This article is a companion to the primary documentation on using the BMS to implement a pool of batteries in a Redflow storage array to be running in Standby Power System (SPS) mode.
Note that successful operation as documented here requires the use of BMS software version 1.9 or later, and the use of Redflow ZBM2 battery controller version 32.18.04 or later in all connected batteries.
You should start by reading that primary SPS configuration documentation first, to understand how SPS mode works and the general strategy and method of configuring it with a Redflow BMS.
In this article, we discuss how to set up and operate a Redflow based energy system where all batteries are in SPS mode.
Applications for a complete SPS based Redflow Energy system
The applications for an 'entirely SPS' energy system are those in which you want to use the energy system as a replacement for another 'backup' energy source such as a standby diesel generator.
If you run a Redflow battery array with all batteries in SPS mode, you achieve an outcome very similar to a standby diesel generator, with some distinct differences:
- The battery array is charged up and then all batteries, as they fill up, automatically go into Hibernation (Standby). At that point, all batteries in the array consume no ongoing energy and have no self-discharge rate - they are genuinely 'asleep'
- When energy is required from the batteries (e.g. as a backup to the failure of some other primary energy source), the SPS mode batteries are activated automatically. This activation can be driven by the BMS (if it remains powered up at the time), or it can be the case that the batteries self-activate via a built-in fail-safe DC bus voltage level trigger point being reached.
- When SPS batteries are brought out of hibernation, the process (like starting a generator) is not instant.
- The startup time for an SPS battery varies, depending on the time since the battery was hibernated. It can be just a few seconds, through to a typical startup time of well below a minute, through to a worst-case of about two minutes. Regardless, this startup time is comparable to the time required by a diesel standby generator to start, come up to operating speed and stabilise, and to then come fully online.
- In the interim period, while starting, the SPS batteries are not providing DC bus power - they connect to the bus (automatically) only once fully able to deliver their full rated output energy.
In effect, a 'fully SPS' battery array acts like a fossil-fuel-free standby generator.
Just like a standby generator, the array can not supply energy until it has fully started up.
This means that the application concerned needs to tolerate the absence of DC energy until the SPS array has fully started and come online. In this 'standby/emergency backup' role, many applications are tolerant of this loss of DC power during the startup period.
If you do require a site to stay 100% online (in terms of DC power being continuously provided), you can achieve this by configuring the SPS system to leave a small number of Redflow ZBM2's in RUN mode. For instance, a site with (say) 8 batteries might decide to deploy 2 in RUN mode for normal operations, with the other six batteries charged in SPS mode and hibernated in case they are required.
Self-Booting of an SPS battery
One issue of relevance in a 100% SPS environment is that energy is required by the ZBM2 to achieve the boot-up process after the external DC bus power has failed.
The ZBM2 requires only a small amount of energy to boot itself up (around 80 watts for the first 30 seconds or so of the startup period), before the stack starts to be able to deliver energy from the incoming electrolyte fluid reaction on the stack and continue the startup process with its own internal energy.
If there are RUN mode batteries on the bus, they can supply this startup energy.
If there are no RUN mode batteries available, and if the hibernation has been for an extended period, there may be no DC bus power left available on the ZBM2's internal stack with which to start itself up.
For scenarios where the SPS batteries will not be hibernated for periods of more than a few weeks, there is typically enough energy left inside the stack plates to achieve self-starting without external assistance.
If the operating environment is expected to include long periods without SPS activations from hibernation (more than a few weeks between activations), then Redflow can supply a modified version of the electronic controller box attached to the ZBM2, which contains a small cluster of ultra capacitors inside the control box. These ultracaps store enough energy to be sufficient for the ZBM2 to bring its pumps up to speed and start to deliver electrolyte to the stack. The stored energy in the stack can then complete the startup process.
When a ZBM2 is running and either charging or discharging, it can generate enough internal heat from normal operation to keep its internal electrolyte temperature at a suitable operating temperature. However, It is important to appreciate that SPS batteries when hibernating are unable to keep themselves warm in this way - and that they are not able to restart and deliver energy in adversity if the internal electrolyte temperature falls below 15C.
To ensure energy is always available, the BMS controlled SPS system will automatically wake SPS batteries up if the electrolyte temperature drops below 17C, in order to avoid the scenario where energy is required but the battery is unable to safely wake itself up.
In environments where the ambient temperature inside the battery enclosure or room is potentially able to fall below 17C for significant periods (more than a few hours at a time), the use of cabinet heaters is required to avoid the need to have the BMS restarting SPS batteries and discharging them, merely to keep them warm.
A small cabinet heater is an inexpensive and simple resolution of this potential issue, where extended hibernation time is required in colder environments.
Configuration of 100% SPS operation
To operate with all batteries in SPS mode, and to avoid the BMS waking batteries too soon, a number of the normal BMS-controlled SPS startup triggers need to be set to 'Disabled'. If this is not done, then the BMS will activate SPS batteries as soon as there are no RUN mode batteries left on the system.
The primary trigger for a fully SPS array is the loss of DC bus voltage, which will trigger automatic battery self-start based on the Low Voltage 'Fail Safe' voltage.
This voltage is calculated to be the BMS-controlled startup voltage minus a configurable additional voltage buffer.
For the 'all SPS' scenario, the BMS controlled voltage start level should be set to no higher than 40V, with the Fail-Safe voltage margin being set to at least 2 volts. This results in an effective emergency start voltage level of 38V (or less).
With this setup, the SPS batteries will hibernate and disconnect, and will all self-activate as soon as the external DC bus voltage falls below that 38V (or similar) trigger point.
The system will then self-energise and devices connected to the SPS array can then start to reboot.
Note that this voltage value is important - it should be below the DC bus voltage normally experienced on the system once all SPS batteries have hibernated and unplugged.
On a Victron based Redflow energy system, the Victron system is configured normally to deliver a 40V 'sustain' DC voltage when all batteries are disconnected. This is why the fail-safe voltage for SPS mode has to be below 40V in order to avoid 'false start' triggers occurring ahead of a real need for the energy.
Here (below) is an example of the BMS SPS page, configured for a one battery SPS based backup-power situation, in the manner described above. This site will self-start its hibernated SPS battery if the DC bus falls below 38V, and will then start to support the site operation until it is fully discharged:
Battery SPS/RUN rotation is disabled in the All-SPS case
In the 'all batteries in SPS mode' scenario as described here - specifically where the 'Target Number of SPS Batteries' on the BMS SPS page is selected to be equal to the total number of physically installed ZBM2 batteries, then the normal SPS-mode battery rotation system (taking batteries out of SPS and back to RUN mode after each discharge) is disabled.
Instead, in this case, all batteries are rotated from RUN mode to SPS mode as their maintenance completes, and then they will remain in SPS mode after each discharge, ready to be recharged and re-hibernated again immediately.
If the number of SPS mode batteries is later selected to be a lower number than the total number of connected ZBM2's, the normal pooling behaviour will operate - where each battery completing SPS discharge is returned to RUN mode, and a new battery completing RUN mode maintenance is later rotated back into the SPS pool to take its place.
Configuring a Victron system to work automatically with a fully SPS based battery array
There is a challenge to be solved, in the 'Entirely SPS' scenario, about how to both allow the battery to be charged (e.g. from a grid) , but how to also allow it to fully discharge into the site load once the grid (or other) external energy source comes back.
If a Victron system is programmed to 'keep batteries charged', this can mitigate at times against the battery being able to complete its own discharge process.
There are 'sophisticated' approaches to managing this. For instance, the BMS's internal 'Digital I/O' rule system can be programmed to sense when SPS batteries need to be charged, and can use that situation to trigger a change in the Victron 'grid set-point' to force grid charging; This can then be de-activated automatically when all SPS batteries are charged. This can be done, for instance, by setting up trigger rules in the BMS that adjust the 'grid setpoint' in the Victron GX system based on the value of the field 'Permitted_Charging'.
However, there is a simpler way, which achieves the desired result in a more pragmatic manner.
The Victron Energy ESS 'Scheduled Charging' feature can be used to solve this. Simply program a 'Scheduled Charge' for a suitable daily time period ( see example screen shot below ). During that time period, the Victron system will grid-charge the battery array if it is capable of being charged.
Then, if one or more SPS batteries need to complete their discharge after activation, they can do this during the remaining hours of each day. Once discharged and maintenance is completed, they will recharge again and hibernate via the next activation of the 'Scheduled Charge'.
The Scheduled Charge function can be found under the Victron GX "ESS" menu (at the very bottom), as per the screen shots below.
BMS and site energy system controller power
It is up to the integrator whether to provide an auxiliary backup power source to keep the Redflow BMS and the on-site energy system controller running across the DC bus voltage failure and SPS startup time.
The BMS is tolerant of being de-powered in this manner, but the disadvantage for the BMS and the site energy system controller of losing power is that there will be no active system logging happening to fully record the power loss and the startup process in local and remote log files.
This can make it quite hard to know when the system has done the right thing!
It is recommended that site installers use a small DC UPS device to provide power to the BMS, to the site energy system controller, and to any other highly critical low-power-draw DC devices, and that this UPS can provide power for long enough to allow the SPS array to start up and start to re-power other devices.
An example of a suitable device for this role is the EdgePower 24V product from Ubiquiti, which can be in turn backed up with a conventional 12V gel-cell backup battery.