5. Operation
5.1. General
In Microgrid (Hybrid Droop) systems, the AC bus is jointly formed and stabilized by all participating Victron Power Banks.
Unlike grid-connected systems with a stiff utility reference, small variations in frequency and voltage are normal and expected during operation. These variations are part of the droop control mechanism and allow automatic load sharing between Power Banks.
Minor frequency deviations under changing load conditions are therefore a functional characteristic of the system.
Operating mode of Inverter/Chargers
During Microgrid operation, always ensure that all inverter/chargers in all participating Power Banks are set to "On".
No inverter/charger may be set to:
Charger-only, or
Inverter-only
Verify the correct operating mode for the physical switch, in VE.Configure, the GX device, and VRM (if applicable).
5.2. Startup & Shutdown procedure
Isolate all loads from the Microgrid AC bus. Ensure the AC bus is unloaded before energizing the system.
Start all Power Banks one by one. Power up each Power Bank individually.
Wait for all Power Banks to connect to the Microgrid. Each Power Bank will either:
Detect an existing energized AC bus and synchronize to it, or
If permitted, initiate a black start and energize the bus.
Reconnect the loads to the AC bus.
During startup, each Power Bank checks for voltage at its configured AC input. It determines whether it must synchronize to an existing Microgrid or initiate operation itself.
Warning
Do not connect additional Power Banks that are already switched on to an energized Microgrid AC bus. This may cause overload or overcurrent protection devices to trip.
If this occurs unintentionally, no damage will result. Switch the affected Power Bank off and restart it following the procedure above.
Ensure the Power Bank's inverter/chargers are switched off
Close the circuit breaker on the Power Bank to energise the circuit from the Microgrid to the AC-IN of the Inverter/Charger.
Switch on the inverter/charger(s).
Power Bank will detect the energised AC bus, synchronise, and join the Microgrid automatically.
Turn off all loads connected to the AC bus before shutting down any individual Power Bank, or the Microgrid as a whole. This reduces the power demand on the remaining Power Banks as they are taken offline and prevents transient overloads during the shutdown process.
Switch off Power Banks one by one. This can be done remotely via the GX device, or physically via the switch at the inverter/chargers. Each time a Power Bank is taken offline, the remaining units automatically take over the load previously supplied by that Power Bank through Hybrid Droop control. Allow a few seconds between each disconnection for the AC bus to stabilise and minimise any spurious warning or alarm messages.
AC bus is de-energised once the last Power Bank is switched off
Verify that no voltage is present on the Microgrid AC bus after all Power Banks have been shut down before performing any maintenance or wiring changes.
Procedure to take a single Power Bank offline while the Microgrid remains operational
Ensure the remaining Power Banks have sufficient combined power capacity to sustain the full system load before removing a unit. Removing a Power Bank without adequate remaining capacity will cause the remaining units to overload and shut down the Microgrid.
Switch off the inverter/charger(s) the remaining Power Banks will automatically take over the load previously supplied by the disconnected Power Bank through Hybrid Droop control.
Open the overcurrent protection device (circuit breaker) between the Power Bank to be removed and the Microgrid AC bus. This safely isolates the terminals at AC input (from the Microgrid) and allows removal of the Inverter/Charger.
Prioritise de-energising the system.
In the event of an equipment fault, safety hazard, or other emergency, switch off all Power Banks as quickly as possible. Do not follow the planned sequential shutdown order.
If individual Power Banks cannot be switched off at the unit, open all overcurrent protection devices (circuit breakers) between the Power Banks and the Microgrid AC bus to isolate the system.
After an emergency shutdown, do not restart the Microgrid until the cause of the emergency has been identified and resolved. Before re-energising, inspect and test all AC and DC wiring, protection devices, and battery systems for signs of damage, overheating, or fault conditions.
When restarting after an emergency shutdown, follow the standard startup procedure described in the Startup procedure section. Begin with a single black-start-enabled Power Bank and add remaining units sequentially, monitoring for correct synchronisation and stable AC bus behaviour at each step.
5.3. Recovering from tripped circuit breaker
If the overcurrent protection device between a Power Bank and the Microgrid AC bus trips:
Switch the affected Power Bank off.
Identify and eliminate the cause of the trip. Verify that no overload, wiring fault, or configuration issue is present.
Reset the circuit breaker, restoring the AC connection to the Microgrid AC bus.
Switch the Power Bank on. The unit will synchronize to the energized AC bus and resume participation in load sharing
If the breaker trips repeatedly, investigate system sizing, protection coordination, and load conditions before attempting another restart.
5.4. Battery Recharging
In a Victron Microgrid system, each Power Bank manages and charges its own battery independently.
While all Power Banks share the AC bus for load supply via Hybrid Droop control, the Microgrid AC bus is not intended for coordinated battery charging between Power Banks.
Each Power Bank must therefore be equipped with its own DC-side charging source, such as:
DC-coupled PV (MPPT charge controllers)
DC genset
Dedicated AC charger supplied from an external source (e.g. grid or separate genset)
Battery management – including state of charge monitoring, charge control, and protection limits – is handled locally within each Power Bank. There is no automatic state-of-charge balancing between different Power Banks.
Note
To enable battery charging from the grid or an AC genset, an additional set of inverter/chargers may be operated in “charger-only” mode.
When using VE.Bus inverter/chargers, their separate VE.Bus network can be connected to the GX device of the Power Bank via USB, using an MK3-USB interface.
5.4.1. Important considerations for simultaneously fully charged Power Banks
When multiple Victron Power Banks are connected in Microgrid there are some additional requirements at the end of the battery charge cycle.
For overall system stability, hybrid droop control needs the ability for at least one of the Power Banks to absorb some power, as the system finds equilibrium during adjustments to voltage and frequency.
If all Power Bank batteries are full and charging continues, the surplus energy has nowhere to go. This can cause power to circulate between Power Banks as they try to find equilibrium between demand and supply, and potentially overcharge the batteries (which will cause the Power Bank to disconnect from the AC bus).
To avoid this, apply at least one of the following charging strategies.
Strategy 1: Don't fully charge all Power Banks at the same time.
Configure charging so that at least one Power Bank always remains below fully charged at all times.
This Power Bank then provides charge headroom and can absorb surplus energy. That allows it to keep participating in droop-based load sharing and helps maintain AC bus stability.
Strategy 2: Minimum base load when simultaneously fully charged
If all Power Banks are fully charged at the same time, other measures are needed.
Maintain a minimum AC base load: Keep a continuous load of at least 2% of the combined nominal power of all connected Power Banks on the AC bus. This can be provided by a dedicated dump load or another permanently connected load.
The base load provides real demand on the AC bus, so droop control can continue to regulate the system correctly.
Note
This minimum base load requirement only applies when battery voltage of all Power Banks approaches or exceeds the configured float voltage and there is active charging.
There is no base load requirement when the batteries are at less than full states of charge, when the batteries can absorb the power instead.
5.4.2. Why imbalances occur
In a Microgrid system, each Power Bank manages its own battery independently.
Over time, differences in battery state of charge between Power Banks will naturally develop due to variations in DC charging supply (e.g. differing PV array sizes, orientation, or shading conditions), differences in battery capacity or age-related degradation, and individual Power Bank loading history if droop parameters have been intentionally offset.
These imbalances are a normal characteristic of Microgrid operation and must be understood and managed to maintain reliable system operation.
What happens when a Power Bank's battery is depleted
As a Power Bank's battery approaches its low state of charge or low voltage disconnect threshold, the battery management system (BMS) or the inverter/charger's internal protection will reduce and eventually cease discharge.
Specific behaviour depends on the battery type and BMS configuration, but the general sequence is:
Once the battery reaches the disconnect threshold, the inverter/charger shuts down and the Power Bank ceases to contribute to the AC bus.
The internal transfer switch opens, electrically disconnecting the Power Bank from the Microgrid bus.
The remaining Power Banks on the Microgrid AC bus absorb the shortfall through Hybrid Droop control. The bus frequency shifts slightly, causing other units to increase their output proportionally.
A depleted Power Bank does not become a load on the AC bus. The inverter/charger does not draw power from the Microgrid to charge its own battery via the AC input in normal Microgrid operation.
When a Power Bank disconnects due to a depleted battery, the total available inverter capacity on the Microgrid AC bus is reduced.
The remaining Power Banks will automatically absorb the additional load. If the remaining capacity is sufficient, the system continues to operate normally with a slight shift in bus frequency and voltage to trigger the increased per-unit loading.
If the loss of a Power Bank causes the remaining units to exceed their rated capacity, the system may become overloaded. In this case, frequency and voltage will deviate beyond normal operating limits and individual units may trip on overload or overcurrent protection. For this reason, system sizing should account for the possibility of one or more Power Banks dropping out due to battery depletion.
Monitoring and early intervention
Where a GX device is installed in each Power Bank, monitor battery state of charge across the Microgrid with a VRM Portal installation group.
Significant divergence in state of charge between Power Banks is an early indicator that one unit may deplete and shut down before the others.
If monitoring reveals a persistent SOC imbalance, investigate the underlying cause. Common factors include unequal DC charging capacity between Power Banks (e.g. one PV array is undersized or partially shaded), a degraded battery in one Power Bank with reduced usable capacity, and droop parameter offsets that are causing one Power Bank to deliver disproportionately more power than its charging source can sustain.
In systems without GX devices or remote monitoring
Establish a regular inspection schedule to check battery voltage or state of charge on each Power Bank. Address significant imbalances before they result in a Power Bank disconnecting unexpectedly under load.
Power Bank has disconnected due to a depleted battery (low battery voltage)
The Power Bank will automatically rejoin the Microgrid when its battery is recharged (e.g from solar the next morning). The unit does not need to be restarted manually, once the battery voltage is sufficient it will detect the energised AC bus and re-synchronise.