Using a Support Power Supply to enable successful ZBM black-start on some inverter/chargers

Hybrid (DC solar and AC grid) inverter/chargers such as the Sol-Ark, Deye and SunSynk products are capable of 'black start' (starting up from zero charge/zero volts in all attached batteries) with a Redflow battery array.

All of these products have a 'Battery Activation' mode that is designed to do this, and that can achieve this black start providing the BMS has the CANBus 'Send Override Values To Support Black Start' flag turned on under the "EMS Integration" menu (CANBus tab).

However, for Deye and SunSynk products manufactured before late 2023, the 'Battery Activate' process has a hardware incompatibility that means it simply doesn't work properly. 

There is a workaround available for these situations, to enable successful operation of these products with a ZBM battery array. 

What is required for this purpose is a power supply that has the following characteristics:

• Supplies DC power at a voltage suitable for connection to the DC Battery Bus
• Has a current-limiting circuit so that it can self-protect if required
• Can be adjusted so that the DC output voltage is on the low end of the normal DC battery bus voltage range (44V has been found to work well). The intention being to provide enough voltage to boot and run the on-board ZBM battery controller and the Redflow BMS, but not a high enough voltage to result int the ZBM's (or the inverter/charger) 'mistaking' the power supply for a charged battery in its own right and drawing significant energy from it.
• Has a dry-contact on/off control input, allowing (optional) automated on/off control via the Redflow BMS. While optional, implementing this means the power supply (and its cooling fan) will not be running for 99% of the time. 

An example of a power supply that has all of these characteristics, and that is suitable for use with Deye and SunSynk "Low Voltage" hybrid products, is the Meanwell RSP-750-48 power supply.

This is the device that has been used for verification testing of this approach by Redflow. This supply has has enough current capacity to actually charge a single ZBM if needed (and if the output voltage is adjusted upward to suit). However, in this black-start role, we are specifically avoiding the need to directly charge a ZBM from this power supply and the amount of current draw by the supply is very low.

Because we are avoiding the need to direct-charge the ZBM from this unit, it is likely that lower output power versions of this power supply (or other brands) could be used equally well, providing they can supply enough power to drive the on-board pumps and fans for the installed ZBM's (around 60W per unit) plus the BMS (around 10W).

The Meanwell RSP-750-48 is (at the time of writing) available from 'the usual suspects', e.g.: RS, Element14, Mouser, DigiKey etc:

This product delivers an adjustable voltage output at up to 750 Watts, with current limiting and optional remote on/off control.

This photo shows a close up of the interfaces on this power supply, with some notes about how to wire and use them:

Notes

• The power supply should typically be powered from the grid side of the system so that it fires up whenever AC grid power is restored to the site. 
• The DC output should be adjusted using the adjustment point shown in the photo, using a volt-meter, to around 44 volts. This has been shown to work well with these inverter/chargers in this black-start role
• It is mandatory to install an in-line diode (not illustrated), in the positive DC output line from the power supply, in the path over to the DC battery bus. This ensures there is no back-feed of higher DC voltages back in to the power supply. Allowing this back-feed will result in the power supply shutting itself down and also has the potential to lead to damage to the power supply unit. An easy way to deploy a suitably rated diode is to source a bridge rectifier and to wire in one of the diodes in that rectifier for this purpose.
• The BMS should also be powered by wiring it in to the DC battery bus, supported (and booted) by this power supply as a part of black-start of a site.
• The BMS must have the 'Send Override Values To Support Black Start' flag enabled for successful operation with these inverter/chargers. This flag is available in Redflow BMS version 1.13 or later:

Expected Outcome

When starting the system from 'black' (inverter/charger powered down, all ZBM's at zero volts), the Power Supply will boot the BMS and will supply 44V to the DC battery input of the inverter/charger.

Once the inverter/charger starts from the incoming AC grid, it 'sees' this 44V energy source as confirmation of a 'live' battery being present.

Providing the unit is programmed to charge from the AC grid (via the Settings/Gear Icon on the on-screen controller, in a page under the System Work Mode menu) then the system will, within a few minutes, raise the DC bus voltage up to the level requested via CANBus by the BMS. This in turn will allow the ZBM(s) attached to the system to compete their initial 'bubble purge' and commence normal charging and operation.

Providing the power supply is connected and running and that the BMS is sending CANBus data to the inverter/charger, the black start and normal charge commencement for the unit then happens on a fully unattended/automated basis. 

Controlling the Power Supply operation via the BMS (optional)

The optional use of remote on/off control of this unit can be effected using the on-board relay on the Redflow BMS, via the BMS "Digital I/O" engine. This can be programmed easily to turn on and off automatically in the presence/absence of the BMS' internal "Support Power Needed" (SPN) flag.  

This flag turns on any time that the BMS detects a need for the battery array to be 'black started'.

Here are some screen shots of the BMS configuration to achieve this outcome - this is set up in the "Digital IO" menu area of the BMS (these examples are from BMS version 1.13).

First, add the BMS 'Device' to the setup in the Device tab to allow access to the BMS relay, hit the 'Save' button and then use the 'Apply Changes' tab:

Next, create a "Periodic" rule that turns the BMS relay ON whenever the support power is NOT needed, use the 'Save' button, and then the 'Apply Changes' tab once again:

The idea here is to wire the power supply enable loop to the Normally Closed side of the relay, and to open this contact when support power is no longer needed.

This approach is designed to be 'fail safe' - if the DC bus becomes de-energised (and the BMS shuts down), then this loss loss of DC bus power will result in the power supply being enabled for operation (and hence it will restore DC bus power).