Also I have been told the BMS should balance the cell on the first charge? Am i missing something?

Posted by: @makia2023

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CT Clamp i will reuse my existing as it is already run (approx 12m) in external trunking to the consumer unit.

As long as you're re-using just the data connection you may be fine. The Solis I had came with an Eastron SDM120CT which I think could be extended with the right kind of (shielded?) twisted pair wire to carry RS485.

Posted by: @makia2023

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If each battery is fuse protected then why do I need another fuse between busbar and inverter?

My reasoning for another fuse would be that if there was a fault in the inverter each battery could push 125A into the inverter continuously without any fuses blowing. I believe some MCBs have 125A DC ratings, so if you did not fancy using another fuse and holder then perhaps this will do as a disconnector.

Posted by: @makia2023

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I have surge protection on the main consumer unit so is this sufficient to cover the inverter?

I think that would be a question for your sparky.

Posted by: @makia2023

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the BMS should balance the cell on the first charge

The BMS finds out the real capacity of the battery by charging and discharging. I think for Seplos it is when there is some over-voltage condition (pack over-voltage?). But the Seplos BMS has a very weak passive balancer (100mA?) which will take a very long time to balance all cells.

Posted by: @makia2023

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If each battery is fuse protected then why do I need another fuse between busbar and inverter?

Selecting a fuse is similar to choosing an insurance policy.
Whilst they all claim to offer protection, some pay out better than others!

Looking specifically at a fuse placed between a storage battery and an inverter, the current rating isn't particularly significant.
Eg a 160A fuse isn't chosen in order to blow if the current reaches 170A for a few seconds.

If you want protection of that sort, then you'd use an MCB/MCCB trip.

The fuse must provide a very fast response in the event of a near short-circuit... in the realm of tens of kiloamps (kA).

Most fuses have a single wire link within an air-filled chamber.
A few have silver-sand within a ceramic cylinder, but there's still oxygen between the grains of sand.

Those fuses 'blow' because the weakest point in the thin wire reaches the melting point of that metal.
In a DC circuit, as the wire melts, an arc forms across the gap, gradually increasing in length as the metal burns away in both directions.
The fuse is demonstrating the same principle as an arc-welder.

That's not really what you want, because a large current is still flowing into the electronic components (in the inverter) which are presenting the near short-circuit.
There's enough time for a fire to take hold.

You should be selecting a Type-T fuse.

There are different Type-T mechanisms used by the (relatively few) manufacturers.
One of them uses a mesh of metal to form the fusible link, similar to steel wool.

But the main feature of a Type-T is the chemistry of the compound which surrounds the fusible link.
The chemicals cause an instantaneous flash-over, which completely vapourises the metal.
No arc can form because the entire length of the fuse body no longer contains any metallic path.

Some of my Type-T fuses are Eaton Bussman FM-type, which they manufacture in India.

At 10kA, the fuse will break the circuit in about 1 micro-second.
At the 40,000A which is more typical of the short-circuit current from a 52v LiFePO4 battery, it takes about 400 pico-seconds.

I also have some Miro BS88 fuses, type RGS4B, manufactured in China.
They 'blow' at about 10x slower for the same virtual-current, but use the same chemical principles for arc-quenching.

Fuseholders for Type-T are difficult to source.
They are more usually built into equipment cabinets with copper/brass busbars.
There is an Eaton BH1133 screw-down holder, but I've not found any within Europe (unless you want a box of 100!)

So I make my own fuse-holders using a 3D printer and chunks of copper!
The bolt-spacing can be arranged so as to suit both the Eaton FM and the Miro RGS4B types.

@transparent can I commission 3 plus some spares from you? 🤣

That's an issue I'm already discussing with MPs and Councillors. 🙂 

As the general public try to further embrace renewable energy strategies there are 'bits' required that are either bespoke or not readily available.
Consider the following:

• constructing a micro-turbine to generate electricity from a small stream or drainage channel
• maintaining and repairing electric bikes/scooters/hover-boards and their chargers safely
• making 3D-printed clips to hold solar-panel wires above an existing gutter (see image below)
• fitting a DC-DC converter and battery to power a landline phone and WiFi hub in a house with a fibre-broadband feed

These sorts of jobs could be readily tackled if we had more community-run maker-workshops.
The skills required are comparable to those used in model-railway clubs or by drone builders/fliers.

The UK needs these skills to be available as technology progress increases in speed.
Employers would also like to recruit from a pool of applicants with hands-on experience rather than paper-based qualifications.
There's no point developing agricultural robots if we don't have anyone who could be employed to run them!

To make the fuse-holders you require is an excellent project for a post-16 student with workshop access to a 3D printer and basic machine tools.
They get to write up the project towards an NVQ, whilst you pay for all the materials.

I'm working within the realm of commercial R&D, so cannot afford time to make up bits to supply to others.

At the moment my company deliberately isn't running a website.
Development work would be severely compromised if people knew who I was or where to find me!

But when we do decide to return to having an online presence, it's likely that we will upload the 3D printer files which we've created in-house over the last few years.
So that will go some way towards helping others to help themselves.

Posted by: @transparent

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At the 40,000A which is more typical of the short-circuit current from a 52v LiFePO4 battery, it takes about 400 pico-seconds.

The chart does not appear to feature 4*10^4, so I would be nervous of an extrapolation to a time which equates to 12cm at the speed of light.

Posted by: @transparent

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If you want protection of that sort, then you'd use an MCB/MCCB trip.

I could not see the trip curves on the Aliexpress MCCBs linked to earlier; it does not appear to be a feature the retailers feel is worth featuring.

Posted by: @chickenbig

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I would be nervous of an extrapolation to a time which equates to 12cm at the speed of light

Ah...

It's a West Country thing.

Time goes more slowly down here 😎 

Posted by: @makia2023

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Also i've spoken to Brian and he can do me a really good price on some 310ah cells so only show stopper will be shipping time - which he is coming back to me with later.

 

hi, what sort of price is he quoting for UK delivery please and time scales (although i actually live near Felixstowe so could go and get them if they allow it?) 

We can calculate the rate of rise of current, if we make an assumption of the inductance which inhibits the current change.  1uH roughly represents the inductance found in a single loop of chunky wire 400mm diameter - this represents the inductance of the current path through all the cells, then the cables, and wherever the short is.  It could be half that or twice that I expect, it's an estimate.

From V=L.DI/DT, we can find:

DT=L.DI/V = 1uH * 40kA / 52V = 0.8ms

The current will start off rising at a rate 40kA/0.8ms, then plateau out at whatever the loop resistance determines.

Was that an answer to my post about fusing @robl ?

@misterb -Although Felixstowe Container Port is close by, I can't think how you could collect a consignment of batteries.
You'd still need an approved agent to handle the passage through UK customs.
The Courier companies do that all the time.

It might be different if you were to buy in a complete container load.
It wouldn't then be necessary to have staff licensed to work in the pre-customs area where containers get unpacked.

I wonder how many LiFePO4 cells you have to order to make it viable for your own container?

Thanks for everything so far (I was away on a last minute work trip so havent been able to get to this for a while).

@transparent for fusing would the following work?

Blue Sea 5007100 Bolt Down Class T fuse holder - Mobile Centre

Blue Sea 5114 Bolt Down Class T 150A fuse - Mobile Centre

Also, looks like i'm getting the version 2 seplos BMS with reduced compatibility so will be going with Growatt 5kW 230V SPF5000, 48V, MPPT CHARGER, PARALLEL FUNCTION, OFF-GRID (voltaconsolar.com).

Any thoughts on this inverter? I wouldn't mind export option in the future but compatibility isn't great and this is very cheap.

Posted by: @makia2023

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Any thoughts on this inverter?

Are you totally sure you want to go down the off-grid inverter path? What loads will this support? Have you sized it to cope with peaks (e.g. machinery turning on)? Will your electrician fit such a system?