MultiPlus-II 5000/48V with small battery pack

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cramik asked Sep 01 2022 at 10:07 AM

MultiPlus-II 5000/48V with small battery pack

Hello all,

I intend to buy a MultiPlus-II 5000/48V to keep a minimum of home power if the grid fails. I already have a SMA SB at 4 kW. I wanted to connect the SMA on the AC-out of the MultiPlus to keep the solar power when there is no more grid power. For the battery pack, I'm currently considering Pylontech modules (US2000C). A unique module would be sufficient to cover my consumption at night (about 50% discharge every day) and in case of grid failure, it should be sufficient to power the boiler stove and the thermal solar system. However, Victron suggests a minimum of 4 modules for such inverter. Is there a risk of damaging the battery if I'm using a small capacity (that can evolve in the future if the energy prices are remaining on the same trend)? The maximum charging current supported by a single module is 90 A for a maximum of 15 seconds which is larger than the time required to force the SMA (4 kW, hence 83 A @ 48V) to reduce the production level.

Is it possible to configure MultiPlus to charge the battery up to a maximum level? Let's say 95%, to keep a certain margin for transient power variations to be absorbed by the battery. When the grid is available, this is not necessary, any excess is automatically absorbed by the grid.

Thanks,

Marc

Multiplus-II Pylontech

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Answer 1 · 2022-09-01T11:33:34Z

nickdb answered · Sep 01 2022 at 11:33 AM

The guidelines exist to guarantee a stable system, when you wander off the beaten path you need to accept that the consequence of getting it wrong is instability and likely some pushback on support.

That said, provided grid is attached, you can get away with a smaller setup to start. If grid is lost, things can get a bit wobbly.

Trade off is that the battery is underspecced for any decent backup power requirements and taking any pack to its limits isn't a great idea.

You can limit inverter output for normal operation (grid on) which helps.

Just be aware of what the consequences are if the system ends up outside it's comfort zone.

Would suggest you check your battery warranty also, it may have exclusions.

2

Answer 2 · 2022-09-01T11:53:21Z

matt1309 answered · Sep 01 2022 at 11:53 AM

I would say it's fine as long as you setup the relevant limits to ensure you remain within the limits stated by the manufacturer of the battery. ie ensure max current drawn from battery is below the spec of the battery.

Not sure I fully understand your last question. If you mean keep batteries full when grid available and only discharge when grid is down. Then yes ESS supports that, the option is called "Keep batteries charged"

Answer 3 · 2022-09-01T20:01:02Z

cramik answered · Sep 01 2022 at 8:01 PM

Thanks for your kind answers. The warranty statements are effectively things that I haven't considered yet. I will dig in that direction.

For my last question, if we are off-grid, if the battery is fully charged (from solar panels or not), if I'm consuming a total of 2.5kW supplied by the solar panels, and if the consumption suddenly drops to 0 (e.g. an oven reached its set temperature), for a fraction of seconds the SMA inverter still delivers its power that must be dissipated somewhere. If the battery cannot absorb this energy, the voltage will probably rise before the actions taken by the inverters (Victron and SMA). From my point of view, the size of the battery pack does not matter. Except that the small capacities are more often full than the larger ones. That's the reason for my question: is it possible to always keep a small buffer? Maybe those aspects are not important, even if they are full, they can maybe still accept a bit of extra energy, and the larger they are, the less significant it is.

4

nickdb ♦♦ commented · Sep 01 2022 at 8:09 PM

You can get away with being undersized on grid. Off grid, not.

0 ·

Alexandra ♦ nickdb ♦♦ commented · Sep 02 2022 at 5:07 AM

@cramik

Then dont use Pylontec then they become balloons.

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matt1309 commented · Sep 02 2022 at 1:13 PM

I cant speak for Plyontec batteries specifically but you could just set the "full" voltage of the battery to be less than actual max of the battery.

ie say 48v LiFePo4 battery set absorption voltage (full voltage) to 55.2v (or even lower) instead of 57.6 which is often used in default profiles for LiFePo4 batteries. It wont add much of a buffer but should be enough to not worry about batteries.

Frequency shifting settings might also be possible. ie maybe you can increase how much the AC coupled inverters are limited, however I'm not familiar with frequency shifting and havent seen much of that on this forum so maybe that wont work.

Another option, more complex option. (I dont know if SMA inverter make this easy or not) but could use Node-red or some alternative to turn off SMA inverter when battery > 95% and then turn back on when below 85% or some other arbitrarily large range.

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matt1309 matt1309 commented · Sep 02 2022 at 1:15 PM

I would go with setting absorption voltage lower than max as i mentioned above, and then simply test it. see what happens in that scenario. Maybe test it with a big load and see if voltage spikes at all. If it goes too high, lower absorption voltage further.

0 ·

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MultiPlus-II 5000/48V with small battery pack

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MultiPlus-II 5000/48V with small battery pack

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