Does a Grid tie, battery only inverter charger exist for time of use arbitrage

[wings2004]

So in the not all in one solution but higher voltage there is something like this - https://www.amazon.com/dp/B01MZ53V7H/ref=cm_sw_r_cp_apa_i_cZxvFbM09VQSD
I believe that could be a good basis to a solution combined with a battery charger and an automatic transfer switch. It would certainly be more components but I'm not sure if it would be more efficient. In general I like higher voltages for less line loss and it's overall efficiency but the simplicity of a all in one solution is nice. I wonder if the Eco mode on this system would work with a compressor where the system needs to see power before it cycles on and then would drop to a very low power usage when the compressor is off. I think that would depend on how long you can set the sense duration too and what the low power cut off can be set to.

 

[GXMnow]

How about this one

https://www.amazon.com/Tripp-Lite-APS2424-Inverter-Hardwire/dp/B00006HNTD/ref=sr_1_51?dchild=1&keywords=24+volt+inverter%2Fcharger+with+transfer+switch&qid=1599439743&s=automotive&sr=1-51

 

[wings2004]

I think if I were to go for an IC that's higher powered I'd probably go with the MPP LV2424 since it's set up for those and more functions but any of these solutions would reduce the efficiency and I notice the tripp lite doesn't have specs on incipient drain.

 

[GXMnow]

My big Schneider XW-Pro does suck about 50 watts when idle. That sounds pretty bad, but when it is running at 3,000 watts, it is a small percentage.

 

[wings2004]

Yeah for a 300 watt load I don't feel great using an extra 20% which is why I was looking for efficient solutions. I'm leaning more towards a 12v system for my first one to work with a simple AIMs IC vs all sorts of separate pieces.

 

[wings2004]

It looks like MPP Solar also has the LV1012MS which would work too but it's on backorder many places and I'm worried about buying on eBay. It's about $100 cheaper than the AIMs but it looks like you get more configuration over the battery charge circuitry which would be useful for not needing a BMS for my LiFePo batteries...

 

[GXMnow]

That was one of the big reasons I went with the Schneider. Everything about the charge profile is fully adjustable. With my repurposed batteries, I wanted to be sure I could properly charge them. I have not been able to find anything small with that kind of programming. The SW is good, but it is still 4000 watts and $1500

 

[Dev-Random]

Hi All!
I've noticed a great amount of inverter related knowledge in this thread and was wondering I you could help me out.
I'm also looking at arbitrage opportunities and am currently looking for inverters / chargers that would fit my use case.

Please keep in mind that my use case is limited to arbitrage only, meaning:
Charge when electricity is cheap or even negatively priced and Discharge back into the grid when prices are at their highest.

It's however the last part, the actual feeding back into the grid, that is giving me a hard time. I cannot seem to find any cheap/simple/dumb battery inverters that are capable of syncing a (fixed) DC voltage back into a 230 AC grid (Western Europe) at around 2500watt.
All I end up with are (hybrid) solar inverters that would theoretically be capable of doing this but they seem to be overqualified or only able to power a separate circuit (not feeding back into the grid)?

I'm OK with handeling switching using external controllers so the most simple on-grid inverter would be sufficient.

Also, let's assume I've got a pretty cheap battery and cycles don't hurt that much so as long as the initial investment of inverters and chargers can be kept low enough it might even be profitable someday!

So, does such a grid tie battery inverter exists ?

 

[GXMnow]

There are not many battery based inverters that are actually meant to feed back into the grid. While some people have had reasonably good results using an MPPT solar inverter fed from a battery, I don't think it is a good idea. You really don't have any control over the power it will try to export. Their whole design is built around trying to get the max from a solar panel. Some will max out their rating when power from a battery, some might actually burn up from it.

I did a lot of searching and I just kept coming back to the Schneider XW-Pro. Yeah, it is a bit more expensive, but it is a beast. It can run up to 6,800 watts all day long. It can back feed the grid while being fully grid code and UL compliant. It meets every safety spec, and it is quite efficient for a unit rated for that much power.

When I first got mine, I just installed it on it's own breaker in my main panel. I would tell it to charge, and then at 4 pm, it would switch to export and push a constant current into my main panel. Either until the battery ran below the set limit, or the time hit where I block export. But the one problem is it won't start a charge cycle by itself. You need to have some other device or a human tell it to charge. It is a dumb software choice by Schneider. I programmed a small PLC controller to manage the charging. I went a big step further. It monitors the energy in my home and commands charging when my solar panels are making more power than my house is using, and adjusts the charge current to keep my grid export to under 100 watts. After 4 pm it switches it to export mode and the PLC constantly adjusts the export current to keep my grid export right at zero.

There are a few other inverters that can do things like this, but they are not any cheaper. Many of the lower cost units can only output power to loads on their output side, not pushing current back to the grid input side. If you can move most of your loads into the sub panel on the inverter output, then this could work for you to nearly zero your grid power. But I have a few large loads back in my main panel, and it really is nice to just have the inverter push the power back to run those loads.

 

[Dev-Random]

Thanks for your insights, I appreciate it.

With your remarks in mind, solar inverts would be able to do the job.
I've came across several cheap grid tie micro inverters like this one.
Wouldn't a matching battery design allow you to stay away from peak output power using lower than max input voltages (less cells in series)?
I'm thinking that could prolong the lifetime of such a micro invert of which multiple can be used in parallel.

The Schneider XW-Pro or similar inverters are probably a bit much for a small pilot/experiment

 

[Zuikkis]

Hmm but there are a lot "hybrid grid tie inverters" which should do the trick. I was just asking about this in another thread.

I was looking at this:

5500W Hybrid Solar Inverter 90A MPPT Grid Tied + Off Grid With WIFI and CT Sensor | eBay

On&Off Grid with energy storage. GRID-TIE OPERATION. External Wi-Fi device optional. GRID OUTPUT (AC). OFF-GRID, HYBRID OPERATION. Wir werden Ihnen so schnell wie möglich antworten. Wir werden Ihr Problem priorisieren.

www.ebay.com

It has solar, but also battery charger / discharge.. And it can discharge battery to the grid.

SolaX also has similar devices, including "retrofit" models that don't have solar. Just battery.

However by default most of these come with a current sensor that's installed in your grid connection. Then it measures how much import/export there is, and only supplies battery power enough for your own use.

I'm thinking maybe the current sensor connection could be abused to force more output?

 

[Dev-Random]

The Solar retro fit looks nice, thank you.
I guess much depends on the software / external interaction capabilities as a on-demand full discharge of battery storage isn't a primary use case for these devices.

 

[Zuikkis]

The Solar retro fit looks nice, thank you.
I guess much depends on the software / external interaction capabilities as a on-demand full discharge of battery storage isn't a primary use case for these devices.

SolaX would be perfect but in latest generation models they have dropped support for user configurable battery voltages. So basically it only works with their own battery packs, that connect with CAN bus to the inverter.

It's just that I have a spare DIY 23kWh lithium pack...

 

[Zuikkis]

SolaX X1-AC seems to be still available and does support custom batteries without BMS connection (70-400V).
Also it supports standard RS485 modbus connection for the current meter, and I found the protocol description online:

https://fastron.com.au/products/sdm630mct-modbus-cl1-v2-growatt-solax

It should be relatively easy to use Arduino or Raspberry Pi with RS485 shield and "simulate" the current meter in software..

I actually have similar electricity meter already connected, SMA Energy Meter. But it uses ethernet connection, not modbus. Would be really stupid to buy another meter..

 

[GXMnow]

I do not know the SolaX, so I can't speak for them. Many of the USA market hybrid "All in One" units can only sell back from solar, not from battery. The battery can only supply to the output side. I think it is mostly because of the regulatory issues here, not any technical reason. To push power from a battery to the grid legally, the inverter has to meet grid code specs. Schneider had to design a complete new logic board from the XW+ to the XW-Pro to be able to meet the grid codes here. I have to run in California Rule 21 mode.

The problem with using an MPPT solar inverter from a battery is that it will seek more and more current and keep seeing more power. A quality one should be smart enough to just "max out" and run at full power, but some will go too high and burn out trying to find the max power point. So unless the inverter has some way of limiting it's power, it will most likely just run at max when fed from a battery. Solar panels are more of a current source, batteries are more of a voltage source.

 

[Surferdudemi]

Check your rate plan. I cannot find it at the moment, but I recall seeing a note on the DTE website about not feeding back from battery-generated power on a net metering rate because it's not from a renewable source. "How would they know?" you might ask. Well, if you've got a couple of cloudy days, and you're generating the full rated or more) output of your solar array, that's a pretty good indicator to the utility that something isn't right. What's totally legit though is using something like a Skybox to zero out your meter during peak time and charging from the grid at the lowest rate. I haven't hooked up my Skybox yet, but it seems to be able to do that pretty easily. You need to install the current transformers on the main.

 

[Zuikkis]

The problem with using an MPPT solar inverter from a battery is that it will seek more and more current and keep seeing more power. A quality one should be smart enough to just "max out" and run at full power, but some will go too high and burn out trying to find the max power point. So unless the inverter has some way of limiting it's power, it will most likely just run at max when fed from a battery. Solar panels are more of a current source, batteries are more of a voltage source.

But these above mentioned hybrid inverters have separate battery connectors, battery is not connected to PV inputs. SolaX retrofit models don't even have PV inputs.

You are right that by default these hybrid inverters have the current sensor in your grid connection, and only output "just enough" from the battery that net current stays at zero. But the inverters have the circuitry to do battery->grid, it's just that the power is limited to your actual consumption.

Anyway. I have my 10kW solar plant which is approved so I can legally sell my surplus electricity. Technically this battery setup is a different device, but I don't think anyone would notice if I occasionally give out a little bit extra.

Actually I would be quite happy with the default setup that matches output with consumption. But for cost reasons I'm going for single phase inverter, while my house has three phase. So the default setup would only cover 1/3 of my consumption... Also I already have the ethernet connected 3-phase SMA Energy Meter, so it would seem silly to add another single phase meter..

So what I'm planning now, a Raspberry connected to ethernet and with RS485 to SolaX, converting the energy data from SMA to SolaX format, also combining the three phases into one. This also allows some flexibility in placing the battery, doesn't have to be close to the main fuse box..