Battery Management Systems at a Glance

• The BMS monitors cell voltages, temperatures, and current flow within the battery system.
• It calculates parameters such as State of Charge (SoC) and State of Health (SoH) and determines permissible charging and discharging limits.
• In critical operating conditions, the BMS can limit power, issue warnings, or initiate a protective shutdown.
• The BMS provides status data and operating limits to inverters, BESS controllers, and higher-level systems via defined interfaces.

How Does a Battery Management System Work?

The BMS collects measurements at different levels of the battery system. These include the voltages of individual cells, temperatures at relevant measurement points, and the current of the battery assembly. Depending on the system architecture, local monitoring units collect these data and transmit them to a higher-level BMS controller.

Based on the measurements and a battery model, the BMS calculates the State of Charge, which represents the battery’s estimated charge level. The State of Health describes the estimated ageing condition and remaining performance capability compared with a new battery or another defined reference condition. Depending on the manufacturer, SoH may be determined using available capacity, internal resistance, power capability, or a combination of these parameters.

SoC and SoH are not directly measurable quantities. Their accuracy depends on factors including sensor performance, the calculation models used, calibration, cell chemistry, and operating history.

Cell balancing is another important BMS function. During this process, the BMS reduces voltage or charge-level differences between individual cells. This helps prevent individual cells from reaching their permissible voltage limits earlier than others and thereby restricting the usable capacity of the entire battery assembly.

The BMS also determines permissible charging and discharging currents or power limits. These limits can change depending on the SoC, temperature, cell voltages, ageing condition, and current faults. Inverters or BESS controllers must account for these dynamic limits when controlling battery power.

If operating limits are exceeded, the BMS can report warnings or fault states, reduce the permissible power, and—depending on the system architecture—open contactors or request a shutdown.

Communication with Inverters, Controllers, and EMS

The BMS exchanges data with other battery storage components to support safe and coordinated operation. Communication with the inverter or Power Conversion System frequently takes place through manufacturer-specific, CAN-based communication protocols. Modbus TCP, Modbus RTU, or other project-specific interfaces may be used to connect higher-level controllers, monitoring systems, or an Energy Management System (EMS).

Typical data transmitted include:

• State of Charge and State of Health
• current voltage, current, and power
• minimum, maximum, and average cell temperatures
• minimum and maximum cell voltages
• permissible charging and discharging power
• operating, warning, and fault states
• contactor and communication status

The available data and whether they may only be read or also used for control depend on the battery manufacturer and the approved interface.

Where Are Battery Management Systems Used?

A BMS is a central component of modern stationary battery storage systems, particularly lithium-based systems. It monitors individual battery modules or racks and consolidates their status information at the system level.

In PV battery systems, the BMS provides the operating limits within which the inverter may charge or discharge the battery. In commercial and industrial installations, these limits are taken into account for applications such as self-consumption optimisation, peak shaving, or time-dependent battery operation.

Current BMS data are also required for grid-supporting applications and the provision of flexibility. A higher-level controller may only implement a planned setpoint if the battery can safely provide the requested power under the current operating conditions.

Benefits, Limitations, and Technical Requirements

The BMS protects battery cells from critical operating conditions and provides the data required for reliable operation. Dynamic power limits make it possible to use the available battery power without exceeding the operating limits defined by the manufacturer. Status and fault data also support monitoring, diagnostics, and maintenance.

However, the quality of this information depends on measurement accuracy and the calculation models used. SoC and SoH in particular are estimates and may deviate from actual conditions due to insufficient calibration, unfavourable operating profiles, or progressive ageing.

Integration into a higher-level system requires a documented and approved interface, unambiguous data-point mapping, and reliable communication. Control systems must also define how they respond to missing, outdated, or inconsistent BMS data. Immediate protection functions must not depend solely on an external platform or cloud connection.

Using BMS Data with EcoPhi

EcoPhi can use approved BMS data for monitoring, visualisation, alerts, and EMS decisions. Charging and discharging setpoints can, for example, be limited based on the SoC, available power, battery temperature, or reported warning states.

The specific integration depends on the available interface, the battery manufacturer’s documentation, and the approved control capabilities. Depending on the battery system, project-specific configuration or additional engineering services may be required.

EcoPhi does not assume direct cell-protection functions. Cell monitoring, contactor control, and protective shutdowns remain the responsibility of the BMS and the battery storage system’s other protection systems.

Conclusion: The BMS as a Protection and Information System

A Battery Management System monitors the condition of a modern battery system and enforces its defined operating limits. Its approved data and power limits also provide an important foundation for monitoring and higher-level EMS strategies.

Frequently Asked Questions About Battery Management Systems

What is the difference between a BMS and an EMS?

The BMS monitors and protects the battery at cell, module, and system level. An EMS uses available operating data to control the battery storage system together with other energy assets according to higher-level objectives. The EMS must comply with the limits specified by the BMS.

Are State of Charge and State of Health measured values?

No. SoC and SoH are estimated using measurements and battery models. Their accuracy depends on factors including sensor performance, calibration, cell chemistry, and operating history.

Does every battery storage system require a BMS?

Not every battery has a BMS. In modern battery storage systems, particularly lithium-based systems, suitable battery monitoring and protection logic are central components of the overall system.

Can an EMS override the BMS protection limits?

An EMS must not override the BMS protection limits. It can optimise setpoints within the approved charging and discharging limits, while the immediate protection functions remain within the battery system.

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Degradation at a Glance

• In battery storage systems, degradation results from both time-dependent calendar ageing and usage-dependent cycle ageing.
• High temperatures, continuously high states of charge, deep discharges and high charging or discharging currents can accelerate ageing.
• The number of charge cycles alone is not sufficient for a reliable assessment of battery condition.
• Long-term data on capacity, energy throughput, temperature and operating conditions can help identify ageing trends and abnormalities.

How Does Degradation Occur in Battery Storage Systems?

The ageing of electrochemical battery cells is caused by various chemical and physical processes. These processes can result in the loss of active material or cyclable lithium. At the same time, the internal resistance of the cells may increase. As a result, the battery storage system may absorb and deliver less energy, produce higher internal losses or provide reduced charging and discharging power.

A general distinction is made between calendar ageing and cycle ageing. In practice, both processes occur simultaneously and cannot always be clearly separated.

Calendar ageing occurs over time, even when the battery is used only minimally. It is accelerated in particular by high cell temperatures and a high state of charge maintained over extended periods. A battery storage system that is continuously kept fully charged and warm can therefore age significantly even without completing many charge cycles.

Cycle ageing results from charging and discharging. Important influencing factors include depth of discharge, the state-of-charge range used, C-rate, cell temperature and the type of load changes. Many shallow partial cycles can affect the battery differently from a small number of deep full cycles. For this reason, the number of full or equivalent full cycles alone provides only limited information about the actual ageing condition.

Why Is Assessing Degradation Important?

When designing a BESS, it is necessary to consider how much usable capacity and power should remain available throughout the planned operating life. Depending on the system architecture, capacity reserves, restricted state-of-charge ranges or technically prepared future expansions may be included. Whether later expansion is actually possible depends on factors such as cell age, manufacturer approvals and the electrical and control-system design.

During operation, degradation is relevant for applications including self-consumption optimization, peak shaving, electricity price optimization and the provision of flexibility. If usable capacity decreases, the storage system can shift less energy. Reduced power may also prevent the system from fully covering short-term load peaks or meeting power requirements.

Ageing data also plays an important role in warranty assessments, residual value evaluations and long-term operational planning. Actual operating conditions must therefore be compared with the assumptions used in the system design, warranty terms and operating strategy.

How Is the Condition of a BESS Assessed?

State of Health (SOH) is a key indicator. It describes the condition of a battery relative to a defined reference state. SOH is often determined by comparing the battery’s current usable capacity with its usable capacity when new. Depending on the manufacturer and method, internal resistance, available power or other cell parameters may also be included.

The comparison must be performed under defined and, where possible, comparable measurement conditions. Capacity and available power depend on factors including temperature, C-rate, the state-of-charge range examined and the applicable operating limits. SOH values from different systems or measurement methods are therefore not necessarily directly comparable.

A meaningful condition assessment should consider several parameters together, including:

• usable charging and discharging capacity,
• maximum available charging and discharging power,
• cumulative energy throughput,
• state-of-charge and temperature profiles,
• charging and discharging rates and operating limits,
• warnings, cell deviations and data from the Battery Management System.

A reliable capacity assessment may require controlled charging and discharging tests. Degradation cannot be determined reliably from a single state-of-charge value or a short-term power measurement.

Degradation of PV Modules

In PV systems, degradation primarily appears as a long-term reduction in available module power. It must be distinguished from temporary or otherwise caused reductions in yield, such as soiling, shading, unfavourable weather conditions, inverter losses, curtailment or technical faults.

An assessment therefore requires normalized long-term data. Yield and power are evaluated in relation to factors such as solar irradiance, module temperature and system availability. A permanent decline in performance can only be assessed reliably after other possible causes have been taken into account.

Monitoring Degradation with EcoPhi

EcoPhi can collect and historically record relevant BESS operating values, provided that these are available through the respective device interface. These values may include state of charge, temperatures, charging and discharging power, energy throughput, operating limits and Battery Management System messages.

Long-term trends can reveal abnormalities, changing operating conditions or indications of potential capacity and power losses. The data can also be used to review operating strategies with regard to temperature, state-of-charge range and battery utilization.

However, a reliable SOH assessment is only possible if suitable battery data and a clearly defined assessment method are available or if additional testing procedures are performed. Monitoring data alone does not replace electrochemical diagnostics or controlled capacity measurements. The specific values available depend on the BESS, its Battery Management System and the supported interfaces.

Degradation Summarized

Over time, degradation can reduce the usable capacity and available power of a battery storage system. Its progression depends on both time and the specific operating conditions. Long-term data supports condition assessment, but an accurate SOH or capacity determination may require additional battery data and defined testing procedures.

Frequently Asked Questions About Degradation

What is the difference between calendar ageing and cycle ageing?

Calendar ageing is time-dependent and occurs even when the battery is used only minimally. Cycle ageing is caused by charging and discharging processes. Both forms of ageing occur simultaneously in an operating battery storage system.

How can battery degradation be reduced?

Suitable temperature conditions, restricted state-of-charge ranges and charging and discharging power adapted to the battery technology can reduce ageing. The appropriate operating strategy depends on the cell chemistry, application and manufacturer specifications.

Is the number of charge cycles sufficient for assessing battery condition?

No. The same number of cycles can cause different levels of ageing depending on depth of discharge, temperature, C-rate and state-of-charge range. Energy throughput, usable capacity, available power and operating conditions must also be considered.

Can a monitoring system determine SOH?

A monitoring system can collect relevant operating and long-term data and provide indications of ageing. However, a reliable SOH assessment additionally requires suitable battery data, comparable measurement conditions, a defined calculation method and, where necessary, controlled capacity tests.

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Virtual cycling refers to trading activities in BESS markets in which electricity positions for the same delivery period are bought, sold or closed out. These positions can offset each other before physical delivery. Therefore, not every virtual cycle results in the battery energy storage system (BESS) actually being charged or discharged.

Virtual Cycling at a Glance

• Virtual cycles result from offsetting trades for the same electricity delivery period.
• A trader or optimizer can modify or completely reverse a planned charge or discharge before delivery.
• Only the net position remaining after trading has concluded is physically implemented through the final BESS schedule.
• Purely virtual cycles do not change the state of charge and therefore do not directly cause storage losses or battery degradation.

How Does Virtual Cycling Work?

Virtual cycling occurs primarily in continuous intraday trading. In these markets, electricity products for specific delivery periods can be traded until shortly before physical delivery. Prices change continuously during this period. A trader or optimization algorithm can therefore close an existing position through an offsetting transaction and subsequently open a new position.

A simplified example illustrates the process: A market participant initially buys 1 MWh of electricity for a specific delivery period at EUR 60/MWh. This position schedules the BESS to charge during the relevant period. If the price rises to EUR 90/MWh before delivery, the same amount of energy can be sold again. The purchase and sale offset each other in terms of volume. No charging schedule remains, the BESS is not physically charged and the price difference represents the economic result of this virtual cycle before trading costs.

If the price subsequently falls again, a new buying position can be opened for the same delivery period. If this position is not closed through another transaction before trading ends, it becomes part of the final schedule and results in the BESS actually being charged.

Trading algorithms can continuously evaluate the order book, current prices, price forecasts and existing positions. Based on this information, they open, modify or close positions. Before physical implementation, all transactions must be consolidated. The operation of the BESS is determined not by the number of intermediate trades but by the final schedule or remaining net position.

Where Is Virtual Cycling Used?

Virtual cycling is mainly used for the short-term market optimization of BESS assets. It allows price changes within the trading window of a delivery product to be used multiple times without physically implementing every adjustment to the trading schedule.

Another application is the continuous adjustment of schedules in response to updated forecasts. If expected market prices, available storage capacity or other market conditions change, an originally planned charge or discharge can be corrected before delivery.

Virtual cycling can also form part of a broader multi-market strategy. In this case, trading positions must be coordinated with other commitments and the BESS’s actual available operating range.

Benefits, Limitations and Technical Requirements

Virtual cycling expands the trading opportunities available to a BESS because price movements can be used without every intermediate position resulting in a physical energy flow. Purely virtual transactions therefore cause no immediate efficiency losses or additional physical battery degradation.

However, the strategy involves trading risks. Market prices may develop differently from the forecast. Low liquidity, bid-ask spreads, transaction costs or trading deadlines may also prevent a position from being closed at the desired time and price. If a position remains open, it must be fulfilled or otherwise balanced in accordance with the applicable market processes.

The final schedule must be compatible with the actual state of charge (SoC), available energy, charging and discharging power, and grid connection limits. Capacity or power already reserved for other applications must also be considered. Implementation therefore requires reliable plant measurements, current operating data, suitable market interfaces and consistent coordination between trading and asset operation.

Virtual Trading Cycle vs. Physical Battery Cycle

A virtual cycle is not a physical battery cycle. It describes offsetting electricity purchases and sales that can fully or partially cancel each other before the delivery period.

A physical battery cycle, by contrast, results from actual energy throughput in the BESS. Only real charging and discharging processes change the SoC, cause conversion losses and contribute to cycle-related battery degradation. Virtual trading cycles must therefore not be counted as full cycles in the BESS’s technical cycle count.

How Can EcoPhi Technically Support Virtual Cycling?

Virtual cycling generally takes place within the trading and optimization systems of an electricity trader, aggregator or specialized market optimizer. EcoPhi is not an electricity trader or market service provider.

On a project-specific basis, EcoPhi can support the technical connection between a BESS and an external market optimization system. This can include processing final schedules or external power setpoints, monitoring the SoC and available power, and implementing the resulting charging and discharging commands. The available functionality depends on the device, market and communication interfaces as well as the requirements of the individual project.

Short Summary

Virtual cycling uses electricity price changes through repeated purchases and sales for the same delivery period. If these positions offset each other, no physical battery cycle occurs. Only the final schedule affects the SoC, storage losses and degradation of the BESS.

Frequently Asked Questions

What is a virtual cycle?

A virtual cycle is a pair or sequence of offsetting electricity trades for the same delivery period. If the positions fully offset each other, the BESS is not physically charged or discharged.

Does virtual cycling affect the battery?

A fully closed-out trading cycle does not directly affect the battery because no physical energy flow occurs. Only physically executed charging and discharging processes affect efficiency and degradation.

What happens to trading positions before delivery?

Open positions are consolidated. The remaining net position forms the basis of the final schedule and must remain within the BESS’s actual operating limits.

Who performs virtual cycling?

Virtual cycling is typically performed by electricity traders, aggregators or specialized market optimizers. A local EMS can subsequently process the resulting schedule or the setpoints derived from it.

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Battery storage monitoring refers to the regular collection, visualization, and analysis of the operating condition, performance, and safety-relevant states of a battery energy storage system. Its purpose is to identify faults and gradual changes at an early stage, ensure system availability, analyze operation, and document indications of possible battery degradation.

Monitoring makes safety-relevant conditions and alarms visible. Immediate protective actions and shutdowns, however, are performed locally by the battery management system (BMS), the battery inverter or Power Conversion System (PCS), and other protection devices.

Battery storage monitoring at a glance

• Monitoring consolidates data from the BMS, battery inverter or PCS, energy meters, and other system components.
• Important key performance indicators include state of charge, state of health, power, temperatures, cell voltages, cycles, and usable or determined capacity.
• Historical analyses and automated alerts support fault analysis, maintenance, and warranty processes.
• Monitoring observes and evaluates operation. Local protection functions are handled by the BMS, PCS, and other protection devices, while an EMS can actively control the battery storage system.

How does battery storage monitoring work?

Operating data is retrieved through the interfaces of the battery management system (BMS), the battery inverter or PCS, and other devices. Typical communication methods include Modbus TCP, Modbus RTU, and manufacturer-specific application programming interfaces (APIs). In addition, an energy meter at the grid connection point can measure grid import, grid export, and current active power.

The most important data includes the state of charge (SOC), state of health (SOH), charging and discharging power, transferred energy quantities, and cell, module, and system temperatures. Cell voltages, voltage deviations, charge cycles, depth of discharge, operating hours, power limitations, alarms, and communication states can also be included.

SOC and SOH are not directly measured quantities. They are generally estimated by the BMS using different measurements and models. Their significance therefore depends on the calculation method used, calibration, and the quality of the underlying data.

A monitoring platform stores the transmitted measurements with timestamps, visualizes current and historical trends, and checks defined thresholds. This makes it possible to identify conditions such as persistently high temperatures, increasing cell voltage deviations, unexpected power limitations, or communication failures.

Sampling, transmission, and storage intervals vary depending on the device, interface, and application. External monitoring typically operates at defined time intervals, while high-frequency monitoring and protection processes are performed locally within the BMS or PCS.

Usable capacity is not provided directly by every system. Determining it reliably may require defined charging and discharging processes, a specified SOC range, and suitable measurement conditions.

Round-trip efficiency is also only comparable when the system boundaries are clearly defined. Depending on the measurement concept, this may refer, for example, to DC-to-DC efficiency, AC-to-AC efficiency, or site-level efficiency including auxiliary consumption.

Where is battery storage monitoring used?

• Operational monitoring: Operators monitor availability, state of charge, power, and current alarms.
• Fault and maintenance analysis: Historical data helps narrow down faults and prepare maintenance activities.
• Performance and degradation analysis: Long-term trends can indicate changes in capacity, efficiency, or cell behavior.
• Portfolio monitoring: Multiple battery storage systems and sites are compared centrally and prioritized according to identified anomalies.

Changes in individual key performance indicators cannot automatically be attributed solely to battery ageing. Temperature, operating strategy, the selected SOC window, power level, or changes in BMS calibration can also affect the results. A reliable root-cause assessment therefore requires comparable operating conditions and, where necessary, further diagnostics or capacity tests.

Benefits, limitations, and technical requirements

Battery storage monitoring provides transparency regarding the actual use and technical condition of the system. It can reduce downtime, make gradual performance changes visible, and provide data for maintenance, warranty, and asset management processes.

The significance of the monitoring data depends on the available interfaces, measurement quality, transmission intervals, and level of detail provided. Not every BMS makes cell-level values, SOH calculations, usable capacity, or internal power limitations externally available. Different measurement points, SOC ranges, and auxiliary consumption can also affect the comparability of key performance indicators.

Reliable communication interfaces, a consistent time base, sufficiently accurate measuring devices, clearly defined system boundaries, and appropriately configured alarm thresholds are therefore required. Monitoring does not replace the local protection functions of the BMS and PCS or proper safety and maintenance planning.

Battery storage monitoring with EcoPhi

Depending on the available device interfaces, EcoPhi can collect data from the BMS, battery inverter or PCS, energy meters, and the grid connection point within a shared platform. This allows BESS, PV systems, loads, and grid power flows to be visualized and analyzed together.

Custom dashboards, historical analyses, and automated alerts support centralized operational monitoring. The measurements available down to cell or module level depend on the respective battery system and its communication interface.

As a local EMS, EcoPhi can additionally transmit charging and discharging setpoints and implement applications such as peak shaving, self-consumption optimization, or feed-in limitation. This requires suitable control interfaces and the release of the relevant functions by the BMS or PCS. Proprietary protocols or specialized analyses may require project-specific integration services.

Battery storage monitoring in summary

Battery storage monitoring makes the operating condition, performance, alarms, and long-term changes of a BESS transparent and traceable. Meaningful analyses require suitable measurements, reliable interfaces, and clearly defined measurement and system boundaries. Monitoring supports technical assessment but does not perform local protection functions. For active operational optimization, it is complemented by an EMS.

Frequently asked questions

What is the difference between monitoring, a BMS, and an EMS?

Monitoring collects, visualizes, and evaluates operating data. The BMS locally monitors and protects the battery, modules, and cells. The EMS controls charging and discharging power in accordance with the operating strategy and within the permissible operating limits.

Which values are particularly important?

Key values include SOC, SOH, charging and discharging power, energy quantities, temperatures, cell voltages, cycles, alarms, and power limitations. The level of detail actually available depends on the BMS and its interface.

Can monitoring determine degradation precisely?

Monitoring data can provide indications of degradation. A definitive assessment, however, requires comparable operating conditions, suitable measurement data, and, where necessary, defined capacity tests or further diagnostics.

Does monitoring control the battery storage system?

Monitoring alone does not control the battery storage system. This requires an EMS or another local controller that transmits setpoints within the operating limits specified by the BMS and PCS.

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