Solar Quotes Blog

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

simon and 2 batteries

Simon Hackett with an early customer’s double Z-cell 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 Z-Cell 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):

Z-Cells, 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 Z-Cells] 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).

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