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Constantin Zaharia avatar image
Constantin Zaharia asked

Multiplus 2 and SOC limit

hello,

sorry if i ask the wrong question, but i have this problem.

I came from a Growatt system, where on settings i have the minimum SOC battery when the inverter will switch back to grid when SOC is below a threshold.

In ESS system, i have found that are "Active state of charge", and SOC (if the grid fails), in this case the battery will be setup accordantly to Dinamic Cut-off.

I have setup the inverter using the tutorial from this https://www.victronenergy.com/live/battery_compatibility:pylontech_phantom

My question, because i don't have started the sistem yet, how i setup the ESS to switch back to grid on specific SOC? How do i know what is the minimum SOC when the sistem will switch back to grid and not drop loads?


My inverter is a Multiplus 2 48/10000 and i am using Pylontech batteries.

Multiplus-IIMultiPlus Quattro Inverter Charger
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matt1309 avatar image
matt1309 answered ·

Hi @Constantin Zaharia


Min SoC unless grid fails is whatever you want the minimum to be under normal conditions with grid connected.

If grid goes down the SoC can drop below that threshold and keep going until low voltage disconnects I believe. When grid comes back online the system will charge from the grid up to that minimum SoC setting if you have dipped below the threshold during the outage.


Active state of charge is used when you have Optimised with battery life enabled I believe:

Here's some more info on all those settings:

4. Configuration (victronenergy.com)

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lifeingalicia avatar image
lifeingalicia answered ·

1. Min SoC - Batteries / Inverter will not go below if grid is available (Reserve for grid failure)

2. Grid fails batteries will use the reserve until depleted (low disconnect)

3. General - ESS will balance available PV - Use for demand - excess into battery up to max allowed charging (can be set under DVCC) - any further excess will be pumped into the grid.

Exceptions SoC below min - loading batteries first using PV & grid

Scheduled charging - will charge battery to requested SoC and not allow battery usage whilst active

Important - if grid is unstable and heavy loads are present / switched it is advisable to keep some reserve at the top end. E.G. PV producing 5kW load present of 3kW - load switched off and grid fails it takes a moment for the system to adjust whilst the production need to go somewhere (batteries)

Hope it helps


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Constantin Zaharia avatar image
Constantin Zaharia answered ·

ok, i understand now, but in the case when i use Pylontech batteries, that have BMS, is not normally to have percentages as unit of measure? What is the reason that in some parts SOC is used and in another places "Low voltage" is used like in Dynamic cut-off?

Is not more easy for Victron also for us something like, because BMS is doing all the heavy work for battery stats:

  • Min SOC - switch to grid when SOC is reached
  • Min SOC (with grid fails) - cut-off the loads when grid failed

To have a toggle option like this, and eventually incorporate other rules, like temperature.

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matt1309 avatar image matt1309 commented ·

I don't really know why, but my guess is it's a safety thing. If you use SoC to cut off loads when grid fails rather than voltage there's a risk that SoC data is incorrect/inaccurate and the battery is discharged too far.

Especially with LiFePO4 cells where SoC is usually calculated by counting amps in/out rather than based on voltage like other chemistries so there's a risk it's not as accurate as relying on voltage.


It's probably far less common feature as well. Like when there's no grid you'd surely want to use all of your battery capacity to cover loads. If not then you're not getting everything out of your battery. Maybe useful to protect the battery but if you're doing that you may as well increase min voltage/dynamic cut off.

(these are just guesses, i dont know the official reason for no min SoC when grid fails).


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nickdb avatar image
nickdb answered ·

SOC is a calculated value that is estimated by the BMS, it is at best somewhat accurate. What matters is voltage, that determines what the battery is capable of delivering, particularly when under load. This is why ESS has dynamic cut off values and the inverter cut-off and restart are all based on voltage.

SOC is just an easy metric for making decisions for grid switching in an ESS, but ultimately all charging/discharging/health is based on voltage.

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lifeingalicia avatar image
lifeingalicia answered ·
  • Min SOC (with grid fails) - cut-off the loads when grid failed ... not necessarily

SoC and low disconnect are two different things. Low disconnect will shut off the inverter.

SoC will use the battery up to max inverter setting. Be aware there is a difference between VA and Watt (power factor).

It also depends very much on how your installation is wired and the grid set point.

PV can feed in before and after multi (rule 1:1 !!). PV after a multi gives black start capability...

Mostly you`ll have AC1 and AC2 one is load shedding, one is powered by inverter/bat if grid fails.

If you use an external CT you could have an AC that bypasses the mullti(s) this drives non essential loads and the AC1 is your "UPS" for essential loads.

Still the unit(s) will provide power up to inverter setting to the whole AC to try to keep the grid set point measured @ CT.

Post an install drawing/sketch for more info.

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Constantin Zaharia avatar image
Constantin Zaharia answered ·

mi main problem is ho i week the batteries to not discharger below 10%, specially if is a power failure. I think the best option is to use node red and drop loads when SOC is around SOC 10%

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nickdb avatar image nickdb ♦♦ commented ·
Being able to run batteries down low, and to pull meaningful power while doing so, are two very different things.

Under load, the battery can dip below voltage thresholds, such as ESS dynamic cutoff, and cease discharging.

This is to preserve the battery.

Too many people focus on a derived metric like SOC, thinking that it is somehow absolute.

It isn't.

Assuming a battery has the same discharge performance at bottom of discharge as it does fully charged, is also a mistake.

Pylon allow you to go low. They don't guarantee what it will deliver when you get there.

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Constantin Zaharia avatar image Constantin Zaharia nickdb ♦♦ commented ·
ok, but what is the risk to go to low and "brick" the battery bank under the normal voltage to boot the BMS?
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nickdb avatar image nickdb ♦♦ Constantin Zaharia commented ·
You can't brick it. They have a good BMS, it will protect the battery, but if you are pushing it that low, you really need to get another one.

Even if they advertise that it can go low, it is a good idea to not do that unless it is absolutely necessary.

You're likely to get more out of it for longer if you look after it.

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matt1309 avatar image matt1309 commented ·
I think the argument is that using 10% as a metric isn't accurate for a weak battery and voltage is better/safer. As if the shunt in pylontech cells drifts slightly that 10% will be a different point each time you charge/discharge.
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