Optimising battery charge rate and maintenance time using BMS Version 1.11

Background: New EMS Menu

As of BMS version 1.11 and later, the old 'Configuration->CANbus' menu has been replaced with a new 'EMS Integration' page (see screen shot below). This menu now contains two distinct tabs, with the old 'CANBus' page now split into a System voltage settings tab and a separate CANBus tab.

'EMS' means "Energy Management System" and it is a reference to the upstream site energy system controller being used to drive the overall energy system operation.

(Other tabs will appear on this page in future software releases, to allow hardware-specific control of various brands of site power handling electronics for special applications)

Improved System Voltage Settings

For BMS 1.11 and later, some significant software improvements have been made to the control of system charge voltage in specific circumstances.

These updates yield some significant system performance improvements (as described in detail below).

For existing sites that are upgraded to version 1.11 and for new sites where the BMS is initially shipped with software older than version 1.11, it is necessary to adjust the two values circled in green on the screen shot below , to take full advantage of the performance improvements delivered by version 1.11

Specifically:

• The "Normal Charge Voltage" can be raised to 57.5V if it is currently lower; and
• The "Discharge/Maintenance Cycle Voltage" should be set to 56.5V if it is currently 'Disabled' or set to a lower voltage value.

The screen shot below shows you how the 'System' tab should be set up for most sites, for optimal performance.

If the values circled in green are not set as shown below, you can change them and hit 'Submit' to bring them up to date:

These new settings in general (as shown above) are recommended as the baseline for all sites.

These values have been chosen with significant care. In almost all cases, changing these values to anything other than those values shown above, without knowing why, will lead to reduced system performance.

We will now explain the specific improvements that can be delivered to your site, once those settings are updated to be 'as above':

Safely maximising battery charge voltage 

Redflow battery maximum charge rate is limited by the charge voltage being sent during normal charging periods. Battery charge rate varies, for a given voltage, based on the time since the last maintenance cycle and (to a lesser extent) upon battery temperature. This means that different batteries charging at once will charge at different speeds, for a given DC bus voltage level. 

If the charge voltage setting is too low, then the average charge rate for the entire array is reduced substantially. 

If the voltage is set too high, the risk exists that at some points in the charge cycle some batteries will sustain a charge rate above the permitted maximum charge rate (50A) for a significant period, leading to one or more batteries protectively disconnecting themselves during charging operations. 

The optimal voltage (one that which allows the highest average charge rate, without the 'highest performing' battery going into over-current disconnect) varies over time.

BMS version 1.11 introduces a new "High Current Voltage Reduction" setting (default: 1 Volt).

Any time that the BMS observes a battery entering the High Current Warning state (indicating that a battery is at near-future risk of being disconnected), the BMS immediately (within just a few seconds) subtracts this value from the Normal Charge Voltage, to avoid any risk of an Over-Current Failure/Disconnection.

After half an hour, the system returns to the normal voltage and 'tries again'. 

Field experience has shown that raising the normal charge voltage to 57.5V and implementing this dynamic backoff approach can improve average site charge rates by as much as 25% compared to the use of a fixed (compromise) value of 57V.

On some sites (especially those with long cabling) that show a voltage drop between the charger and the batteries, the 'Normal' value can be raised to a higher value (up to 58V or in some cases 58.5V). If you elect to do this, we recommend that you also adjust the 'Voltage Reduction' value by the same amount, so that the 'backoff' outcome is 56.5V (or lower)

Maximising battery discharge-for-maintenance speed on a 'charging bus'

The "Discharge/Maintenance Cycle Voltage" value existed in earlier BMS versions but the functionality had not been implemented. In BMS version 1.11 and later, this feature is fully operational.

When a voltage is configured into this field (default and recommended value: 56.5V), then this voltage becomes the target charge voltage any time that 1 or more batteries are undertaking their pre-Maintenance discharge process with the Energy Extraction Device (EED) turned on ("E" Flag engaged on the battery status page).

The EED is designed to extract all of the energy from the battery by raising the internal stack voltage up to an external value of 57 Volts. This 57V energy supply (at up to 1kW) helps to empty the battery in a timely manner, sending the extracted energy into the site load and/or into adjacent batteries. 

However, if the DC bus is in a 'charging' state at the time that a battery is trying to empty itself using an EED, a system charging voltage at or above the EED voltage (57V) will 'block' the EED, and stop the battery being able to discharge.

By limiting the maximum charge voltage to 56.5V whenever an EED is turned on, the EED is still able to extract energy from a battery and deliver it to the site and the site remains capable of charging batteries in general (albeit at a mildly reduced maximum rate).

In overall performance terms, it is much better to allow the EED's to complete their work as fast as possible, and to return all batteries to being chargeable, instead of having batteries get 'stuck' for an extended period being unable to use their EED's to discharge themselves. 

Note that if this behaviour is not desired, this setting can simply be set to 'Disabled'. 

Summary

BMS version 1.11 delivers significant software-driven site performance improvements for practically all Redflow sites. 

These improvements may be activated as simply as adjusting the values shown in the screen shot above and hitting 'Submit'.