As battery owners and operators seek to maximise the returns from their assets, they simultaneously face the Herculean challenge of managing degradation. This remains one of the most prominent challenges in the industry, where assets are expected to last around 15 years before reaching End-of-Life (EoL).
Degradation manifests itself in several ways leading to reduced energy capacity, power, efficiency and ultimately return on investment.
Put simply, battery degradation is a serious economic problem which will vary according to how the battery is used. It is therefore essential to monitor factors which drive degradation. These include temperature, ramp rate, average State of Charge (SoC) and Depth of Discharge (DoD).
Analysing the impact of these factors is vital to assessing the cost-benefit of decisions to charge or discharge a battery in response to different market signals.
This is especially important as single/multi-service batteries have the option of participating in a variety of markets, such as frequency regulation or the Balancing Mechanism (BM), and each market can have a different risk level according to the asset’s load profile and cycling behaviour.
Back to basics: what ‘exactly’ is a charge cycle?
Unfortunately, and confusingly, the industry has different definitions for what ‘a cycle’ actually is. In commercial documents, such as warranties, a cycle is calculated via energy throughput. This tallies the energy going in/out of the battery and divides total energy throughput by capacity. Even though this is a relatively simple calculation, it actually only tells you the number of ‘Equivalent Full Cycles’, or EFCs.
EFCs do not quantify DoD, which factors how deep charge cycles are. As can be seen below, EFCs would be unable to distinguish 1 cycle of 100% DoD vs 2 cycles of 50% DoD vs 10 cycles of 10% DoD. Cycle depth is completely ignored in EFCs! For this reason, KiWi Power utilises the Rainflow algorithm as a tool for profiling each ‘real cycle’ in terms of DoD.
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