ESS mode exhibiting charge/discharge oscillation.

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gprado asked Feb 14, '22

ESS mode exhibiting charge/discharge oscillation.

Hello Victron peeps,

I successfully setup the ESS function with a Multiplus II, charge controller, smart shunt, cerbo GX, and a 48V lithium bank. I set it to sell excess DC coupled energy, which it does well. it is also setup not to discharge the battery unless the grid is lost The problem is when the charger (Multiplus) reaches the absorption voltage, the system starts oscillating between discharging the battery (sometimes by several hundred watts) and immediately charging it back up to the set voltage. I know that can't be good for the battery and it seems unnecessary. I also noticed that the Multiplus really doesn't want the battery voltage to go up or down from that set level, so it keeps oscillating in order to "clamp" that voltage. How do I tell the Multi to chill and leave the battery alone once the set voltage is met? Meaning, let the voltage go down on it's own according to the chemistry of the pack. Can it not bypass the battery and just use the charge controller current to invert back to the grid?

Multiplus-II grid

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Answer 1 · 2022-02-15T06:15:36Z

Alexandra answered · Feb 15, 2022 at 06:15 AM

@gprado

That is a common thing you are seeing in the absorption stage. It is the stage where there is balancing of cells mostly. Sometimes the batteries discharge a bit to keep high cells from running away. And it is not a problem or does it damage batteries.

Since your system is DC based PV the behaviour is to be expected. Remember it is converting a variable DC source (solar) to DC then up to AC, there is going to be alot of tracking retracking and calculating.

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gprado commented · Feb 15, 2022 at 02:02 PM

Thanks for the answer Alexandra. The interesting things is that the system works pretty stable if I completely disconnect the battery bank. I wish there was a feature in ESS to stop charging or discharging the battery once it is 100% SOC

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Alexandra ♦ gprado commented · Feb 15, 2022 at 07:21 PM

@gprado

It only "appears" stable (when you have disconnected the battery) however when things go wrong then it will be quite bad. in a Victron system the Battery is the heart/base of the system not the inverter.

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Answer 2 · 2022-02-15T06:23:19Z

nickdb answered · Feb 15, 2022 at 06:23 AM

The battery BMS sets the voltage set point that the MPPT's will track towards.

With keep batteries charged they will target a slightly higher voltage. What often can happen is there is a little voltage loss in the system (generally wiring), so under load the voltage tends to track above the batteries set point. Once load reduces, the mppt's throttle and the battery discharges back down towards the set point again, so you can see these charge/discharge phases. Various causes, part of which is how ESS works.

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