Hack Week 21: Keeping the Battery Full

As described in some detail in my last post, we have a single 10kWh Redflow ZCell zinc bromine flow battery hooked up to our solar PV via Victron inverter/chargers. This gives us the ability to:

  • Store almost all the excess energy we generate locally for later use.
  • When the sun isn’t shining, grid charge the battery at off-peak times then draw it down at peak times to save on our electricity bill (peak grid power is slightly more than twice as expensive as off-peak grid power).
  • Opportunistically survive grid outages, provided they don’t happen at the wrong time (i.e. when the sun is down and the battery is at 0% state of charge).

By their nature, ZCell flow batteries needs to undergo a maintenance cycle at least every three days, where they are discharged completely for a few hours. That’s why the last point above reads “opportunistically survive grid outages”. With a single ZCell, we can’t use the “minimum state of charge” feature of the Victron kit to always keep some charge in the battery in case of outages, because doing so conflicts with the ZCell maintenance cycles. Once we eventually get a second battery, this problem will go away because the maintenance cycles automatically interleave. In the meantime though, as my project for Hack Week 21, I decided to see if I could somehow automate the Victron scheduled charge configuration based on the ZCell maintenance cycle timing, to always keep the battery as full as possible for as long as possible.

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Go With The Flow

We installed 5.94kW of solar PV in late 2017, with an ABB PVI-6000TL-OUTD inverter, and also a nice energy efficient Sanden heat pump hot water service to replace our crusty old conventional electric hot water system. In the four years since then we’ve generated something like 24MWh of electricity, but were actually only able to directly use maybe 45% of that – the rest was exported to the grid.

The plan had always been to get batteries once we are able to afford to do so, and this actually happened in August 2021, when we finally got a single 10kWh Redflow ZCell zinc bromine flow battery installed. We went with Redflow for several reasons:

  • Unlike every other type of battery, they’re not a horrible fire hazard (in fact, the electrolyte, while corrosive, is actually fire retardant – a good thing when you live in a bushfire prone area).
  • They’re modular, so you can just keep adding more of them.
  • 100% depth of discharge (i.e. they’re happy to keep being cycled, and can also be left discharged/idle for extended periods).
  • All the battery components are able to be recycled at end of life.
  • They’re Australian designed and developed, with manufacturing in Thailand.

Our primary reasons for wanting battery storage were to ensure we’re using renewable energy rather than fossil fuels, to try to actually use all our local power generation locally, and to attain some degree of disaster resilience.

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