Going off grid with Redflow’s ZCell? Here’s how to handle the ‘maintenance purge’.

simon and 2 batteries

Simon Hackett with an early customer’s double Zcell off grid system.

Yesterday, Aussie solar veteran Tony Smith left an interesting question at the bottom of our ZCell battery review. Tony asked, for an off grid application, how to handle the fact that the ZCell battery needs to shut down so it can perform maintenance mode every week or so:

Hopefully not a too stupid question ….


Ok so we use it in an off grid application. So far so good.


To avoid assumptions, off grid inverters need to be connected to the battery bank to stay alive. Disconnect the battery bank, they instantly turn into a boat anchor.


Our Redflow Wunderkind happily does its things for 2 weeks and there is great rejoicing. The owner issues a smug warning to his off grid neighbours laughing derisively at their prehistoric lead acid systems.


But then! Our redflow decides it’s time for its fortnightly shut down and purge. So it shuts down….


Can you see where this is going yet?


Our smug owner now has zero power for 2 hours. The off grid inverter died at the same time the redflow started its purge cycle. Once it’s dead, it can’t start a generator and as far as I can tell, nothing in the redflow did either.


Now our redflow has zero power available to it because its killed the off grid inverter and the generator auto start function in one fell swoop.


So where does our redflow get the power from to do its purge cycle? Remember it’s killed all potential sources of power…..


As far as I can see it can’t do its purge because there is no longer any power anywhere.


Maybe I’m missing something?

It’s a good question, the maintenance mode is unique to Redflow’s Zinc Bromide technology, and is something installers have not had to think about before.

So I thought a comprehensive answer would be useful and I popped the question to Redflow Executive Chairman (and Chief Geek), Simon Hackett.

Here’s Simon’s very comprehensive reply (headings added):

ZCells, Maintenance Cycles and ‘Black Starts’

The maintenance cycle timing depends on activity level. If a battery is cycling continuously 24×7 the maint happens every 4 days. If its being lightly used and mostly idle, yes, it can take as long as two weeks. The battery management system handles this automatically (and minimises it automatically).

At face value you’re right – but only if you don’t design in one of multiple ways to resolve that. And there are multiple ways to resolve that.

Most offgrid systems include a generator – because there is always one more run of bad weather days (i.e. because statistically the battery will always run out sometime, unless you had an outrageous number of them). This can also happen if you under-size the solar array or over-size the loads.

So there always needs to be a way to black-start an off-grid system anyway – you’re just describing another circumstance where a black start is needed.

The Redflow ZBM’s [AKA ZCells] aren’t fussed by being allowed to run flat (or staying that way for ages) – unlike other batteries – so this doesn’t damage them. Perfect for an offgrid system on a ‘weekender’ where you might only be there sometimes, and where having the batteries run flat is thus not necessarily a problem when you’re away. And if a battery does go dead flat with no external power source, no harm is done, and it wakes up and finishes the maintenance cycle once power does return.

Ok, so what are the ways around your scenario below? Multiple of them;

Option #1: Two Batteries

Have two or more Redflow batteries (as per the system pictured). The Redflow BMS automatically sequences maintenance so there is only ever one battery in a maintenance cycle at once. The system pictured is precisely such a system:

  • off-grid,
  • two ZBM2’s into a Victron Multiplus and CCGX
  • solar panels via an SMA Sunny Boy AC inverter.

Option #2: Smart Generator Control

Configure the generator to be auto-started just before the maintenance cycle. In other words, run the generator across the maintenance cycle to keep the power ‘on’. Easy to do (e.g. with a Victron CCGX energy controller, which has a sophisticated generator control function and a trigger relay for it, built right in). Not needed in the system pictured because it has two batteries.

Option #3: DC Coupled Solar + Auto Black Start

Switch over to DC coupled solar. This is the subtle but very cool answer. If you DC couple the solar (e.g. use a Victron MPPT unit to charge the DC batteries directly, alongside the Multiplus inverter in the example pictured – or anyone elses’ equivalent), then you get automatic ‘black start’ the next time the sun comes up.

MPPT DC coupled solar (unlike an AC solar inverter) auto-starts and auto-charges batteries autonomously whenever solar energy is available. So a system can run entirely flat (e.g. middle of the night) and DC coupled incoming solar will reboot it when the sun next rises. Note in this case you can use the BMS to schedule the maintenance cycle start time to be at the solar maximum, so that the solar keeps the place alive while the battery doesn’t. Viable if your solar array is very good and the weather is good. But if that fails, well, it’ll auto-restart again later.

Option #4: Ignore it – you’re on holiday!

For a ‘weekender’ – just ignore the issue. When you turn up next time, manually start the generator, run your site on that, and once the maint cycle finishes the battery will start recharging from the generator (and solar) too.


Obviously the nicest answer is to put two batteries in, but the above (especially DC coupling and an auto-start generator) are quite viable as well if your needs and $$ only run to one battery.

Naturally on-grid its a non-issue with one battery – just schedule the maintenance time to suit a point where you can have the battery empty anyway. Typically in solar-self-consumption on-grid, we configure to start the pre-maintenance discharge cycle at sunset (when it starts discharging anyway) – the battery sends 100% of its energy into the load, runs down completely, does maintenance, and its back again before the sun rises the next time (making the cycle invisible in practice).

About Finn Peacock

I'm a Chartered Electrical Engineer, Solar and Energy Efficiency nut, dad, and the founder and CEO of SolarQuotes.com.au. I started SolarQuotes in 2009 and the SolarQuotes blog in 2013 with the belief that it’s more important to be truthful and objective than popular. My last "real job" was working for the CSIRO in their renewable energy division. Since 2009, I’ve helped over 700,000 Aussies get quotes for solar from installers I trust. Read my full bio.


  1. Jim Raleigh says


    I really enjoy reading your blog as I am keen to invest in Solar. My plan is for about an 8 KW array and I would very much like to make use of the Reposit capability but I see the gradual deterioration of the Tesla as a real downer. For Tis reason I am becoming a fan of the Z Cell but have been unable to get any information on whether the Reposit device will work with it and if not when such capability may become available.

    Do you have any information?

    • Simon Hackett says

      Its up to Reposit to support a given brand of inverter/charger, which is an independent (and non-conflicting) question compared to whether that same inverter/charger is supported by/works with the ZCell BMS (and hence ZCells).

      I.e. these are independent questions and they don’t conflict with each other.

      To put it another way:

      1) Either Reposit supports talking to (and controlling) your inverter or it doesn’t
      2) Either the ZCell BMS has been tested/confirmed to work with that brand of inverter or it doesn’t.

      (1) and (2) are independent variables. There is no need for Reposit to support a ZCell or vice versa. Both Reposit and the ZCell BMS talk to the same inverter, but via different physical interfaces. One is for battery status, the other is for inverter output control.

      So the trick is to find an inverter/charger that Reposit supports and match that with the list of inverter/chargers that work with a ZCell (2 officially so far, but another two in the wings and close to release, and more on the way beyond that).

      I’d love to see Reposit support the Victron inverter/chargers, and I’ll drop them a line to see if we can help to grease the wheels there for them somehow (not withstanding that its not directly our problem space – as above).

  2. Tony Smith says

    Awesome – thanks Finn (and Simon) and BTW (i) Sunergy has been an Internode customer since 2010 and (ii) we’ve filled in the webform to become Redflow installers because we want to be able to offer this technology to our off grid customers.

    I think the question has already been answered, but for the avoidance of doubt, can we please have it confirmed that the Redflow is self powering during its purge cycle?

    • Simon Hackett says

      The last two hours of the maintenance cycle requires around 100W externally from the ZCell undergoing the maintenance cycle.

      That can come from any external source of power – a generator (if running); The sunshine or wind (if DC coupled via an MPPT to the DC bus), or from a second ZCell.

      If power is totally lost during the last phase of the maintenance cycle then it will pick up where it left off, when power is later applied.

  3. Would it not be better to engineer this out of the battery design?

    E.g. Split the cell so that one half continues to provide power while the other half purges?

    • Simon Hackett says

      Yes, it would.

      You might (correctly) imagine that this is something that has occurred to us already and that might well be the subject of some R&D work at present.

      That isn’t a commitment to anything. Its just me agreeing with you.

  4. john nielsen says

    Hi Finn,
    Again a very well articulated description of this battery, even though I didn’t fully understand how it works except for the two battery system. I seem to think that it isn’t a problem with maintenance if you are on the grid which means you will have power when the maintenance occurs,,,, I am not sure I am right on that one, not because of your explanation, but because I don’t know anything in detail about these types of batteries. If the ‘one battery’ maintenance problem is solved by being on the grid, then that seems an easy solution, if you can feed the battery as well as the load at once. Why anyone would go off grid is beyond me to understand. I have 36 kWh of lead gel battery bank and run my off grid system 24/7 from solar only, EXCEPT night time air cons. I don’t know any affordable battery system that can run 4 split system air cons, for that purpose I am on the grid with a separate system, separate power points. Perhaps in years to come, I might be completely off grid, but not today with any of these fancy new batteries which will be completely unable to cope with all night air cons. So you quit the grid and use a generator for night time air cons! Is that any cheaper than using the grid? I know it is a lot more noise.
    John Nielsen, Silkwood

    • Simon Hackett says

      Yes, in practical terms its a non-issue on-grid, if the aim of having the battery is primarily solar self-consumption.

      I have two ZCell batteries on my home system but I’ve been intentionally running with just one of them online of late, to prove out this scenario in practice. All works very nicely.

  5. Hi Finn – thanks for another great article. You say “statistically the battery will always run out sometime” so I assume that also applies to two batteries in which case it would be better to spend my money on a backup generator [option #2] than a second battery [option #1] – unless of course I need that much storage to get me through the night / winter /etc.

    • Simon Hackett says

      With more than one battery, its a non-issue in practice. The ZCell BMS (see the zcell web site for details) automatically orchestrates maintenance cycle timing for maximum availability so that there’s always at least one battery online.

      The BMS also allows flexible control over when any maintenance cycles happen, to let you swing them around to a time of the day where they effectively become invisible (relative to absolute time, or to subset, or to sunrise – which the BMS calculates correctly for your location).

      It does a lot more stuff too – again see the zcell web site (the FAQ) for a better sense of that. It can also be remote monitored and (if needed) adjusted, securely over the Internet.

      All that said – if you’re off-grid, a backup generator (preferably remote-start attached to your energy controller unit) is always a good idea if you’d like 100% uptime – because regardless of the battery technology, there is always a run of ‘one more’ rainy day to accommodate.

      That said, you can engineer automatic restart after a run of terrible weather in another way – which is to have your renewable energy source DC coupled into the batteries (which is self-restarting) rather than AC coupled into it. This is also discussed on the ZCell web site FAQ section.

      These latter things are independent of your choice of battery type, however – and are really questions about off-grid system design in general.

      Just to make one more thing clear – the maintenance cycle is easy to accommodate in the real world using the BMS, and its presence is actually the secret to a long and productive lifetime for the flow battery. Each time the battery comes out of maintenance, its been completely wiped clean – and its like its like having a brand new battery again.

  6. Maurie Wedlake says

    In an attempt to get a ZCell anywhere near affordable ($18,000 gives me a 22 year payback period) I would appreciate comments on the following:
    I understand that a single ZCell can be charged from flat to fully charged at 50 amps max, which is conveniently that amount of current that a 2.5 kW PV array could deliver.
    ZCell literature states that the only charge limitation is that the terminal voltage never exceed 57 volts.
    I propose that a simple voltage controlled cut-off (or diversion) device triggered at 57 volts would achieve that. I am a retired electronics tech and could prob design and make such a device on a quiet weekend!
    This would negate the requirement for the battery charge-controller function in a combined charger/controller (Redflow installers appear to be suggesting $3,000 plus for such a unit – two of which may be needed for reliability if off-grid)!
    One could buy half a dozen el-cheapo Chinese 48 volt inverters for $3,000, giving some degree of peace-of-mind regarding inevitable inverter failures at sometime, and the ability to run more than one of these units in parallel, opens up all sorts of options – although I have yet to see a ‘stackable’ pure sinewave inverter capable of this, but modified square wave (yuk!) devices appear to be readibly available.
    I installed a 1.2 kW PV grid connected system 7 years ago across the river from sunny Mildura, in the good old days of the 60c FIT which ends this December. I haven’t had an electricity bill in that 7 years and I am $3,000 in credit and keen to go off-grid.
    My average daily useage is less than 4 kWh / day, and the Service Charge component of my (pending) electricity bills, will be considerably more than the actual energy-cost, and the total will be in the order of 63 c /kWh!!
    Comments / discussions welcomed.

  7. Adrian Schulz says

    Hi Ron
    I noted from online install photos that the unit is installed outside and the specs mention operating temp min 15 degrees C. Does this mean that in areas where the overnight ambient goes below 15 degrees they need to be housed inside with 15 degrees C plus temperatures?
    Also in the FAQ “What will it cost and what is included in that cost?” I noted that the system does not use DC-DC coupling. Is this possible whilst still supplying an inverter feeding household and possibly grid?
    Thanks Adrian

    • Ronald Brakels says

      Hello Adrian. I was definitely concerned about ZCell’s operating temperature, as in places like Canberra the average high is below 15 degrees all winter. I contacted Redflow and they told me that provided the installation was suitable for the location it wouldn’t have a problem with temperature in Australia. I am a still little concerned because if you are in Canberra and go on two weeks holiday I don’t see why it wouldn’t be below 15 degrees when you came back. But Redflow does use this battery in things such as remote communications towers, so I presume they know how it handles a wide range of temperatures. If I was getting a ZCell in a cold location, I would want the installer to guarantee that it wouldn’t have a problem with low temperatures.

      Redflow covers DC coupling here:


      You can DC couple the ZCell to a compatible multimode (hybrid) inverter and use to to provide power to a home or potentially to the grid. (Although it only makes sense for a battery to provide power to the grid if you are being compensated for it through Reposit or something similar.)

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