Synchronized battery checks traced to recurring power-grid frequency dips
Researchers at Forschungszentrum Jülich have traced recurring, minute-by-minute frequency dips on electricity grids to synchronized state-of-charge checks by stationary batteries. The pattern has appeared in measurements from Europe, Australia, the United States and Malaysia. On Ireland’s grid, the size of the one-minute drop rose from 3 milli-Hertz in 2016 to 12 mHz in 2021, with some more recent events exceeding 20 mHz.
The investigation began after Tennet Germany brought the anomaly to Dirk Witthaut. Grid frequency must remain within a defined operating range, and unexplained oscillations can complicate operators’ response. Large swings can contribute to blackouts; unchecked fluctuations can also harm power-plant equipment, particularly hydropower turbines.
From a hydropower hypothesis to a timed load
Hydropower was initially a plausible explanation. Turbines vibrate, and resonant mechanical behaviour can amplify those vibrations into ultra-low-frequency oscillations visible on the electrical system. Witthaut’s team modelled a hydropower plant control system on the Scandinavian grid and found conditions capable of producing a once-per-minute resonance.
That model did not explain the persistence of the signal. Mechanical resonances normally appear and recede, whereas the observed drop returned every 60 seconds. The regular timing led the researchers to look for equipment governed by synchronized clocks rather than an intermittent physical process.
More than 80 meters identify the battery signal
The team installed more than 80 smart power-quality meters around the Jülich campus. The site includes medium- and low-voltage distribution lines, transformers, solar panels, battery energy-storage systems, a high-performance computing system and office equipment supported by smaller uninterruptible power supplies.
Sampling frequency, voltage, current and power every three seconds revealed a matching peak every minute at the transformer connected to the battery energy-storage system. Many stationary batteries, including data-centre UPS fleets and grid-scale systems, perform state-of-charge checks at that interval. Because internet-connected devices keep closely synchronized time, a tiny action that is imperceptible from one unit can become measurable when many systems perform it together.
Ireland shows the scale of the exposure
Grid-operator data from Ireland strengthened the explanation. Data centres accounted for 18% of the country’s grid load by 2022, and each facility uses fleets of UPS units. Ireland’s 50 Hz system is designed to withstand deviations of plus or minus 200 mHz, although its operator prefers to limit swings to 100 mHz. A 20 mHz event is therefore within that range, but it can add to fluctuations caused by other loads.
The researchers also detected periodic power fluctuations in IoT equipment, including a smart oven and Raspberry Pi clusters. They propose a software change: manufacturers should randomize the timing of battery checks rather than schedule them exactly on the minute. Many IoT devices remain unpatched, and a cited study found that fewer than 3% run the latest firmware.
A practical requirement for operators and buyers
For data-centre operators, battery owners and procurement teams, the finding makes firmware behaviour a grid-reliability question. They should establish whether UPS and battery-management systems randomize routine checks, maintain update processes for connected equipment, and include that requirement in supplier reviews, because coordinated loads can span organizations and national grids.