Wow, that is quite an impressive setup! You would not have a drawing lying around (even if handwritten) that you could share?

I had a quick look at the Solas Manual and found the below diagram that is perhaps applicable..

A few questions: 

- do you have export capability and is that a consideration?

- is you Home assistant based management working well?

- you would generally not want to have long DC cables for the batteries but is extending with Solax batteries not an option? 

Posted by: @weoleyric

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We cannot upgrade the Solax battery system, as it is both loft-installed and our existing installer isn't interested in lugging any more heavy batteries into the loft or for building safety regulations.

Well, quite!

Have you come across PAS 63100, available free from British Standards?
This document provides Guidance on domestic battery installations, and is not mandatory at present.

It's written by the IET, who also control the Wiring Regulations. They're not the optimal body to provide advice on domestic storage batteries which operate below 75v, but it's welcome none-the-less.

PAS63100 does advise that all domestic storage batteries should be outside the property, which might be of interest to you.

It also strongly advises against battery storage being housed in an attic/loft !
That's partially right. But the risk of fire is greater for the inverter than it is for the battery itself!

Your inclination to use Fogstar for additional external storage is sound. They are a British company with a good reputation.

Your inclination to use Victron products for the storage inverter stage can't be faulted on technical grounds...
... but they are known to be expensive.

You don't have to buy-in additional battery storage which is already enclosed in a waterproof cabinet.
You pay a lot of money for the cabinet and the labour to build the assembly in a factory.

I buy in the raw LiFePO₄ cells and make the connections myself.

Although mine are sited within an internal plant room, I could just as easily use standard steel racking components for an external enclosure, and clad the outside with cementitious board. Look online for a local merchant who stocks STS Boards.

Such a DIY approach would probably halve your costs, consequently reducing the payback period.

Posted by: @weoleyric

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We have an 80A grid connection, and use the Octopus Intelligent Go tariff

We are on the same tariff, on 3.5p/kwh. So until early 2027, when it expires, I see no scenario where I would want to switch.  

Posted by: @weoleyric

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So, we're exploring options to expand our battery setup to completely time-shift energy use, even during the coldest, darkest winter days.

That makes good sense. Like @transparent suggested, I would recommend moving onto DIY. I cannot imagine finding an installer that would support even my significantly simpler system. 

I will not assemble cells onto batteries again (long story..). I have however been casually researching options to add ready made waterproof batteries externally, to supply an ASHP using the Intelligent Go. 

Additionally to being waterproof, you would probably want them to include a heating system. This is if you want to charge them while outdoor temperatures are below 0C.       

Posted by: @weoleyric

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* We cannot upgrade the Solax battery system, as it is both loft-installed and our existing installer isn't interested in lugging any more heavy batteries into the loft or for building safety regulations.

As you probably know, Solax is a leading manufacturer of inverters. I cannot recall if they make or rebadge LiFePO4 batteries. But in any case, if that includes IP65 or similarly rated batteries, you may be able to connect them via an external 48V DC cable (which essentially does not require an electrician). This is assuming the Battery Management System of one of the batteries would be able to manage the others, essentially what you already have, like the below. Hence my question about whether you may have suitable wall space near the Solax inverter. 

Thanks for the ideas, @batpred.

I don't have a diagram, but I could explore creating one?

Posted by: @batpred

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A few questions: 

- do you have export capability and is that a consideration?

- is you Home assistant based management working well?

- you would generally not want to have long DC cables for the batteries but is extending with Solax batteries not an option? 

• I have export capability with G98, and am open to G99 if needs be.
• The Home Assistant setup is working reasonably well. It's not too intelligent, but thanks to Wonder Watt, our EV's don't drain the batteries when an Octopus Intelligent Go smart session kicks in. I've only got minimal automation setup with the Ecoflow plug-in batteries, but I'm hoping I can automate that in the future, too.
• While the Solax capacities could technically be expanded outside the loft, if I understand the technical limitations in the Solax system, I could only add one additional slave battery (1 Master + 2 Slaves (maximum 3 batteries total).

However, your suggestion did prompt me to do more searching, and a SolaX BMS Parallel Box would apparantly expand that limit to 6 batteries (~34.8 kWh).

But... SolaX officially recommends adding extra batteries within one year of the initial installation. Because lithium-ion cells degrade naturally over time, adding a brand-new battery to an older stack can cause capacity mismatches, where the older cells limit the performance of the new ones.

It's been three years since the original Solax system was installed, so I think we're back to square one and exploring a 3rd system in order to expand our capacity by another 30kWh.

@transparent Thanks! This is great feedback.

I think that going to the DIY route is probably a step too far for my skillset right now, but I'd be open to working with my trusted local electrician to install a Fogstar system, if I couldn't find a reliable Fogstar installer.

Posted by: @weoleyric

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• I have export capability with G98, and am open to G99 if needs be.

That could be worth, as a new inverter may give you more ability to export more when it makes more financial sense. 

Personally I avoided full DIY but managed to go further by doing some of it myself. 

Like I am with the UKPN DNO and when I approached them to register my inverter, they "offered" 5kw export and said "just fill in the details in the initial G99 form as if G98". 

Posted by: @weoleyric

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• The Home Assistant setup is working reasonably well. It's not too intelligent, but thanks to Wonder Watt, our EV's don't drain the batteries when an Octopus Intelligent Go smart session kicks in. I've only got minimal automation setup with the Ecoflow plug-in batteries, but I'm hoping I can automate that in the future, too.

Personally I try to use the inverter builtin local capabilities whenever possible and home assistant for monitoring. 

Posted by: @weoleyric

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• While the Solax capacities could technically be expanded outside the loft, if I understand the technical limitations in the Solax system, I could only add one additional slave battery (1 Master + 2 Slaves (maximum 3 batteries total).

However, your suggestion did prompt me to do more searching, and a SolaX BMS Parallel Box would apparantly expand that limit to 6 batteries (~34.8 kWh).

But... SolaX officially recommends adding extra batteries within one year of the initial installation. Because lithium-ion cells degrade naturally over time, adding a brand-new battery to an older stack can cause capacity mismatches, where the older cells limit the performance of the new ones.

It's been three years since the original Solax system was installed, so I think we're back to square one and exploring a 3rd system in order to expand our capacity by another 30kWh.

Certainly if you would replace some cells within a battery the BMS would need to be able to manage that. Arguably a good BMS needs to be able to do that as no two cells even if the same age perform exactly the same. My battery has a Seplos BMS but uses a jk balancer to balance the cells. 

From reading other contributions in this forum, I understand many (most?) BMS systems allow batteries to be charged independently including batteries of different nominal capacities.
I am surprised Solax if could not do this. But I could also imagine some BMS system that would automatically account for some battery degradation and would need batteries to have the same age (and supposedly been used the same number of cycles). Something to check carefully. I believe Solax is mainly an inverter manufacturer. Perhaps put less focus on battery management features and instructions on using them. 

I posted a bit about adding more batteries to a single BMS here, but I have not yet done that. @bash has that his Solis inverter, the menus may be different from ASolax but it is essentially the same. @transparent has multiple inverters and the ability to switch battery banks across them! 

Not exactly what you are looking for, but hopefully of some help.

Posted by: @weoleyric

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Posted by: @weoleyric

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• I have export capability with G98, and am open to G99 if needs be.

That could be worth, as a new inverter may give you more ability to export more when it makes more financial sense. 

Clarification needed. Here goes...

G99 approval is required if the home has more than one G98-certified device which has the potential to export to the grid.

Here I've taken the terms used in the G99 documentation, and presented them as diagrams:

Thus in your case a G99 (Fast Track) application would be required if you were to add a second inverter. Your Solax is the first one, and the Ecoflow systems don't have export capability and are therefore excluded.

Adding more storage outside the house wouldn't require G99 per se, but the inverter which connects to that storage does.

The problem I foresee is how any new inverter could operate satisfactorily in tandem with the existing Solax in the attic. Their protocols are proprietary, so the new inverter must also be a Solax.

There is a PARallel port in the Solax X1 inverters, but I don't know if they have limited the length of data cable which goes between the inverters.

Secondly, the diagrams I've found online suggest that installers are fitting Solax X3- Hybrid-G4 inverters when parallel operation is required.

You have an X1 unit, and those diagrams appear to suggest an additional box is required in order to facilitate parallel operation. In most diagrams it's called a Solax G1 Boost.

Thirdly, the storage batteries designed by Solax are 'high voltage' which means they operate above 75v DC.

That means they can only be installed by a qualified electrician.

Buying a second Solax inverter means that you'd also need to buy Solax storage batteries. There is a dfedicate communications cable which runs between the inverter and the BMS electronics in the batteries.

In short, choosing the first inverter to be Solax has left you 'locked in' to a Solax-only system.
That's compratively expensive, and a DIY route isn't an option.

Posted by: @transparent

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"Have you come across PAS 63100, available free from British Standards?
This document provides Guidance on domestic battery installations, and is not mandatory at present.

Is it not now referenced by BS7671 and the latest book (I'm too cheap to buy a new copy to find out). If it is as I understand then it's now got the force of building regs.

Only thing I really wanted to add though - if you get a sparky to do the install get them to buy the kit too. Firstly they get VAT off, secondly they can sometimes (and understandably) be really funny about components they don't know definitively are to spec and from a proper trusted source.

@transparent 

I also double checked and it does not seem this Solax inverter can use low volt 48-52V DC batteries. If it did, it could be an option to sell the Solax HV batteries (>100V). 

So yes, probably locked into Solax if wanting to avoid G99. My suggestion would be to pursue G99 with a second inverter able to use low voltage batteries..

Posted by: @etchedpixels

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Is it not now referenced by BS7671 and the latest book (I'm too cheap to buy a new copy to find out). If it is as I understand then it's now got the force of building regs.

I hope that's not the case.

PAS63100 has a number of deficiencies.

1: It is concerned with protection against fire from storage batteries, and yet lumps all cells with lithium chemistries into the same category.
As such it over-emphases issues that should rightly be applied only to Li-MNC cells.

2: It makes assumptions about the battery being the potential source of fire, but omits the possibility of the inverter (or a charger) being a source of fire risk.

3: It assumes professional installation is a prerequisite, despite a qualified electrician not being legally required for voltages below 75v (ELV).

4: It doesn't apply to dwellings with a floor area in excess of 200m² (mine is over 300m²)

5: It states that DC fuses must be interlocked to an isolator, such that the fuse cannot be removed without the isolator being in the OFF position. That ignores all those installations where the fuses and isolators are external to the storage battery enclosure(s).

6: It introduces the term Power Conversion Equipment (PCE) which has no universality or legal definition. PAS63100 uses PCE to include both inverters and DC-DC converters.

7: It stipulates three instances where visual and/or audible warnings must be implemented. But battery condition monitoring is often a feature on the inverter, not integral to the battery unit.

8: It refers to Arc-flash protection to prevent burns. But then states that the battery manufacturer should be responsible for the methodology. That overlooks requirements for on-site maintenance, monitoring and fault diagnosis.

9: It asserts that the stored energy of all batteries should not be greater than 40kWh, or 80kWh if they are outside the dwelling, such a garage or outbuilding.

It's a fair attempt at creating a standard, but its wording is clearly created by those who haven't got sufficiently wide hands-on experience of how storage batteries are sold, installed and maintained. The sort of batteries it is trying to address are going to be a part of a larger installation which includes items not mentioned in the document.

There is no indication that the IET has the authority to produce such a document. Putting the DESNZ logo on it and having it published by BSI doesn't automatically endue it with that authority.

Posted by: @etchedpixels

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Only thing I really wanted to add though - if you get a sparky to do the install get them to buy the kit too. Firstly they get VAT off, secondly they can sometimes (and understandably) be really funny about components they don't know definitively are to spec and from a proper trusted source.

That is interesting. My understanding was that, to be VAT-free, it would need to be for renewables, so the installer would need to be registered with MCS. And when I reached out to an installer for my Fogstar+Solis system, I was given an estimate of £3200 (the installer was listed in the Fogstar website). They alleged there is a lot of paperwork involved. The VAT was significantly less than that, so I gladly paid HMRC.   

Or is any registered electrician now able to claim it?