This article discusses some characteristics and issues that need to be understood when designing a system using Redflow ZBM2 modules, compared to other battery technologies.
Integrators considering working with Redflow ZBM2's should study this article carefully and should also engage in Redflow product training by contacting Redflow directly before commencing their deployments of Redflow products in the field.
Periodic self-maintenance cycles
The Redflow ZBM2 needs to periodically undertake a 100% discharge and to spend a couple of hours offline doing self-maintenance. This process is the secret to the long life and 100% discharge capability that we have - each time this happens, the battery is returned to service pretty much 'like new!'.
The maintenance cycle usually runs after a maximum of 72 hours of battery operating time. The 72 hour 'clock' runs when the battery is running normally.
That clock does 'stop' in specific situations (extending the time between maintenance cycles):
- When the battery is empty
- When it is offline
- When it is fully charged and hibernated in Standby Power System (SPS) mode
Where multiple batteries are installed in a system in parallel, these maintenance cycles are automatically controlled by the Redflow BMS, to automatically maximise overall system availability. This is done by sequencing maintenance cycles in an appropriate manner (and in a timing regime configured and controlled by the BMS).
Even with a single battery, in most applications, the BMS makes the maintenance cycle practically invisible by causing it to trigger at the most appropriate point in the daily usage cycle of the energy system concerned (typically in a solar-driven site, the start of discharge for any "maintenance-due" modules is commenced, automatically, at sunset)
This self-maintenance cycle, its implications, and its management (via the Redflow ZCell BMS) is explained and discussed in detail in the training courses that are held for Redflow system integrators.
The Redflow BMS provides for simple handling of this cycle via adjustable system configuration settings that take advantage of (and synchronise in with) the nature and charge/discharge timing of the customer application cycle concerned.
In most cases (especially in multi-battery situations) the maintenance cycle becomes effectively invisible.
That said, there are specific combinations of battery count and application that are particularly well suited and/or not well suited, to this product.
Here is a table illustrating the mapping between application type and suitability for the use of the Redflow ZBM2 product:
Power Conversion Electronics (PCE) must support continuous provision of DC power onto the DC bus to bootstrap a Redflow battery array from completely empty
The Redflow ZBM2 maintenance cycle requires that each battery in a parallel battery array periodically spends a few hours totally empty and totally depleted (no energy left in the unit). At this time, the maintenance cycle is completed with the support of energy drawn by the module from the DC bus.
In a multi-battery system, the BMS ensures that (as much as is physically possible), at least one battery is 'not empty' at any given time. It orchestrates maintenance cycles with this specific intention, and it does this automatically. Hence, on a normally running multi-battery system, this requirement is effectively 'hidden' (avoided) almost all of the time.
Of course, in a single battery system, this maintenance cycle cannot be 'shielded' from the inverter/charger and has to be handled properly by the power conversion electronics.
Even on a multi-battery site there are still edge cases - including specifically 'black start' (and also some specific potential failure modes) where this automatic 'support power' is required. The BMS can signal when this is needed, and/or the power conversion electronics can simply provide some DC energy onto the bus on a routine basis all the time as a 'baseline' (e.g. 'rectifier' style DC systems)
No matter how many ZBM2's are in a system, the inverter/charger must be able to cope with an 'all batteries de-energised' scenario on the DC bus.
It should ideally do this with the PCE continuously providing that energy (supporting an on-board controller and the fluid pumps) until the ZBM2's return to normal 'battery' operation. The requirement is not for a great deal of power - of the order of 2-3A per ZBM2 module on the bus. But it has to be there, and it has to be there on a continuous basis.
The BMS will tell the PCE what is needed, in terms of volts and amps... but the PCE needs to cope with providing that energy to an array that presents (initially, during bootstrap) as a simple DC load, not as a battery.
Alternatives/Workarounds for the supply of 'sustaining' energy to the DC bus
The Redflow batteries don't mind if they wind up completely de-powered for an arbitrary period of time. This will do them no harm at all, but obviously they can't support site loads at that time.
There are a few potential workarounds if the inverter/charger doesn't supply this 'sustaining' voltage by itself:
1) This can be achieved by using energy supplied from DC MPPT-attached solar panels on the DC bus when the sun comes up tomorrow; or
2) This can be achieved using an 'auxiliary power supply' (with a diode in-line to protect against back-flows of energy). The Redflow BMS can be easily programmed to use its on-board relay to signal when this auxiliary supply is needed, and when it is not.
The second option (an auxiliary power supply) is the least preferable answer, because there are edge conditions that can develop around the process of switching this supply into, and later out of, the DC bus environment.
There is also the potential for the inverter/charger to 'mistake' it for a battery and attempt to support building loads from it, leading to a need, typically, to also have the inverter/charger commanded into an 'idle' state when the auxiliary power supply is on. In summary, there is a 'dance' here and the dance happens at some distinct risk of the dancer tripping on their own shoelaces.
PCE needs to handle the scenario where all batteries are full and electrically blocking further charge
As each battery in a Redflow battery array fills up completely, and also during some parts of its discharge cycle, the battery blocks further charge by switching a diode into the DC energy path.
The PCE in use needs to cope with the scenario where some, and at times all, batteries are blocking further charge. The BMS will present a charge limit of 0 to the PCE at this time, to request that no further charging occurs.
The point, though, is that the PCE needs not to become 'upset' by a scenario where all batteries are blocking charge. Some PCE assumes that there is always some capacity for transient absorption of charging energy from the PCE, even when the charge limit is advertised as zero. In a Redflow array, this is not the case.
In some scenarios, the addition of an Ultracapacitor onto the DC bus can be considered. This acts as both a transient 'charge surge buffer' and also as a buffer to assist with transient (very) high energy draw surges from the PCE to the load.
Requires an inverter/charger compatible with flow batteries
The previous points have critical design consequences in terms of the choice of Power Conversion Electronics (PCE) in Redflow energy system designs. Dealing with the two scenarios noted above, smoothly, is the essence of what is required for PCE to be correctly described as 'compatible with flow batteries'.
Pure DC (e.g. Telco) customer applications
For pure 48V DC applications, the Redflow ZBM2 works easily and automatically with standard 'Telco' industry rectifiers. These are designed to impose and maintain an appropriate DC voltage and current based on a nominal 48V bus. Other than configuring the appropriate target DC charge voltage, such rectifiers generally work without change on a Redflow based deployment
AC inverter/charger based customer applications
For applications where the 48V DC storage modules need to be converted to and from AC power systems (or or off grid), it is important to appreciate that only a limited number of inverter/charger products on the market currently work with flow batteries (including Redflow ZBM2 modules).
At the time of writing, in particular, there are only two product lines that work with (and are supported to work with) Redflow ZBM2 modules:
1) The Victron Energy product line including the MultiPlus and Quattro inverter/chargers
The Quattro and Multiplus product lines, used with the Victron Energy "GX" energy management system devices, work really well with Redflow ZBM2 battery strings.
Victron Energy's products automatically generate the appropriate 'sustain' DC voltage and current source on the DC bus during Redflow battery maintenance cycles. The Redflow BMS CANBus interface works on a 'plug and play' basis with the Victron Energy "GX" energy management system devices.
2) The TRUMPF Hüttinger "TruConvert" product line
The TRUMPF products are purpose-designed for flow batteries. They automatically provide support to raise the DC bus from 0 volts to the pre-charge voltage required by the Redflow ZBM2 and fully support black-starting of flow batteries in general.
The Trumpf flow battery DC1008 and DC1010 energy converters transform the 48V DC bus to a high voltage (750-900V) DC bus.
This high voltage DC bus can be connected either to a cluster of TRUMPF AC3025 inverter/charger modules or to any suitable third party high voltage DC inverter/charger (e.g. ABB PCS100, Dynapower, etc).
Redflow has created a product that integrates a cluster of ZBM2 batteries, BMS and a cluster of Trumpf DC/DC converters into a single outdoor cabinet called a "Pod-Z" (https://simonhackett.com/2021/05/17/pod-z-the-redflow-grid-scale-hvdc-energy-pod/)
Using one or more Pod-Z units, a Redflow battery cluster can be integrated with AC inverter/chargers that can operate somewhere in the 600-900 volt DC range on the DC side.
Pod-Z presents a high voltage DC interface for battery energy storage and delivery (and a BMS interface) in a manner that avoids any need to be concerned about the issues the edge cases in the ZBM2 operating cycle.
Pod-Z units can be combined to create very large systems.
A variety of high voltage DC inverter/chargers exist that have the capacity to work with Pod-Z units (e.g. Dynapower, ABB PCS100 etc).
Additional Devices Under Test
Redflow is currently evaluating additional brands of inverter/charger operating in the 48V DC range for potential addition to the list of Redflow supported and compatible energy handling devices.
Once Redflow has successfully integrated with additional brands, these will be listed on this page.
It is crucial to appreciate that use of ZBM2s with inverter/chargers that are not specifically supported by Redflow is a really bad idea, with a high potential to end in tears.
If you are interested in using another specific brand of inverter/charger product with Redflow ZBM2 modules, feel free to get in touch. However, please be aware that this is impossible to achieve without the direct, high level, support of the inverter/charger vendor concerned, working directly with the Redflow integration lab to create a compatible setup.
A number of other brands have been tried by installers without Redflow support, and 100% of those integrations have failed to work without this high level manufacturer-direct support.
Selectronic Inverter/Chargers: Special Note
Redflow had progressed substantially with an integration into the Australian designed "Selectronic" product line, but unfortunately Selectronic elected to terminate the integration process with Redflow without resolving some critical firmware issues that exist on the Selectronic platform and ceased cooperation with Redflow toward that outcome.
We believe this is due to a perceived lack of sufficient market opportunity to justify the software effort on their part, not withstanding that the firmware changes required are relatively minor.
The issues that Redflow encountered - requiring change by Selectronic to accommodate them - are only relevant to flow battery integration and have no impact for conventional battery types. However, these issues make it impossible to complete the integration process without Selectronic's active cooperation.
If you are interested in a Selectronic<->Redflow integration, please request this from Selectronic, and perhaps they may be prepared to reconsider this decision. We would be open to them doing so and we stand ready to complete the process with them.
Not a solid-state device
A flow battery moves electrolyte fluid around using pumps.
There are moving parts (albeit only two of them - two pumps) and there is fluid in tanks that creates a theoretical risk of electrolyte spill if one of the tanks developed a leak.
The electrolyte is a zinc-bromide fluid that is far less challenging to handle than, say, battery acid from a lead-acid battery, but nonetheless (and as is appropriate), the ZCell enclosure includes internal 'secondary fluid containment' (the industry term is 'bunding') to catch and hold the fluid in the unlikely event of a leak in the primary HDPE plastic electrolyte tanks.
ZBM2 modules must be wired in parallel (series wiring not supported)
The ZBM2 happily coexists with other ZBM2's on the same parallel-wired bus at different states of charge at the same time. This is quite unusual for batteries - and it allows system designs to be quite flexible in terms of sequence charging/sequence-discharging ZBM2 modules over time. It also means there is no requirement for 'balancing' of multiple batteries in a system - the concept is simply not applicable to the Redflow product.
The ZBM2 includes on-board CPU and pump systems that require DC power to operate, including to complete the periodic maintenance cycle. The ZBM2 is designed to work in parallel-wired configurations, where the other ZBM2s and/or the PCE driving the DC bus can supply this power to support the Redflow battery maintenance cycle automatically when required.
The ZBM2 stack is entirely de-energised during the core of the maintenance cycle. As a result, ZBM2 modules cannot be wired in series, because any battery undertaking maintenance then 'breaks the chain', resulting in a disconnected DC bus. In addition, that 'broken chain' means it becomes impossible to power the internal on-board systems on the battery (CPU and pumps).
Typically, series wiring is used to raise the DC voltage of the overall system from 48V to some higher voltage.
High voltage, including grid-scale, DC storage arrays can be built with Redflow batteries, using clusters of parallel-wired ZBM2's connected to clusters of flow-battery-specific bidirectional high voltage DC/DC converters from Trumpf (see above).
Suitable only for stationary energy applications
The use of moving fluid, and the manner in which the battery uses that store and recover energy (by depositing zinc onto plastic electrolyte layers to charge, and recovering the zinc into solution to discharge) depends upon the battery being stationary and level while it is operating.
Hence this battery can not be used in moving vehicles or in situations where the battery will be subjected to changes in level or changes in g-force during when actively charging or discharging energy.
Not totally silent
As the (very quiet) pumps move fluid through the electrode stack during normal operation, the battery makes a small sound. Its a low volume, low intensity sound - a 'burbling' noise not unlike a garden water feature. Its arguably quite a relaxing sound - and it is also attenuated by the ZCell enclosure.
There is also a fan included in the system that can start up during high temperature / high workload operations, to cool the fluid electrolyte with ambient air drawn in from the outside. This fan has a rated maximum noise level of 55dB when operating at the highest operating speed normally expected in this application. It is, however, variable speed (runs slower when it has less work to do), and it only runs when needed.
The maximum operating speed of the fan can also be limited (as a software-adjustable setting) in scenarios where very low operating noise is a requirement, with the tradeoff being the potential for the battery to be less capable of remaining within its normal thermal operating limits during severe weather.
An 'energy' battery (not a 'power' battery)
Our battery is a marathon runner, not a sprinter.
The maximum charge/discharge rate of this battery (in kW) is below its nameplate output capacity in kWh (the industry term here is that "C" is less than 1).
In other words it typically takes up to circa 3-4 hours to drain the battery completely from full, depending on the discharge rate being used.
We actually consider this an advantage, but it is also a key technical differerence compared to lithium batteries.
Lithium chemistry can support the output of peak energy levels in kW (for short periods) at significant multiples of their 'nameplate' storage capacity figure in kWh).
That said, when Lithium is deployed in stationary applications it typically is configured to be output-rate-limited (in effect, to have a 'C' value held at levels quite similar to ZCell) to try to maximise its operating lifetime in the stationary energy/deep cycling environment.
Low temperature operating limit
The battery (currently) has a minimum electrolyte operating temperature of 10-15 degrees Celcius (10 during charge cycles and 15 during discharge cycles). It has a maximum electrolyte operating temperature of 50 Celcius. Outside of this range the battery will self-protect (disconnect) until the environmental conditions change.
When running, batteries keep themselves warm, and in particular can easily maintain their own internal electrolyte temperature in the optimal operating range (mid 20's C). Redflow ZBM2 modules have been demonstrated to stay at optimal internal temperature 'year round' even in very cold and elevated remote site situations, if they are running continuously.
In some sites where batteries are to be installed during very cold periods, or where batteries will be turned off and/or operated in Standby Power System mode for extended periods, the installation of a simple and conventional cabinet heater and/or the use of an under-battery heating pad (easily obtained and installed by your integration partner) can easily ensure that the electorate temperature remains at the optimal level (15C or warmer) at all times.
These devices are routinely used in electronic cabinets in cold environments for various reasons. Note that actual cabinet heating is only needed until the battery is running.
It is very important to appreciate that this is an internal electrolyte operating temperature limit and that outside ambient temperature can be substantially outside of this operating range for an extended period before this is an issue in practice.
Note that if the electrolyte temperature does reach an operating thermal limit then the battery will self-protect, meaning that it will disconnect from the system load to protect itself and then reconnect when conditions improve. Hence while your energy system may lose access to the battery in this scenario (as a worst case), the battery will come back online automatically, without damage, once the temperature has shifted.
When external ambient is beneficial in terms of adjusting internal electrolyte temperature (up or down), the battery uses its internal software-speed-controlled fan and pumps automatically to adjust its electrolyte temperature.
Lower round trip efficiency than conventional batteries
The battery has a round trip DC-DC efficiency quoted around 80%. Typical Lithium packs can run at a round trip DC-DC efficiency in the mid 90's.
This means it takes more energy to fill up our battery than to fill up an equivalent Lithium battery, before that energy is then delivered into a customer application load.
The offset here is that generally, finding more input energy isn't the problem in an energy system - if you have a solar array, its often feasible to make it (more than) large enough to get the job done.
In addition, as our battery ages, the expectation is that degredation will be expressed as a gradual lowering of that DC-DC round trip efficiency, but not as a loss of the capacity to output a full 10kWh of energy into your application load.
This means the battery does not need to be 'over-sized' up front to anticipate a long term loss of output capacity.
Suggested Further Reading
Here are some good links to explore next, to continue to explore the interfacing and operation of Redflow battery installations with the Redflow BMS:
Querying the Redflow BMS using MODBUS-TCP or JSON:
SPS operation guide:
Using SPS mode exclusively (no RUN mode batteries on the system):