My rule for myself:
Thou shall NEVER solder anything on a solar system battery.
Thou shall crimp all wire connections and bolt them as required.
So it shall be. 
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Lovely! Wish the same rule has been followed in the battery I’ll take apart. Though I highly doubt it. Only one way to find out!
Correct. It is a 12V 800VA inverter. The Mecer is supposedly an 800W 1200VA unit, so a pretty close match in terms of being apples for apples.
Right here - wondering, what BMS and where from? Going to need one of those 16S ones. Bluetooth suffices or I need CAN/RS485 to get battery to talk to Multiplus?
Also in calculating the amps required - 50Ah cells, assuming 1C max means 50A total + 20% safety factor to give me 60A. Math is fine?
I recently bought from these guys. JK BMS are very good, in no small part, because of Andy and his subscribers in The Off Grid Garage.
This one now interfaces with Victron “off the shelf”:
Note: Because they can go as low as 8S, if you have to take out cells, it can still work for 24v and 48v banks.
assuming a starting size of a 2.56kWh battery pack, any way that the 5kVA can be an overkill? I know it can charge a much bigger battery pack, so maybe have to set a max battery charge current?
Let me give you the two reasons, and then you can decide for yourself.
One reason such a big inverter is not advised on a small battery pack, is DC ripple. It’s a downside of the LF designs (low frequency), they create a 100Hz ripple on the battery, and the smaller the battery, the more pronounced such a ripple will be. You can avoid it, of course, with the relevant limits, but it becomes something that always have to be kept in mind, accounted for.
Another reason it is not recommended, is inrush current. The larger the battery, the bigger the capacitors, the bigger the inrush current. A small pack might disconnect when you connect the inverter (unless it has a precharge circuit of sorts).
There is no reason you can’t run a 5kVA from such a small battery, it’s just not going to be a “install and forget” setup
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Not sure I understand fully (due to my lack of knowledge in this field). What are the mitigations for these?
As an alternative what would be the max inverter size for this. I am expecting the 2.56kWh to be temporary until I get to more cells salvaged somewhere and then parallel to a bigger pack once this prototype is done.
Happy to learn, but I still want install and forget apart from the reason of cells failing. Those I understand I need to deal with.
I agree with Plonkster.
Once had a 5kva on a small bank, it is all as he says.
My case, it was a temporary bank … and a tiny bit of “here, hold my beer”.
So if you want to end up with a 5kva, then manage the small bank.
As a matter of fact, if the 5kva is not what you are after long term, get a 2nd hand 3kva if you are interested in one.
Reduce the max charge amps.
Reduce the max draw of the inverter.
I’d get the 3kVA if I’m faced with a good deal for sure. Otherwise if I’m buying new I’m leaning towards 5kVA because… long term.
From the spec I see the 3kVA can do 2.4kW sustained - I’m thinking 2kW heater and 300W tv setup that goes up to 400W. Any advice against?
Of course this would mean this is like a 1C load 
but I’m also thinking parallel banks later and so on.
It depends a little. If you use ESS (ie you run that inverter in parallel with the grid), the answer is a bit different to when you’re using it as a UPS or off grid.
If you’re just using it to run some loads when the grid is down, off-grid in other words, or as a UPS, then there isn’t really too much of an issue. You would have to limit your loads to no more than 1kW, and you’d have to set the charge current to about 20A (about C/2, the capacity divided by 2 hours).
When running it grid parallel, it becomes a bit more complex. If the grid is perfectly stable, you should still be fine. You’d set the maximum discharge power to about 1kW, and again make sure the output loads are no more than 1kW. But the software loops are always a bit slower, and a large load starting in the output, or a large change grid voltage, or things like that, can cause a larger inflow or outflow out of the battery, which could trigger a low voltage condition, put it into sustain mode, and so on. When running grid-parallel, it is a bit more important to have the battery sized proportionally.
I’m sure you will be fine if this is just temporary.
There you go: 48/3000/35 Multiplus for sale
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Thank you - I’ve reached out. Says something about old chipset and firmware update not possible. Let me find out more
Makes no difference on your project.
Chances of you adding more than one inverter to that battery is zero.
Not that use case. More that I’d want any improvements from Victron not related to parallel running.
Coming back to the project, here’s the story @TheTerribleTriplet and @plonkster :
- Start with basic UPS - inverter and the DIY 2.5 kWh battery (now)
- Eventually grow storage as needs grow or deals arise with parallel 51.2V banks (3-6 months from now)
- Move storage away from living room and ideally connect in parallel to existing inverter to increase kW and kWh for the house. Alternatively find a way to supplement existing Deye inverter with the diy battery via gen port or something. (Also 3-6 months from now if not sooner)
2 and 3 can switch around.
So I know #3 is what might decide my inverter size. That’s why my head was at 5kVA.
Then stick to 5kva, manage smaller battery.
It also gives you a much deeper insight into solar systems in general, managing a smaller battery on a bigger sized inverter.
You can build up towards Victron. Piece by piece, new and/or second hand.
Any ideas on how I achieve my #3 though? This vendor lock in thing is not cool.
Do you want full control, always?
Can use any battery you want?
If you are interested, and you are, your DIY project, then seriously consider Victron.
All of it.
I take it you mean replace the Deye right? Or disable it and use a Victron in its place or something like that?