Standby Power System (SPS) mode is a novel software-controlled operating mode that is supported by Redflow's ZBM2/ZCell battery technology.
A battery in SPS mode acts less like a battery, and more like a software-driven, non-fossil-fuel, backup generator.
Once it is fully charged, an SPS battery can remain in its 'hibernation/standby' mode for extended periods (months) with no internal self-discharge.
Once called upon, the SPS battery starts up rapidly and then delivers all of its stored energy to support site energy requirements. Once fully discharged, the SPS battery completes routine maintenance and is then ready to recharge.
The typical use case for SPS mode is in multi-battery sites where a subset of batteries are charged and held in SPS mode as a 'reserve power supply'. Routine site operation occurs with batteries in the standard 'RUN' mode, until or unless a deficit of site energy exists. This may be because of an extended period of low solar generator, or because of the failure of the on-site grid and/or generator systems.
In response to such an energy deficit, the BMS automatically activates one more more SPS batteries to deliver energy and support site operations with their stored energy.
Use of this feature requires the Redflow BMS to be running version 1.9 and Battery Controller firmware version 32.18.04 or later.
You can upgrade the BMS using the "Tools->Upgrade BMS" BMS menu.
Once the BMS is upgraded, use the "Tools->Upgrade Battery" menu to load the latest battery controller firmware into each connected battery.
SPS operational sequence
Once placed into SPS mode, a ZBM2 initially acts like any other ZBM2. It starts up and commences charging when energy is available on the DC bus to allow it to do so.
SPS batteries are shown separately, below batteries in the standard 'RUN' mode, on the BMS status page, like this:
Once the battery is completely full, it automatically enters the SPS 'Hibernation' mode.
In SPS hibernation mode:
- The battery flow pumps are completely stopped and electrolyte drains away from the stack
- The battery retains its stored energy with no background self-discharge of stored energy, just like a standby generator (but with no fossil fuel required!)
- When called upon at a future time to provide energy, the battery restarts its pumps and becomes capable of supplying its full rated energy output within 15-20 seconds (typical) and within 60 seconds (worst case)
Once the battery is re-activated to supply energy to the site load, the requirement is that the battery must fully discharge and undergo a normal ZBM2 maintenance cycle before returning to service.
To ensure the battery energy is delivered fully to the site loads and to allow the battery to recharge as soon as practicable (ready for next time), the discharge process is assisted with the use of the EED (Energy Extraction Device) included with all new ZBM2's. This device assists in attaining a minimum output rate of 1kW per ZBM2, and a maximum discharge rate consistent with ZBM2 specifications. If site demands are below 1kW, the EED can also 'cross-charge' into other running batteries.
SPS Battery Designation, Activation, and Rotation
When a subset of batteries on a site are to be operated in SPS mode, the BMS arranges to automatically 'rotate' batteries between 'RUN' and 'SPS' mode over time. This rotation is intended to ensure that there is 'wear levelling' across the site, with all batteries taking turns to be designated as 'RUN' vs 'SPS' batteries over the passage of time.
The desired number of SPS batteries is configured as a target value on the SPS configuration page in the BMS.
Transitions between RUN and SPS mode occur only at the end of a battery maintenance cycle.
Each time a battery in RUN mode completes maintenance, it becomes a candidate to be added to the SPS pool. If the SPS pool needs more batteries to reach its target count and a battery in RUN mode completes its maintenance cycle, the BMS can move it to SPS mode.
Each time an SPS battery (having been awoken from hibernation) completes energy discharge and maintenance, it becomes a candidate to leave the SPS pool at the end of the maintenance cycle.
Accordingly, it can take several days for changes in the target number of SPS batteries to be reflected in the running system when it is first configured.
Activations of SPS batteries to service site load, that result in the battery being rotated out of the SPS pool, can (again) take a few days to be replaced in the SPS pool by other RUN mode batteries.
SPS Mode Configuration
There are two pages on the BMS that are central to the operation of SPS mode under BMS control.
The primary configuration of SPS operation is achieved via the "Configuration->SPS page":
The second other page related to SPS automatic operation is the "Configuration->Battery Optimisation" page:
To enable BMS controlled SPS operations, go to the "Configuration->SPS" page.
Select the Enable SPS checkbox at the top to start the configuration process. Once all desired changes have been made, press the Submit button at the bottom of the page to commit them.
SPS Battery Rotation Settings
Choose the Target Number of SPS Batteries to suit your application. If you select 'Disabled' the system will progressively move all SPS batteries back to RUN mode as they complete their maintenance cycles.
The Time Until Automatic SPS Rotation defines how long a battery can be in SPS Hibernation before the BMS activates it proactively (without an external energy demand). The intention of this function is to ensure that SPS batteries progressively cycle back through to RUN mode in the medium term.
The default is 30 days, and it can be raised or lowered to suit site requirements up to a maximum of 60 days
There are two mechanisms that will trigger the waking of an SPS battery due to abnormal conditions.
The Fail Safe Voltage Margin sets a buffer voltage that is the specified number of volts below the value in the Low Bus Voltage Threshold setting. For instance if the Low Bus Voltage Threshold setting is 45V and this Margin value is 2V, then the Fail Safe Voltage will become 43V.
This value is programmed into the battery controller directly. If the DC bus voltage ever falls below this value then all SPS batteries will self-activate. The intention of this function is to ensure that if the BMS is unavailable or disconnected, then all SPS batteries will self-wake before the DC bus entirely depletes.
The Fail Safe Electrolyte Temperature defines the temperature (of battery electrolyte, not ambient air temperature) below which an SPS battery will be automatically activated and discharged into the site load.
If SPS mode is intended to be used in a physical environment in which the ambient temperature experienced by the battery array will be below 15C for any significant period of time, a cabinet heater is required in order to ensure that a suitable electrolyte temperature for SPS batteries can be maintained.
Redflow ZBM2 batteries require an electrolyte temperature of 10C or above to charge, and 15C or above to discharge. Batteries in RUN mode can generally keep their electrolyte warm even during extended low ambient temperature periods, due to internal heating effects that are the normal consequence of their routine operation.
However, batteries in SPS mode are not consuming energy and hence are not capable of creating any internal heating to maintain their electrolyte temperature during extended periods of low ambient temperature.
Failure to keep the electrolyte temperature in the appropriate range means that the SPS battery would not be able to successfully awaken and deliver energy when subsequently required.
If this trigger activates, it should be taken as confirmation that cabinet heating needs to be installed in the site concerned.
SPS Battery Discharge/Maintenance Control
Once activated for any reason, an SPS mode battery is automatically placed into 'Discharge-Only' mode. This ensures that no battery recharging takes place until the SPS battery has completely discharged and undertaken a self-maintenance cycle.
The initial Discharge-Only mode allows battery energy to be consumed but it does not actively drive energy out of the SPS battery if there is insufficient site load to require that energy. Hence the battery can remain in the Discharge-Only mode, slowly self-discharging, for an extended period of time.
In most applications, having an SPS battery at a low state of charge, unable to recharge, and yet not actively discharging, creates the potential for reduced storage array availability over time. It is generally preferable to proactively discharge the battery in order to allow it to complete its maintenance cycle and recharge again - ready for the next demand upon its stored energy.
The Enter Maintenance Below (SoC) setting defines the SPS battery state-of-charge below which the battery will be switched into Maintenance Discharge mode. In this mode, the on-board Energy Extract Device (EED) on the battery is activated. This device is a DC/DC converter that delivers a higher DC output voltage (typically 57V) to the DC bus.
In most sites, this results in battery energy being proactively delivered into site loads, or into other ZBM2 batteries, or (where present) into an external grid, at a minimum output level of 1kW (hence a maximum of circa 10 hours to fully drain the battery), rather than having the battery potentially remain idle for an extended period in discharge-only mode.
Note that this function does not limit output power to only 1kW; If the site energy demand is higher, the battery will deliver energy out at up to the full rated output power possible for the ZBM2 to supply.
The default (and normal) value for this setting is 100%, which means the battery (once activated) is immediately promoted into Maintenance mode, for the most timely delivery of energy and the most timely return to service of the SPS battery.
However, in sites where an extended period in non-EED 'DIscharge-Only' mode is desirable, this SoC threshold can be reduced to a lower level. The battery will then operate in Discharge-Only mode initially, before switching to EED-augmented 'Maintenance' mode when the SoC threshold is reached.
Startup Triggers for SPS Batteries in Hibernation
This configuration section controls the normal start triggers to take one or more batteries out of SPS hibernation and commence their delivery of energy to the site. Note that the BMS dynamically selects the most appropriate SPS battery or batteries to activate when required, favouring the activation of those batteries that have been hibernating for the longest period to date.
The first pair of settings, Batteries To Start At Low SoC and the Start State Of Charge Limit will activate the specified number of SPS batteries if the aggregate SoC of all actively running batteries in the system falls below the specified level.
This is the typical start mechanism for SPS batteries - causing them to be activated in response to a low level of remaining energy in the active batteries on the system, in just the same way that a generator can be triggered due to low site SoC. You should consider the interaction between this setting and the start thresholds configured any actual generators on the site, to ensure that the desired energy delivery sequence occurs.
By activating a subset of the overall number of SPS batteries, site energy demand can be progressively supported by multiple SPS batteries in an energy-efficient manner.
The next three settings, namely Batteries To Start At Low Bus Voltage, Low Bus Voltage Threshold and Low Voltage Start Delay collectively define the criteria to activate one or more SPS batteries in response to sustained low DC bus voltage.
Low DC bus voltage can be caused by imminent depletion of running battery energy or it can be caused by a substantial increase in site energy demand that in turn drives DC bus voltage lower regardless of running battery SoC.
The Low Voltage Start Delay setting is intended to allow for transient high energy demand on the battery array without necessarily activating SPS batteries in immediate response. A sustained Low Bus Voltage condition (longer than this delay setting) will result in the activation of the specified number of SPS batteries (if available).
The High Discharge Current Threshold functions independently to the previous settings, and is a distinct activation control for SPS batteries. If the BMS observes any running battery delivering energy at above the specified current level (default 100A) for more than one minute, the BMS will activate one SPS battery (if it is able to), in order to help to support the site load. In sustained (very) high load conditions, this may result in more than one SPS battery being activated over time (on a progressive basis) until the highest observed battery discharge current falls below this threshold.
SPS battery Charge Optimisation
The BMS supports an optional Battery Optimisation feature set ("Configuration->Battery Optimisation"). This feature set is able (amongst other things) to optimise charging outcomes on sites where the maximum charge power available is not sufficient to charge all site batteries at once.
To allow the BMS to assist with the timely recharging and hibernation of SPS batteries for later use, on sites that can not charge all connected batteries efficiently at once, it is possible to selectively prioritise SPS vs RUN-mode batteries during the battery charging process.
This prioritisation is achieved using the BMS Battery Optimiser page, and specifically the Charge Optimisation settings on that page:
If the Battery Optimiser is activated, the Charge Settings panel (as shown above) can be used to accelerate the charging of SPS batteries, so they can be fully charged and hibernated as a priority, if desired.
First, consider changing the Number Online For Charging value from its default (All). Setting a lower number here will result in the BMS permitting charge for only that specified number of batteries.
As battery charge level reaches the specified Charge Maximum SoC Threshold, additional batteries are allowed to charge. Additional batteries can also be added and removed from the set being charged on a dynamic basis, using the Charge Current Upper Threshold and Charge Current Lower Threshold settings. In this way, optimum charging rates can be achieved in sites with insufficient charging energy sources to charge all connected batteries at full rate at once.
These are general Charge Optimiser functions which apply to any site with batteries in RUN mode.
SPS Specific Charge Prioritisation
The Run SoC Before Charging SPS is used to prioritise the timely charging of SPS mode batteries, relative to the charging of RUN mode batteries.
This setting drives specific behaviour to optimise overall energy availability across the entire charging cycle, and it works as follows:
Initially - when the overall SoC of the overall system is below this threshold (10% by default), all RUN mode (non-SPS) batteries are charged first, with SPS batteries left disconnected. The intention of this initial charging phase is to ensure that at least some energy is stored in active, instantly available, RUN mode batteries to ensure that energy is available for instant and/or transient site energy demands.
Once the system SoC reaches the specified threshold level, however, the optimiser then switches to proactively charging SPS batteries as the priority. It dynamically defers further charging of RUN mode batteries to the extent required to achieve this prioritisation of SPS battery charging.
This results in all SPS batteries being fully charged and automatically hibernated as soon as practicable, so the SPS battery pool can be available to be called upon if required.
As SPS mode batteries become fully charged and hibernated, RUN mode batteries are progressively added back to the charging set until all of the batteries in the whole energy system are fully charged.
Manual Configuration and SPS Startup Diagnostics
If required, batteries can be manually transitioned at any time between RUN and SPS mode using the BMS.
This is achieved by clicking on the required battery on the BMS status page, selecting the 'Operations' button at the top right of the battery data screen that appears, and then selecting the option to 'Take Battery Online' or 'Take Battery to SPS mode' (as appropriate).
If an SPS battery is triggered to leave hibernation and the reason for this is unclear, note that these event triggers are logged in the primary BMS event log.
This log can be found by selecting the 'Logs->BMS Logs' menu item and then clicking on the 'Event' link on the page that appears.