Multiplus II and one BYD Box 13.8 for off-grid?

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s-forsberg asked Jan 28, '20

Multiplus II and one BYD Box 13.8 for off-grid?

Hi guys,

I am planning to install a system according to the overview below where I normally use three Multiplus inverters + B-Box 13.8 kWh for self-consumption but also for occasional emergency power.

I read the advice on the Victron/BYD minimum battery size in the link below and there was some confusion! The minimum battery size table for Multiplus 48/5000/70 Three-phase states:

Self consumption minimum battery = 1 x 13.8kWh
Off-grid minimum battery= 3 x 13.8kWh (my calculations give 2 x 13.8kWh here)

https://www.victronenergy.com/live/battery_compatibility:byd_b-box

I am struggling to understand if this is a requirement or a recommendation.

So my questions are,

Will this system work?

Why does Victron recommend using 3x13.8 kWh for off-grid use?

Does the battery capacity have to match the inverter maximum currents? I'm ok with not getting up to 3x5 kVA out from the inverters.

Thanks in advance! :)

Sven F

Multiplus-II offgrid BYD

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Answer 1 · 2020-01-29T01:01:02Z

Hi @S_Forsberg,

Those minimum battery sizing requirements are required, and specified by BYD (in consult with Victron).

Installing fewer than that may work, however you potentially void the battery warranty (for not following BYDs installation requirements), could lead to tricky to troubleshoot system instability, or cause other issues that will frustrate any support attempts due to not following that minimum requirement.

The original minimum battery sizing requirements document from BYD is available here.

In a fully off grid situation, if you don't need the full 3x5K power, consider 3x3K units, which only require 2 x 13.8 kWh batteries.

Inverter Maximum Operating Currents is one of the considered factors when designing minimum battery sizing requirements, but not the only one. Another is inverter inrush current to charge the capacitors. And there are others as well.

The question topic mentions off grid, but your actual question text mentions a grid connection, with occasionally grid outage operation. This is fine, and covered by the self consumption model.

Off grid system design must be significantly more robust, as it is relied on for 100% of the power requirements, 100% of the time.

In an a predominantly grid connected situation, peak loads, low voltage shutdown situations, overvoltage from PV overshoot, and startup issues are shared by the grid, reducing the demand on the batteries. So you are able to use fewer.

If this system was pre-dominantly grid connected, aside from the occasional grid outage (where the system would provide backup), then I would be comfortable with the installation of 2 x 13.8 kWh batteries on 3 x 5 kW MP-II.

Though a confirmation from @BYD_service is always appreciated :)

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