question

Hi Nick< Not a expert here but I would suspect they are refering to the Lead Acid Battery cemistry there,

As the Lithium Phosphate batteries can deliver HIGH instant load current (within reason) I would suspect that this figure if lithium is used would be reduced by 2/3rds (best Guess)

so dont worry about the battery side

I do stand to be corected in relation to the above statement

just keep the max solar at or under the 1 to 1 rule

Yeh, guidelines are guidelines. if you want BYD, talk to BYD. There may be all sorts of factors that could bob up to affect a particular supplier's recommendations - and warranty cover.

@BYD_service may summon the forum's local BYD man..

Thank you for 'summoning' me. For grid connected systems don't mind the minimum configuration. Only in the case of backup make sure that the battery surge power can support the systems potential surge power.

Hi!
I have 3x Multiplus 2 3000 and a 14,8kWh Lifepo4-Battery with 200A BMS. (16S / 48V)

Now i would connect 9kWp on AC-Out 1.
According the minimal battery-size from Victron i would need a 28,8kWh battery.

But: 9000W / 48V = 190A
So in this case everything would be fine.

An example:
Battery is fully charged, 9kW coming from the inverter on AC-Out 1 and grid is shutting down:

I think the only problem could be that single cell-voltages can rise to fast (because of the too small battery-size) and the BMS cut off the battery befor the Multiplus and Symo reduce the power.

BUT: How risky is that with an CVL at 55V (= 3,44V per cell)?

How fast reduce the system the power? 0,5 seconds? 2 seconds?

Next question: I can connect 3000W on the AC-Out 1 of one MP2, charge-power is only about 2000W (35A) - what happens to the remaining 1000W in the moment the grid is shutting down?

Hi people, I am new to forum so please excuse if silly suggestion, but I also want to know about minimum battery sizing when using grid tied inverter in ESS off grid mode.

Wouldn't a bank of reasonably priced super capacitors placed accross the battery be capable of absorbing the initial spike from load switching until frequency shifting kicked in?

it's unlikely that super capacitors will do what you expect them to do.

LFP cells impedance is not only low, it's also relatively flat versus frequency.

https://www.researchgate.net/figure/Impedance-Bode-plot-from-LiFEPO4-cell-Moreover-linear-Kronig-Kramers-compliance-test-is_fig2_334286881

The high current rating of the supercap (SC) won't help if they have higher impedance.

let's start DC (sub 1Hz) :

My LFP battery (230Ah, 16cells) has a drop of 1V between 100A load and no load (impedance = 10mΩ). That's about 2% of the nominal voltage between no current and full current.

This same 2% will be the votlage change across the SC before the battery enter protection. Thing is, 2% voltage change across a capacitor is 3.7% change of internal energy. Less than 4% of your capacitor is actually "used" during a single event load jump. Not exactly cost effective.

you would end up with an insanly large SC to actually increase the power rating of your LFP battery when you could reach the same effect at lower cost with simply more battery. As a side effect you'd get also more energy stored.

In the frequency domain it gets more complicated. There you could reduce the AC 50Hz current ripple from the battery. Due to the (once again) low impedance of LFP and the abscence of need to provide a ripple free DC current most inverter provide only little filtering of the battery current. My own Victron multi RS has about 10% ripple (Idc/Iac) at 50Hz, measured during discharge without solar with a Hall current clamp.

this current mean some added aging for the battery. Exactly how much is hard to tell but it's certainly not 10% reduced lifetime. 50Hz current is less problematic then DC current as it doens't cause much diffusion (AKA charging).

There, you'd need 1F at 60V rating to reduce the AC current by 50%. 1F of capacitance is not that expensive. For a load that takes 200ms to start you'd then need 10F and 100F for 2s. 100F is not cheap but 10F would still do for many loads. Maybe / why not ?

Last warnings :

- in the frequency domain you will care a lot about the SC ESR. reaching 10mΩ (Zc + ESR) impedance seems easier on paper than it is in reality.

- you seriously do NOT want your BMS to re-engage on a discharged super capacitor. That would call for some fancy currents.

- I wouldn't spend money on reducing 50Hz AC currents without more understanding of the Lithium chemistery. No one know for sure which kind of charge profile best extend LFP life and not all AC currents are bad : https://res-mdpi-com.webpkgcache.com/doc/-/s/res.mdpi.com/d_attachment/energies/energies-13-02458/article_deploy/energies-13-02458-v2.pdf

all of that can change if you use lead batteries.

TLDR : don't use Super Capacitor with LFP. save you money for something else.