Degradation refers to the progressive loss of performance of a technical system due to operation and ageing. In a Battery Energy Storage System (BESS), it primarily manifests as a reduction in usable storage capacity and/or available charging and discharging power. Increasing internal losses can also reduce battery efficiency. PV modules likewise experience a gradual decline in performance over their operating life.
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.