing' the whole load to the one doing the powering. If something large on the opposite leg turns on which is temporarily greater than what the other Skybox's load, the discharge switches to the other Skybox.

Oh, I was talking about using a relay, not a flip switch. Although it's not the same, the contact switch time should be much smaller than a manual switch.

- Steve
Brian_B
Forum Member
Posts: 23
Joined: Fri Jan 29, 2021 8:33 am
My RE system: Skybox
3.25kW Canadian Solar
40kWh Samsung SDI
Predator 8.25kW Gas Backup

Re: Calling all Multi Skybox users, External CTs and battery discharge

Post by Brian_B »

SHunsader wrote: Sat Apr 03, 2021 6:16 pm Everyone,

Thanks for all the thoughtful responses. Over the past couple of weeks, I've learned that using CT for L1 on Sky 1 and CT for L2 on Sky 2 has worked quite well. The only time both batteries discharge at the same time is when PV doesn't quite cover the load in the late afternoon. Once a significant amount of battery needs to be used to power the load, it makes enough power on the opposite leg (either L1 or L2) to 'swing' the whole load to the one doing the powering. If something large on the opposite leg turns on which is temporarily greater than what the other Skybox's load, the discharge switches to the other Skybox.

- Steve
Good to know - thanks for sharing. It sounds like a decent work around until Outback can fix their software to get slave units to share load. The way the programming manual reads you shouldn't have to do this - but there are a lot of things that aren't quite working as written in the programming manual...
mhspohr
Forum Virtuoso
Posts: 86
Joined: Sun Jun 30, 2019 8:56 am
My RE system: 6.6 kW SolarWorld/Enphase and 2.6 kW Sunpower/SMA in Main house
4.4 kW REC string into second Skybox in detached Cottage
Skybox with 10 kWh SLA (used for power outage backup only)
Tesla 2015 Model S 85D charged from the sun.
Skybox name "Bristlecone"

Re: Calling all Multi Skybox users, External CTs and battery discharge

Post by mhspohr »

Did a little testing of my stacked Skyboxes this am. (Leader/Follower configuration)
Backup power does not kick in unless both units are switched on and batteries connected. Second unit still doesn't do anything except waste about 200 W power all the time but it must be on for the backup to kick in. Second unit runs from battery all the time even when the grid is connected.
Entire load when on backup power (less than 5 kW) is met by first unit.
Each of these units has the same battery size and type so it would be nice if they shared the load rather than just draining the first battery and then shutting down.
I'm thinking I'll set these up as two independent units (as described by @shunsader) and test some more to see if they will share the load in an independent configuration.
SHunsader
Forum Expert
Posts: 52
Joined: Mon Feb 15, 2021 2:54 pm
My RE system: Pair of Outback Skybox Systems each with 12 - LG 340 panels, one with 4 - 14kWh Lishen batteries, and the other with a pair of Discovery 6.5kwh batteries, both running as Masters. External CT's, one using L1 CT input and the other using L2 CT input so as to help share load more evenly (Called 'Split Mode')
Location: SF Bay, California
Contact:

Re: Calling all Multi Skybox users, External CTs and battery discharge

Post by SHunsader »

Brian_B,

Yesterday I finally implemented your suggestion of using a CT series connection between both Skyboxes. Although the systems haven't been running even 24 hours in this new configuration, the system is behaving quite well. Gridzero for my system has never been tighter. I"ll run it for a few days this way, and report back to this thread.

I recognize that you aren't running multiple Skyboxes, so you haven't experienced the pain in things not working as expected. There are a number of shortcomings running the Leader/Follower configuration. The 2 that stand out are:

1. If ANY system needs to be taken offline, the entire system is brought offline when the watchdog timer fails to respond from the system that's being brought offline. Thus, so much for "Protected Loads"....in this case, anything connected to the Load breakers should be called "Vulnerable Loads", because they'll be blacked out anytime a system needs a hard reboot.

2. Skyboxes are configured differently than its predecessor. It's my understanding that Radians and their older brothers also have a stacking feature which allow the Leader to tackle most of the loads first, and queue up additional Radians as the load becomes too great for a single box to handle. This has to do with minimizing the amount of wasted inverter power while handling the loads. But from a battery perspective, Radians have a single battery pool, and Skyboxes require individual battery banks for each unit. This means that using the Radian scale up approach using Skyboxes, the Leader battery will take the full brunt of the load each and every time the battery is used, and Follower units might seldom get used. Unfortunately, this style of use affects battery life. The Leader battery will be subjected to twice or even 3 times the discharge rate (2 or 3 Skyboxes, respectively) in this mode. Alternatively, if the load was spread evenly amongst systems, battery life could be more predictable and life expectancy significantly increased. After reviewing the whitepaper you suggested reading, (reviewed again yesterday before making the switch), apparently the first battery to be exhausted in the system determines how much uptime is available for the system (weakest link syndrome). Again, using one battery instead of spreading between systems essentially cripples the longevity of the system.

There are other issues, but these 2 are the most critical.

If I'm correct, your suggestion of using series CT connections so that all Skyboxes see the same external loads should allow BOTH of my Leader configured boxes to act on those loads independently, and share in battery discharges more evenly. I'll report back when I have more complete results. - Steve
Brian_B
Forum Member
Posts: 23
Joined: Fri Jan 29, 2021 8:33 am
My RE system: Skybox
3.25kW Canadian Solar
40kWh Samsung SDI
Predator 8.25kW Gas Backup

Re: Calling all Multi Skybox users, External CTs and battery discharge

Post by Brian_B »

SHunsader wrote: Mon Apr 19, 2021 7:41 am Brian_B,

Yesterday I finally implemented your suggestion of using a CT series connection between both Skyboxes. Although the systems haven't been running even 24 hours in this new configuration, the system is behaving quite well. Gridzero for my system has never been tighter. I"ll run it for a few days this way, and report back to this thread.

I recognize that you aren't running multiple Skyboxes, so you haven't experienced the pain in things not working as expected.
Glad to hear - hope it helps out some.

As far as multiple Skyboxes, you are entirely correct. But I wouldn't say I haven't experienced growing pains with Skybox - it's a quirky beast and still needs a lot of refinement on all fronts. My system has plenty of gremlins of it's own.
Xeroiv
Forum Member
Posts: 8
Joined: Fri Mar 01, 2019 4:30 pm

Re: Calling all Multi Skybox users, External CTs and battery discharge

Post by Xeroiv »

Hi Steve. I am interested in how your all leader configuration is going. I have 4 skyboxes that are currently in a leader/follower configuration and am looking at ways to improve my setup. I don't like the fact that if I take any unit offline to either mess with the battery or other changes it drops my whole stack. I have other issues where power blips on the grid are resulting in brownout as my skyboxes are not picking the loads. I recently updated all four of the skyboxes to 1.5.23 and haven't done any significant testing since yet though on grid transfer.
SHunsader
Forum Expert
Posts: 52
Joined: Mon Feb 15, 2021 2:54 pm
My RE system: Pair of Outback Skybox Systems each with 12 - LG 340 panels, one with 4 - 14kWh Lishen batteries, and the other with a pair of Discovery 6.5kwh batteries, both running as Masters. External CT's, one using L1 CT input and the other using L2 CT input so as to help share load more evenly (Called 'Split Mode')
Location: SF Bay, California
Contact:

Re: Calling all Multi Skybox users, External CTs and battery discharge

Post by SHunsader »

Xeroiv,

It should be pointed out that Outback does not support the way I'm using the Skyboxes, but I have mentioned repeatedly that until they 'solve' the blackout situation when one needs to reboot a single system, I'll need to continue operating using my own, partially redundant system.

Things have gone well, but I'm still not done testing on all fronts. I'm still in need of an electrician to connect my backplane of the protected load circuits. I have learned a considerable amount of information, that probably even Outback doesn't recognize. Before giving advice, I need to completely understand how your system is configured, as well as what your goals for your installation are. There are benefits and shortcoming for each configuration I've tried, and until some additional firmware changes are made, I'm living with what I have until the electricians finish the backplane installation.

My installation: 2 Skyboxes (Sky 1 & Sky 2), both in Leader mode, each having a string of 12 - 340w LG panels. Sky 1 now has 4 banks of 14kWh of Lishen Grade A (56kW) batteries, while Sky 2 has 2 6.65kWh Discover (13.3kWh) batteries. My home has a 400A main, which distributes power to 3 Generac subpanels (200/200/100) to 2 house locations and a workshop. The subpanels have a built in transfer switch. The load breakers will/are to be distributed along a backplane to all subpanels. The system has a 25kW Whisperwatt diesel generator for low battery condition. External Current Sensors (CTs) read the current at the meter to know how much power to offset to GridZero the property. Each Skybox receives 1 of the 2 External CT sensor data, effectively splitting the task of load management (wishful thinking). I call this "Split Mode". The system is running Self Consumption mode, allowing system to export excess energy to grid, use PV to charge batteries, and only use grid for power when absolutely necessary.

My goal: Initially, for system batteries to cover loads during peak pricing period (3p-9p), and exercise batteries during that time for better system confidence during a real outage. The NEW GOAL is to become self sufficient if the new CA Net Metering 3 agreement begins to charge $10/month for each 1kw of panels on the roof. ($81.60/month, ~$1,000/year), and thus, using batteries to cover ALL loads during a 24 hour day without hitting the grid except for covering large inductive loads.

Here's where I am: I am back to running "Split Mode", where each system gets one of the 2 external CT readings (think L1 to Sky 1 and L2 to Sky 2). The system which receives the largest load will most often 'handle' the load for both systems. This has to do with HOW the Skybox creates power to offset the load. At least for my system, I've recognized that if Skybox sees a 1kw load on one of the CT sensors, it creates 1kw on the leg to offset the load, and another 1kw on the opposite leg (maybe because it can't see anything on the other leg, or just because that's the way Skybox works). This extra power generated on the opposite leg offsets whatever power the other Skybox was needing to provide, and most of the time, some residual power is sent to the grid, thereby creating a Net Export condition. Note: My utility charges upwards of 1c per .01kWh every 15 minutes of use, so being a Net Exporter is important in my configuration. What's the downside to this configuration? That I'm really only getting about 1/2 the peak power capacity on 120v loads. 240v loads seems to work really well.

Why split mode? After using a member's suggestion of running CT data to both systems in series (it works) for a few weeks, I've abandoned that concept due to both systems trying to tackle the same load at the same time, overdriving the handling of the load, and causing fatal errors, requiring system reboots. Most were caused by inductive loads starting, but sometimes something as simple as a computer UPS 'refreshing', as they do a couple times per month.

Note: Self Consumption mode is currently the ONLY choice, if I don't want the system to charge FULL FORCE once the sun comes up. Netmetering "with Backup" should really be deemed "Netmetering w/o Generator", meaning that the system NEEDS to keep batteries kept at full capacity when batteries begin to charge, to ensure that batteries are full if there's an outage. No provision if one has a backup generator.

Although I haven't tried this yet, I have considered running CT sensors in series, but using current sensors that only read 1/2 the real load, thereby having each system contribute 1/2 the amount of power when a load presents itself. I'm relatively confident that this will work, but don't have time at the moment to implement. Instead, I've fallen back on something that's worked well for the past couple of months.

What would I do with 4 systems? I'd STRONGLY consider a hybrid system having a PAIR of Leader/Followers, using Split Mode CT sensor input. It is VERY IMPORTANT that you IDENTIFY YOUR GOAL before you embark. Without this, there will be frustration.

- Steve
Xeroiv
Forum Member
Posts: 8
Joined: Fri Mar 01, 2019 4:30 pm

Re: Calling all Multi Skybox users, External CTs and battery discharge

Post by Xeroiv »

Thanks for the detailed reply Steve. Here is my current setup. 4 skyboxes (1 leader / 3 follower) each with a 189AH NMC 48v battery (still need to properly config the settings since updated firmware). My house has two 200A feeders from the utility. One goes directly to my 200a non critical loads panel and the other goes to the line input on all 4 skyboxes. The loads all feed my 200A critical loads panel. Each skybox has two string of 8 315w LG solar panels. Since I don't have TOU issues here in Ohio my goal is at least net metering with backup working correctly. Id have to check my NEM agreement but I think that it would be advantageous for me to do self consumption.
SHunsader
Forum Expert
Posts: 52
Joined: Mon Feb 15, 2021 2:54 pm
My RE system: Pair of Outback Skybox Systems each with 12 - LG 340 panels, one with 4 - 14kWh Lishen batteries, and the other with a pair of Discovery 6.5kwh batteries, both running as Masters. External CT's, one using L1 CT input and the other using L2 CT input so as to help share load more evenly (Called 'Split Mode')
Location: SF Bay, California
Contact:

Re: Calling all Multi Skybox users, External CTs and battery discharge

Post by SHunsader »

Xeroiv,

Four Skyboxes must mean that you either need to replace a lot of power, ensuring that you have enough power over the winter, selling the power and SRECs, or something I haven't thought about. I completely failed to ask how much power you need on a daily basis, and what the peak loads look like.

Something to compare to....I have an elaborate home automation system, a deep freeze, a couple computers, lots of devices that need charging, and a few other things that need power 24 hours per day. I use 32-36kWh/day when not doing any construction projects outside, which can significantly change home demand....upwards of 60kWh/day. My home's base load is between 900w @ night, and around 1,200w when everyone is awake. Thus, of that 34kWh/day, about 24 of that is just background loads. Peak demand for us is either the electric oven, range, dishwasher, microwave, quick coffee pot, hair dryer, and outside equipment like a skill saw, 240v saw heavy duty stone saw, demolition hammer, rotary hammer, planer, lathe, etc. My family is getting better at understanding how to manage loads better. It's really dinner and weekends that loads test the system. The only time I've seen 5kW peak loads is when my family turns on the oven, range, microwave and washer/dryer on all at once. Other than that, I don't really ever use more than 5kW. Maybe sometimes outside when it's sunny and equipment is being operated, but PV usually helps at that time. - Steve
Xeroiv
Forum Member
Posts: 8
Joined: Fri Mar 01, 2019 4:30 pm

Re: Calling all Multi Skybox users, External CTs and battery discharge

Post by Xeroiv »

I would say my best guess is somewhere between 60-90 kWh per day for my house. Its new construction and I don't have great metrics yet which also means I don't have a great idea of what the peak demand is. My best guess is it would be whatever the startup draw for my geothermal HVAC would be plus my normal background draw worked out to be. Given that AC units are fairly intensive on the startup draw is the reason I have the 4 units stacked rather than just being AC coupled together.
Post Reply