Deploying a Support Power Supply on the DC bus

In some cases, there can be a need to provide a backup source of electrical energy on the DC bus to support the ZBM maintenance cycle.

This is not required for Victron Energy based systems, where the MultiPlus/Quattro products will provide a DC power source to the bus even when all attached ZBM2's are empty and/or performing their maintenance cycles.

It is also not required for Trumpf TruConvert based systems.

For the rare cases where there is a need to do this (including to support other sorts of energy system that do not provide a 'support' power feed when all ZBM2's are empty or undertaking maintenance), it is possible to deploy a 'Support Power Supply'.

Note that this approach is not recommended in general and there are various edge cases it creates that may require firmware changes to the inverter/charger systems concerned, to properly handle them. 

There are two ways to do this - a simple way requiring no system interfacing, and a more sophisticated way that allows the support power supply to be actively switched on and off as required under BMS control. 

In both cases, the power supply that Redflow has used successfully in this role, and one we recommend, is a MeanWell RSP-750-48. This is available from various suppliers such as Digikey and Mouser.

The RSP-750-48 has the following useful features in this context:

• 750W capable - powerful enough to slowly charge a ZBM2 on a standalone basis if/when required (which can be useful for initial bootstrap of some sites as well as for the 'Support Power Supply' role described here)
• Can have the voltage adjusted with a simple screwdriver-adjusted potentiometer on the unit, down as low as 40V DC (optimal for supporting batteries during maintenance without the power supply conflicting with other devices on the bus at a higher voltage)
• Can be switched on and off under the control of a dry-contact relay output (e.g. via the Redflow BMS' on-board relay) if required

Support Power Supply - general considerations

In all cases, the support power supply should be wired by an appropriately licensed electrician and should include the following in the deployment:

• Mounting in a suitable enclosure to prevent access to dangerous voltages during operation
• An appropriate circuit breaker on the AC input supply path to the power supply
• The use of a suitably rated inline diode in the DC positive output line, to prevent higher voltages on the DC bus from attempting to 'back feed' back into the power supply.

A common approach to source a suitable protection diode is to use suitably rated Bridge Rectifier (such as this one) and just run the DC output cable through two pins on the diode bridge (to access a single diode out of the bridge):

Support Power Supply - Version 1 (Self Managing) - 40V  - Maintenance Support Only

In the 'self-managing' approach, the power supply is set (with the voltage set-screw on the unit) so that it outputs 40V DC, and it is wired to the DC bus and turned on and left on.

When there is no other source of DC energy on the bus, the 40V feed from the power supply, via the diode, is automatically presented on the DC bus as a 'fallback' energy source. 

This 40V supply is sufficient to keep the on-board ZBM2 controller operating (and can also power other control devices on the DC bus such as the Redflow BMS). This 'baseline' power feed is sufficient for the ZBM2's to complete their periodic maintenance cycle, and return to being ready to recharge.

This scenario requires that the main energy system controller and charger can then 'take it from there', and apply a charge-capable voltage (55V or higher) that will start to recharge the ZBM2's. 

As soon as there is a higher voltage on the DC bus than the 40V being emitted by the support power supply, the diode automatically shuts off the supply of energy from the support supply, and it simply remains on-but-idle, consuming very little energy, ready for the next time that it is needed. 

The external energy system should be configured so that it considers that 40V is below its cutoff voltage for discharging battery energy. This means that the external system won't mistake the power supply for a battery and try to use it to power the entire site.

Support Power Supply - Version 2 (BMS Controlled) - 55V - Maintenance plus initial Pre-Charging

Some energy systems are not capable of maintaining the necessary stable 55V output voltage, for long enough to get the ZBM2's to perform their pre-charge and initial battery charging phases. 

The 'Version 2' approach is intended for this situation. It is far more complex and should not be used if it can be avoided. 

In this scenario, the support power supply must be set to deliver a 55V output power level, so that it is the support supply that achieves the initial pre-charge of the ZBM2(s) in the system.

The challenge here is that the external energy system will mistake this voltage source (at the 55V level) as being a battery, and hence the energy system will generally then (incorrectly) try to use that 55V voltage source to power the entire site (via the 750W power supply). This won't end well. 

To avoid this, we have to construct an 'interlock' scenario such that when the support power supply is on, the main energy system is prevented from discharging from the DC battery bus.

This entire approach is fraught with challenges, and should only be deployed if you are 100% sure you know what you are doing and why you are doing it.

The choice of 55V (not higher, not lower) as the output voltage is very specific and very important.

At 55V, a Redflow ZBM2 will be brought through the 'bubble purge' and 'pre-charge' phases and can be charged slowly (at around 10A) by the power supply, without over-drawing on that power supply (which will occur at higher output DC voltages).

However, 55V (as noted above) will fool an energy system into thinking the power supply is a battery, and will overload it, unless you actively disable that external system from discharging DC energy until the support supply turns off.

Here are the deployment steps required:

• Physically construct the power supply setup (as noted for version 1), and set the output voltage on the power supply to be 55V
• Cut the power supply enable loop on the Meanwell Power supply (see the manual). This is a loop on a removable header plug on the end of the unit. 
• Wire the two ends of the cut loop into the output from the Redflow BMS' on-board signalling relay, using the 'Normally Closed' output pin on the BMS relay (so that a loss of power supply to the BMS will lead to the support supply being automatically enabled to restore power to the DC bus).
• In the Redflow BMS, configure a 'BMS Relay' endpoint in the Digital I/O section.
• Create a 'Periodic' rule in the Digital I/O section that send the value of the field 'Support Power Not Needed' to the BMS relay endpoint, updated (say) every 5 seconds.

The 'Support Power Not Needed' boolean is false (0) when the Support Power Supply is required by the Redflow battery array (when there is no battery online capable of delivering DC bus power).

With this output false, the normally-closed relay signal engages the support power supply.

Once at least one ZBM2 battery has completed maintenance and has pre-charged, the support power supply then charges the battery at a circa 10A rate. 

The BMS monitors the charge in the battery system, and it keeps the supply engaged until at least one battery has reached around 2% State-Of-Charge.

At that point, the BMS changes the status of the 'Support Power Not Needed' signal - it sets it true (1).

With the signal true, the relay is triggered, opening the dry-contact circuit. This, in turn, switches the support power supply off.

The last necessary step is to disable the external energy system while the support supply is on. As noted above, this is essential to avoid the support supply being drawn upon by the site loads and overloaded.

There are multiple ways to do this, and we will discuss this in detail below.

Disabling external battery discharge while the support supply is enabled

Version 2 requires that an approach is adopted that avoids any other use of DC energy from the bus, by the external energy system, until the support power supply is no longer in use.

There are two common ways to achieve this.

1. Use the CANBus 'BMS Alarm' signal to disable the external energy system

You can enable the checkbox on the BMS CANBus page called 'Support Power Alarm':

This flag, when set, causes the BMS to emit a CANBus protocol alarm called 'BMS Alarm', whenever the support power supply is required.

The "BMS Alarm" signal is one that tells the external energy system there is 'something' (unspecified) wrong with the BMS and so it is signalling that it is 'not happy'.

Many energy systems will respond to this alarm by suspending their normal operation and going into an idle or 'pass-through' mode instead.

If this approach works, then the external energy system can be 'idled' in this way, with no extra cabling - using the existing CANBus status protocol connection that is already present in a properly built Redflow based energy system.

      2. Use the a relay output to achieve suspension of the external system operation

If approach 1 is not effective, then the BMS 'Support Power Needed' signal can be used, via another dry-contact relay or via a MODBUS-TCP command, to disable the external system (assuming that it can in turn be disabled with a dry-contact relay output).

The BMS can drive a separate relay to achieve this outcome, using either a direct-attached USB relay module, or a MODBUS-TCP attached relay module, and another 'Periodic' rule in the BMS' Digital I/O section.

It may also be possible to use MODBUS-TCP 'write' commands to directly shut down the external energy system. The BMS can flexibly send MODBUS-TCP write commands to any valid MODBUS-TCP endpoint, based on the value in the 'Support Power Needed' signal. 

The precise approach here depends on the nature of the external energy system concerned.

If any of this is unclear, chances are that you should not be using this approach at all.

Finally - no guarantee is made that this approach will be effective for you - we are providing this information as a suggestion only, not as a guarantee of anything.

The best approach to use in most cases is 'Version 1' (if support power is needed at all).