ESS: MPPT charges battery although more power is needed by consumers

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herzigwi asked Apr 4, '24

ESS: MPPT charges battery although more power is needed by consumers

Hello,

I have an ESS system with the following components:
- Multiplus II
- Victron MPPT
- PV inverter connected to AC in
- PV inverter connected to AC out

My minimum SOC is set to 40%.

Is the following a normal behavior of an ESS system?

- MPPT charges the battery even when minimum SOC is reached and PV is not enough to satifify the load of all consumers? See the following picture:

SOC is 40.1% and the MPPT charges the battery even when energy is needed from the grid. Only the PV inverters are used to reduce the energy needed from the grid.

This way the battery is charged until the SOC of the battery reaches a certain level. If this is reached the battery is discharged down to the minimum SOC.

Is this the normal behaviour of an ESS system?

Multiplus-II battery charging ESS

1712234876475.png (115.9 KiB)

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Answer 1 · 2024-04-04T14:03:54Z

luphi answered · Apr 04 at 02:03 PM

I think, it's a kind of hysteresis. It will stop charging at min. SOC + 3% (at least my installation does)

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Answer 2 · 2024-04-04T14:12:24Z

nickdb answered · Apr 04 at 02:12 PM

Yip, normal. The battery needs a buffer, usually 3%, so it doesn’t keep bouncing into/out of charge states.

Your system clearly shows ess#1 which is documented in the manual and will lift at 43% and then start serving loads.

Many, many, existing topics on the forum available by searching.

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Answer 3 · 2024-04-04T14:29:49Z

herzigwi answered · Apr 04 at 02:29 PM

Thanks for the answer. There was a similar question 4 years ago. I have hoped that this battery consuming and energy consuming behavior is no longer normal.

Charging and discharching the battery results in a loss of energy. I do not see a reason why the MPPTs load ist just not used by the Multiplus inverter. Fronius inverters just convert DC to AC without needing a battery as a buffer.

Looks like is will be better to extend by system by using AC coupled inverters.

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nickdb ♦♦ commented · Apr 04 at 02:40 PM

ESS is an Energy Storage system, its focus is in maintaining minimum charge levels and charging it from solar, using surplus PV to power loads.

If you don't need a battery, you don't need an ESS.

The minimum SOC algo has a +-3% hysteresis where it either won't charge from grid or dedicates DC PV to rapidly charge the system.

The 3% isn't a problem unless there is insufficient PV available, in which case it will preserve that minimum +-3% and use AC for loads.

It is the most effective way to keep the system operating efficiently, otherwise every time a load was activated the SOC would drop and the system would change states again.

If the PV is incapable of charging the battery, it isn't going to be much good at serving loads.

For mobile users (where ESS isn't supported) there is solar priority which prefers PV for loads and holds the battery at a minimum SOC, unless it remains that way for too long.

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Answer 4 · 2024-04-04T15:16:24Z

herzigwi answered · Apr 04 at 03:16 PM

Thanks for the information.

Your system clearly shows ess#1 which is documented in the manual and will lift at 43% and then start serving loads.

In which manual is this documented? I just looked into the ESS manual and did not find any information about a 3% buffer which is needed if minimum SOC is already reached.

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