DIY AC coupled storage?

[Hamalot]

Hi All, I’ve had a 4kw solar system for 8 years now and benefiting from the feed in tariff in the UK. I recently took my first steps to making more of my solar power an built a DIY power diverter, using a Shelly EM to monitor when i’m exporting power and a shelly uni to control the settings on a PWM controller feeding my immersion heater. Really pleased with the result, but i’m now looking at the next step towards making the most of my solar power and that’s battery storage. Like i said i get FIT payments so any system would have to be AC coupled.

Option 1 – Buy a retrofit system:
While money isn’t the only factor here, it has to make some sort of sense. Purchasing an ac coupled system will probably never pay back.

Option 2 – Buy a retrofit AC coupled inverter/controller and build the battery and BMS myself
I’ve read and watched so much about DIY powerwalls and can see that DIY batteries can save huge amounts and seem straightforward enough, my problem is the control that goes around it.
The battery packs are the real cost right, so, why not buy something like the SOLAX X1 RETROFIT AC COUPLED BATTERY INVERTER and build the battery and BMS myself? I think all of the off the shelf controllers/inverters like this will only work with their own batteries. Is this right? Does this option exist with any manufacturer?

Option 3 – Full DIY
For all the videos i’ve watched about DIY powerwalls, 99% of the focus seems to be on the batteries, there doesn’t seem to be much about of AC coupled control equipment (maybe i’ve just been looking in the wrong places). I recently found “Mikes DIY Tesla Powerwall” and the Grid Tie Inverter with Power Limiter he’s using seems like it would fit the bill.
For charging, i have seen how you can use a solar charge controller (not an option for me). I’ve seen how to charge using the mains (i will do this in the winter months). What i haven’t seen is how i can charge from the mains but only using what would be exported. Has anyone done this? If so how?

Has anyone built a DIY AC coupled system? Or are there information sources out there already on this? Do options 2 and 3 both fall foul of building regs?

Any thoughts/experiences welcome. Thanks in advance

[Ampster]

I have what I consider to be a DIY AC coupled system. I purchased an Outback Skybox and wired it to solar panels, LFP cells and a BMS.
I do not know of a way to AC couple a grid tie system to another inverter unless that other inverter has the firmware to AC couple.
In the USA building regs require UL approved esuipment. AC coupling is perectly legal as long as you get a building permit.

[Hamalot]

Thanks for the reply. I have panels then inverter then generation meter. That system must stay if I want to get paid for the generated power, my system must be standalone and after the generation meter.

[Ampster]

... I have panels then inverter then generation meter. That system must stay if I want to get paid for the generated power, my system must be standalone and after the generation meter.

Maybe we are talking about two different concepts. If your new system with batteries has to be standalone then it would not be AC coupled in the traditional sense that the term AC coupling is used in the USA.
It sounds like you have a grid tied system that exports. I reread your post and I am not clear what you mean by "making the most of" your solar power. Are you saying the FIT is not enough and you would rather store it for use at a later time? That seems to be what you are accomplishing by using an immersion heater. Storing energy in batteries is a similar concept as storing heat in a well insulated vessel. I call that concept load shifting and it can easily be done with batteries and and inverter, It does not require AC coupling. The economics depend on the rate you are paid for energy which you send to the grid and the rate that you would pay for energy after the sun goes down.
Is your goal to store some of your solar energy in batteries and use it later so that your consumption of energy from the grid is less?

[Hamalot]

Sorry, I should have been clearer. FIT payments are based on generation, not export. Once the power has gone through the generation meter I'm free to use as much or as little as I like. I can export to the grid or use what I want. This should expian why I can't start diverting to batteries until its been converted to ac.

So I have the IoT infrastructure in place, I just need a charger that can adjust its output based on the available power (that would otherwise be exported), or a charger that won't mind having a pwm voltage fed into it.

[Ampster]

....... FIT payments are based on generation, not export. Once the power has gone through the generation meter in few to use as much or as little as I like. I can export to the grid or use what I want. But this should expian why I can't start diverting to batteries until its been converted to ac.

Now, that makes sense. it is not AC coupling in the way I have become accustomed to thinking about it.

So I have the IoT infrastructure in place, I just need a charger that can adjust its output based on the available power (that would otherwise be exported), or a charger that won't mind having a pwm voltage fed into it.

I have faced a similar issue trying to change the charging rate of my EVs. I have no problems turning the power on and off based on how much solar I am producing. Getting the onboard charger or the Charging station to modulate is the difficult part. Some hybrid inverters can be controlled by CANBUS signals and their internal charger could vary its output, but that adds another layer of networking. What you want is a charger that runs off the mains but only draws power based on the energy available from solar.

[solardreamer]

Sorry, I should have been clearer. FIT payments are based on generation, not export. Once the power has gone through the generation meter I'm free to use as much or as little as I like. I can export to the grid or use what I want. This should expian why I can't start diverting to batteries until its been converted to ac.

So I have the IoT infrastructure in place, I just need a charger that can adjust its output based on the available power (that would otherwise be exported), or a charger that won't mind having a pwm voltage fed into it.

I am not aware of an off the shelf battery charger that supports solar diversion. Do you have an EV? If you have one then it's probably the easiest way to divert surplus solar energy to charge a battery. OpenEVSE can do it but since you are in the UK you may also want to consider MyEnergi Zappi especially if you are already using their diversion controller for immersion heaters. If you want to setup your own battery systems then you can look at battery chargers with programmable current limit (e.g. Victron Multiplus) but you will likely need to roll your own solar diversion controller to integrate with it.

[ajvok]

Hi Hamalot.
I realise that this thread is 2 years old now but I came across it as my situation is the same as your was.
What did you end up doing? I would be very helpful to know. Thanks.