Custom made UPS/ Energy Storage

DIY
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Hi all,
this is my first post in the forum, I would like to thank you beforehand for your great contributions.

I looking to build a “custom” made ups/energy storage to replace all my PB UPS units when the power is cut and also to use it as power source when I have no PV production for my electronic equipment only (computers, cameras e.t.c).

I am living in a remote area in Attica Greece, with a real terrible power line, where we may have multiple voltage sags in a week for no apparent reason, the situation is worse with bad weather and It gets terrible in hot summer days. I had the unfortunate luck to buy multiple from the same APC UPS , which have two PB batteries in series without balancing , where always one them dies in a very short period. After multiple PB batteries failures I have decided that I had enough.

I have already a PV system based on Fronius Symo Gen24+ with 22kWp installed with a net metering contract, I bought the hybrid inverters with the intention to add batteries later but they are only compatible with the Fronius inveters are the unobtainium BYD HVS batteries 1000€ per Kwh (or something near that) , so I never got to install the batteries and I am using a diesel Generator which I already had as a backup power source.

I made small research and Ι found that the Victron Energy MultiPlus-II 48/3000/35-32 supports switching to battery in less than 20ms in case of power loss so it should probable be suitable to act as UPS.

For charging the battery from excess solar, by looking in the Victron ESS, I understand it can be programmed to use excess solar via a power meter, programmed in the appropriate modbus registers, enable / disable battery charging e.tc. I still have not exactly figured out how to automate the grid A/C enable/disable from inverter to force it to battery when on demand but I think it should be possible.

For battery I was looking a 5kWh 48v battery, initially I was looking for the Polytech U5000 but I read about the 15cell situation and some negative comments about the reliability so therefore I would not go for it. Another battery is the Deye SE-G5.1 Pro-B, 5.12kWh. Any other battery proposals available the EU area with similar cost would be very useful.

I am trying to keep the total system cost about 1500€ + VAT for (Victron Energy MultiPlus-II 48/3000, Deye SE-G5.1 Pro-B , Cerbo GX MK2 ) for the main system components.

I am asking for your advice, if I can accomplish my intentions with the above setup.

Thanks,
nkef

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I can confirm that the Victron inverters do switch from grid to invert sufficiently fast to keep a standard pc running, even when not in UPS mode.
the US5000 battery is also reasonably reliable, pylontach is one of the big shippers. Just get them from a reputable source.

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Thank you @MikeD for the information, I am intending to use the inverter in ess mode
I would to ask what is the opinion of the victron community for the Deye SE-G5.1 Pro-B battery.