Each new Redflow ZBM2 comes with a built in 'EED' (Energy Extraction Device).
This unit is a DC/DC converter that extracts energy from the internal battery stack down to as low as 5V, and up-converts it to 57V (at up to 20A max … approximately 1kW).
The purpose of this device is to achieve a timely full discharge of the energy in the battery during the discharge cycle ahead of the regular battery maintenance cycle (undertaken with the battery stack entirely empty and at 0V).
This full discharge is a requirement of the preparation for the regular ZBM2 battery maintenance cycle (described elsewhere).
You can tell when the EED is enabled - the 'E' flag is 'lit' on the battery entry on the BMS status page.
When on, the EED provides energy onto DC bus just like a third party 57V (max) DC MPPT would provide energy into the DC bus.
Like an MPPT, the EED will cease output if something else on the bus brings the voltage, in turn, above 57V.
The operation of the EED is typically invisible to the rest of the energy system (because the EED-extracted energy flows into another battery and/or into the site load). It only becomes visible as grid export energy if the total site load is below 1kW.
Note that even when the EED is on, the battery can still discharge at more than 1kW; However, any time there less than 1kW of 'natural' energy discharge demand, the EED’s role is to try to keep the discharge rate of the battery concerned at ’not below’ 1kW. This so that even a 100% full 10kWh module can be fully discharged in 10 hours (worst-case) before the 2 hour battery maintenance cycle is run.
The point here is that if the battery is not discharged (ahead of maintenance cycles) in a timely manner and if (for instance) the sun comes up the next day in the meantime, then you can wind up with the battery ‘deadlocked’ (still trying to discharge, and hence not able to take in any new charge, and spending the entire day ’stuck’ before completing discharge the following evening).
This doesn’t harm the battery, but it does lead to a day or two when you aren’t achieving any real benefit from having that battery present.
All of this is far less visible (or totally invisible) if you either have (a) routine site loads above 1kW and/or (b) more than 1 Redflow ZBM2 module on the site.
During maintenance cycles, the requested max DC charge voltage for the upstream energy system is being requested to be a value below 57V by the BMS (intentionally). This is done in order to promote and enable that discharge and (as observed, if needed) also export-to-grid (as a last resort energy destination from the discharging battery).
This behaviour is highly configurable. If you would like to try not having the EED operating until near the very end of the discharge cycle, that is as simple as going to the BMS’ ‘Battery Maintenance’ page and un-checking the box labelled "Immediate maintenance for batteries with an EED “.
The actual voltage target that is advertised in various operation modes is also adjustable via the 'Configuration->EMS Integration' menu on the BMS, if required.
If you do turn off the immediate EED activation (and hit submit!) then the battery will block further charge when it is time to start discharging, but will only discharge ’naturally’ (as and when it can)... until the SoC is below the “Start Maintenance when the SoC is below” threshold on the same page (see screen shot below).
Turning off that checkbox will minimise the time that the EED is on, but it will also make it much more likely that your battery will wind up ‘deadlocked’ for a day or so, every time your overnight energy requirement is below 10kWh.
Hence in the overwhelming majority of cases, having the EED on as early as possible is a good thing!