Something Interesting Happened

Our 5.94kW solar array with Redflow ZCell battery and Victron Energy inverter/charger system is now slightly over three years old, which means it’s time to review its third year of operation. There are several previous posts in this series:

If you’ve read the above you’ll know that the solar array was originally installed back in 2017 along with a Sanden heat pump hot water service. That initial installation saved us a lot on our electricity bills, but it wasn’t until we got the ZCell and the Victron gear that we were able to really manage our own power. The ZCell allows us to store our own locally generated electricity for later use, and the Victron kit manages everything and gives us a whole lot of fascinating data to look at via the VRM portal.

There were some kinks in the first two years. We missed out on three weeks of prime solar PV generation from January 20 – February 11 in 2022 due to having to replace the MPPT solar charge controller. We also had no solar PV generation from February 17 – March 9 in 2023 on account of having our old tile roof replaced with colorbond steel. In my last post on this topic I wrote:

In both cases our PV generation was lower than it should have been by an estimated 500-600kW. Hopefully nothing like this happens again in future years.

…and then at the very end of that post:

I’m looking forward to doing another one of these posts in a year’s time. Hopefully I will have nothing at all interesting to report.

Alas, something “like this” did happen again, and I have some interesting things to report.

In early December 2023 our battery failed due to a leak in the electrolyte stack. It was replaced under warranty, but the replacement unit didn’t arrive until March 2024. It was a long three months. Then in August when we were looking at finally purchasing a second ZCell, we discovered that Redflow had made a commercial decision to focus exclusively on large-scale deployments (minimum 200 kWh, i.e. 20 batteries) and was thus no longer selling individual ZBMs for residential or small business use. As an existing customer we probably would have still been able to get a second battery, except that in late August the company went into voluntary administration after failing to secure funding to build a new factory in Queensland. The administrators attempted to seek a sale and/or recapitalisation, but this was ultimately unsuccessful. The company ceased operations on October 18 and subsequently went into liquidation. This raises several questions about the future of our system, but more on that later. First, let’s look at how the system performed in year three.

Here are the figures for grid power in, solar generation, power used by our loads, and power exported to the grid over the past three years. As in the last two posts, the “what?” column here is the difference between grid in plus solar in, minus loads minus export, i.e. the power consumed by the system itself, or the energy cost of the system.

YearGrid InSolar InTotal InLoadsExportTotal Outwhat?
2021-20228,5315,64014,17110,84975411,6032,568
2022-20238,9365,74414,68011,53479912,3332,347
2023-20248,8785,62114,49911,1621,48912,6511,848

Note that in year three our grid power usage and solar generation are slightly down from the previous year (-58kWh and -123kWh respectively), so the total power going into the system is lower by 181kWh. Our loads are happily down by 372kWh, a good chunk of which will be due to replacing some old always-on computer equipment with something a bit less power hungry.

What’s really interesting here is that our power exported to the grid is close to double the previous two years, and the energy cost of the system is noticeably lower. In the first two years of operation the latter figure was 16-18% of the total power going into the system, but in year three it’s down to a bit under 13%.

The additional solar export appears to be largely due to the failed battery. Compare the following two graphs from 2022-2023 and 2023-2024.Yellow is direct usage of solar power, blue is solar to battery and red is solar to grid. As you can see there’s way more solar to grid in the period December 2023 – March 2024 when the battery was dead and thus unable to be charged:

Why is there still any blue in that period indicating solar power was going to the battery? This is where things get a bit weird. One consideration is that the battery is presumably still drawing a tiny bit of power for its control circuitry and fans, but when I look at the figures for January 2024 (for example), it shows 76.8 kWh of power going to the battery from solar. There is no way that actually happened with the battery dead and unable to be charged.

Here’s what I think is going on: when the battery went into failure mode, the ZCell Battery Management System (BMS) will have told the Victron gear not to charge it. This effectively disabled the MPPT solar charger, which meant we weren’t able to use our solar at all, not even to run the house. I asked Murray from Lifestyle Electrical Services if there was some way we could reconfigure things to still use solar power with the battery out of action and he remoted in and tweaked some settings. Unfortunately I don’t have an exact record of what was changed at this point, because it was discussed via phone. All I have in my notes is a very terse “Set CGX to use Victron BMS?” which doesn’t make much sense because we don’t have a Victron BMS. Possibly it refers to switching the battery monitor setting from “ZCell BMS” to “MultiPlus-II 48/5000/ 70-50 on VE.Bus”. Anyway, whatever the case, I think we have to assume that the “to battery” and “from battery” figures from December 2023 – March 2024 are all lies.

At this point we were able to limp along with our solar generation still working during the day, but something was still not quite right. Every morning and evening the MPPT appeared to be fighting to run. Watching the console at, say, 08:00, I’d see the MPPT providing solar power for a few seconds, then it’d stop for a second or two, then it’d run again for a few seconds. After some time it would start behaving normally and we’d have solar generation for the day, but then in the evening it would go back to that flicking on and off behaviour. My assumption is that the ZCell BMS was still trying to force the MPPT off. Then in mid-Februrary I suddenly got a whole lot of Battery Low Voltage warnings from the MPPT, which I guess makes sense – the ZCell was still connected and its reported voltage had been very slowly dropping away over the past couple of months. The warnings appeared when it finally hit 2.5V. Murray and I experimented further to try to get the MPPT to stop doing the weird fighting thing, but were unsuccessful. At one point during this we ended up with the Mutli-Plus II inverter/chargers in some sort of fault state and contacted Simon Hackett for further assistance. We got all the Victron gear back into a sensible state and Simon and I spent a bunch of time on a Saturday afternoon messing with everything we could think of, but ultimately we were unable to get the MPPT to provide power from the solar panels, and use grid power, without the battery present. One or the other – grid power only or solar power only – we could do, but we couldn’t get the system to do both at the same time again without the battery present. Turns out a thing that’s designed to be an Energy Storage System just won’t quite work right without the Storage part. So from February 15 through to March 14 when the replacement battery arrived we were running on grid power only with no solar generation.

Happily, we didn’t have any grid power outages during the three months we were without a battery. Our first outage of any note wasn’t until March 23, slightly over a week after the replacement battery was installed. There were a few brief grid outages at other times later – a couple of minutes one day in April, some glitches on a couple of days in August, but the really bad one was on the 1st of September when the entire state got absolutely hammered by extremely severe weather. Given there was a severe weather warning from the BOM I’d made sure the battery was full in advance, which was good because our grid power went out while we were asleep at about 00:37 and didn’t come back on until 17:28. We woke up some time after the grid went down with the battery at 86% state of charge and went around the house to turn off everything we could except for the fridge and freezer, which got our load down to something like 250W. By morning, the battery still had about 70% in it and even though the weather was bad we still had some solar generation, so between battery and solar we got through just fine until the grid came back on in the afternoon. We were lucky though – some folks in the north of the state were without power for two weeks due to this event. I later received a cheque for $160 from TasNetworks in compensation for our outage. I dread to think what the entire event cost everyone, and I don’t just mean in terms of money.

Speaking of money though, the other set of numbers we need to look at are our power bills. Here’s everything from the last seven years:

YearFrom GridTotal BillGrid $/kWhLoadsLoads $/kWh
2016-201717,026$4,485.45$0.2617,026$0.26
2018-20199,031$2,278.33$0.2511,827$0.19
2019-20209,324$2,384.79$0.2612,255$0.19
2020-20217,582$1,921.77$0.2510,358$0.19
2021-20228,531$1,731.40$0.2010,849$0.16
2022-20238,936$1,989.12$0.2211,534$0.17
2023-20248,878$2,108.77$0.2411,162$0.19

As explained in the last post, I’m deliberately smooshing a bunch of numbers together (peak power charge, off peak power charge, feed in tariff, daily supply charge) to arrive at an effective cost/kWh of grid power, then bearing in mind our loads are partially powered from solar I can also determine what it costs us to run all our loads. 2016-2017 is before we got the solar panels and the new hot water service, so you can see the immediate savings there, then further savings after the battery went in in 2021. This year our cost/kWh (and thus our power bill) is higher than last year for two reasons:

  1. We have somehow used more power at peak times than during off-peak times this year compared to last year.
  2. Power prices went up about 8% in July 2023. They actually came down about 1% in July 2024, but most of our year is before that.

I should probably also mention that we actually spent $1,778.94 on power this year, not $2,108.77. That’s thanks largely due to a $250 ‘Supercharged’ Renewable Energy Dividend payment from the Tasmanian Government and $75 from the Federal Government’s Energy Bill Relief Fund. The remaining $4.83 in savings is from Aurora Energy’s ridiculous Power Hours events. I say “ridiculous” because they periodically give you a bunch of time slots to choose from, and once you’ve locked one of them in, any power you use at that time is free. To my mind this incentivises additional power usage, when we should really be doing the exact opposite and trying to use less power over all. So I haven’t tried to use more energy, I’ve just tried to lock in times that were in the evening when we were going to be using more grid power than during the day to scrape in what savings I could.

One other weird thing happened this year with the new battery. ZCells need to go into a maintenance cycle every three days. This happens automatically, but is something I habitually keep an eye on. On September 11 I noticed that we had been four days without running maintenance. Upon investigation of the battery logs I discovered that the Time Since Strip counter and Strip Pump Run Timer were running at half speed, i.e. every minute they were each only advancing by approximately 30 seconds:

I manually put the battery into maintenance mode and Simon was able to remotely reset the CPU by writing some magic number to a modbus register, which got the counters back to the correct speed. I have no idea whether this is a software bug or a hardware issue, but I’ll continue to keep an eye on it. The difficulty is going to be dealing with the problem should it recur, given the demise of Redflow. Simon certainly won’t be able to log in remotely now that the Redflow cloud is down, although there is a manual reset procedure. If you remove the case from the battery there is apparently a small phillips head screw on the panel with the indicator lights. Give the screw a twist and the lights go out. Untwist and the lights come back on and the unit is reset. I have yet to actually try this.

The big question now is, where do we go from here? The Victron gear – the Cerbo GX console, the Multi-Plus II inverter/chargers, the MPPT – all work well with multiple different types of battery, so our basic infrastructure is future-proof. Immediately I hope to be able to keep our ZCell running for as long as possible, and if I’m able to get a second one as a result of the Redflow liquidation I will, simply so that we can ensure the greatest possible longevity of the system before we need to migrate to something else. We will also have to somehow figure out how to obtain carbon socks which need annual replacement to maintain the electrolyte pH. If we had to migrate to something else in a hurry Pylontech might be a good choice, but the problem is that we really don’t want a rack of lithium batteries in the crawl space under our dining room because of the fire risk. There are other types of flow battery out there (vanadium comes to mind) but everything I’ve looked at on that front is either way too big and expensive for residential usage, or is “coming soon now please invest in us it’s going to be awesome”.

I have no idea what year four will look like, but I expect it to be interesting.

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