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March 31, 2021

ESS Commissioning and the Battery Management System

This article was written with copious amounts of support from Nuvation Energy battery management system designers Nate Wennyk and Alex Ramji.

By now most people in the energy storage industry know what a battery management system does – or to be more precise, what one is used for. The distinction between “does” and “is used for” is important because it opens up a conversation about what an intelligent BMS designed specifically for large-scale energy storage applications can be used for.

After being on site at numerous ESS commissionings and troubleshooting the same system bring-up problems over and over again, Nuvation Energy decided to solve these problems in the BMS. With Nuvation Energy’s BMS you can:

  • Check that all battery cells and modules are properly connected.
  • Check that all voltage sense wires are connected and conducting properly.
  • Dampen power oscillations that can arise from control loops between the BMS and PCS.

Can all battery management systems be used for these tasks?

No, most battery management systems stay exclusively focused on ensuring safety through cell balancing and the enforcement of voltage, temperature, and current limits.

Then why would I want to do those things in the BMS – aren’t there already other ways to solve those issues?

Yes there are, but they are onerous and time consuming. Let’s take a closer look:

Cell and Module Connection Quality

When two apparently flat pieces of metal are pressed against one another (like a bolt against a flat plate), they look to the human eye to be securely touching each other. Under a microscope however, one would see that these seemingly flat surfaces are actually uneven. The two surfaces do not form a seamless connection until they are torqued with a level of force that “flattens” them against each other. Insufficient torqueing of battery connections increases electrical resistance, which inhibits the flow of electricity to and from the battery, and causes excessive heat generation at the connection point.

It is easiest to see this issue visually in this image of 2-Volt large-format lead acid batteries. The less visible connections between series-connected lithium-ion cells, or between modules, also require the same precision in connection quality.

This issue is of particular importance because:

  • Battery modules can emerge from a factory with connection issues between cells.
  • Connection issues between modules and/or cells can occur during ESS assembly (large-format cells may be connected manually on site, as opposed to smaller cells which are shipped as pre-assembled modules by the battery manufacturer).
  • Connection issues may arise as batteries undergo jostling during shipping.
Figure 1. Seemingly flat surfaces are actually uneven, and do not form a seamless connection until they are torqued with a level of force that “flattens” them against each other.

Nuvation Energy’s BMS detects the performance issues that arise from improperly connected cells and modules, and can trigger a warning (alerts you that there is a problem) or fault (disconnects the batteries from the power path) condition in response. The ESS operator can then use the Nuvation Energy BMS to identify the precise location of the connection issue, and provide detailed instructions to service personnel for quick resolution.

This feature has also been used by Nuvation Energy customers prior to ESS commissioning, to detect and return improperly assembled modules to the battery manufacturer for warranty replacement.

BMS Voltage Sense Wire Conductivity

Data is the heart and soul of a BMS. In a large energy storage system there are hundreds of sense wires connecting battery cells to BMS components. Voltage data obtained from sense wires are used by the BMS to set current thresholds, which are provided to the PCS to enable it to manage the batteries safely. An incorrectly connected sense wire can result in inaccurate voltage readings, which can lead to battery control issues. This can also impact State-of-Charge and State-of-Health estimations, and manifest as other issues that can be difficult to troubleshoot.

Diagnosing this sort of problem can be tricky, especially in a large ESS. Or the issue may simply be too small to get noticed until it manifests as battery damage not covered under warranty. That’s why all Nuvation Energy battery management systems:

  • Perform a voltage sense wire conductivity check on boot-up.
  • Immediately detect any connection problems.
  • Prevent connection of the battery to the power path if a connection problem is detected.
  • Continuously perform this check during battery operation.
  • Disconnect the battery stack if a connection issue is detected.
Figure 2. The cell connection quality verification feature of Nuvation Energy’s BMS has been used to detect and return improperly assembled modules to the battery manufacturer for warranty replacement.

A system operator can use the Nuvation Energy BMS Operator Interface to identify the exact location of any “open wires” (we call the feature “Open Wire Detection”) and dispatch service personal to repair the issue, providing exact directions regarding the physical location of the problem.

Control Loop Filtering

A battery management “control loop” can occur between the PCS and BMS, which causes fluctuations (“oscillations” to be more precise) in the voltage and current flowing to or from the battery system. This challenge stems from the effect of current on cell voltage. When current flows through the battery in response to a request from the PCS, battery voltage immediately changes in response to the presence of the current. In response to that voltage change, the BMS alters the current thresholds it provides to the PCS, which results in an updated current request from the PCS, which in turn produces another change in voltage. This circular phenomenon is called a control loop. Un-tuned control loops can create an endless cycle of power oscillation, which must be resolved (“dampened”) before the ESS can be used.

Power oscillations due to control loops are a fairly normal occurrence. Energy storage systems that experience it the most are those in which cell voltages react in a pronounced manner to changes in current. It can be particularly pronounced in systems that are near the end of their operational lifespan, or are using Second-Life cells (i.e. used electric vehicle batteries).

Oscillation is usually managed by the ESS controller, which adds a filter to the voltage data to dampen the oscillation. An ESS can have a separate controller unit, or a smart PCS can include a controller inside the PCS. Programming this filter is usually an arduous process that takes place on site during ESS bring-up.

Figure 3. An incorrectly connected sense wire can result in inaccurate voltage readings, which can lead to battery control issues.
Figure 4. Nuvation Energy’s battery management system includes control loop filters via three programmable registers that are tuned during ESS commissioning.

Nuvation Energy’s battery management system includes control loop filters that can be easily tuned during ESS commissioning via three programmable registers. This electrically simpler solution removes the need to have a controller sending filtered data to the PCS and BMS. It also means the PCS has fewer incidents of oscillating power going through it, which can help improve the lifespan of the switching power electronics and/or battery.

Engineers with a Product vs. Product Engineers

At Nuvation Energy we have spent thousands of hours using our own BMS in energy storage applications. We have taken it to the desert and operated it in blistering heat with sand in every crevice. We have traveled around the world to an island that is only reachable by weekly supply barge, and found ourselves torqueing battery cells one by one. We have turned on megawatt-scale energy storage systems during commissioning and heard the distressed whine of the PCS experiencing power oscillation. In each case we said “we’re going to make this issue easier to resolve.”

All battery management systems are designed to protect batteries and many do that job well – but in large scale energy storage, that’s not always enough.

Contact Nuvation Energy to learn more about the BMS designed to support all aspects of energy storage system management and control.

Nuvation Energy Engineers
Figure 5. The Nuvation Energy BMS is designed to support all aspects of energy storage system management and control.
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